U.S. patent number 3,702,973 [Application Number 05/072,982] was granted by the patent office on 1972-11-14 for laser or ozone generator in which a broad electron beam with a sustainer field produce a large area, uniform discharge.
This patent grant is currently assigned to Avco Corporation. Invention is credited to Jack D. Daugherty, Diarmaid H. Douglas-Hamilton, Richard M. Patrick, Evan R. Pugh.
United States Patent |
3,702,973 |
Daugherty , et al. |
November 14, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
LASER OR OZONE GENERATOR IN WHICH A BROAD ELECTRON BEAM WITH A
SUSTAINER FIELD PRODUCE A LARGE AREA, UNIFORM DISCHARGE
Abstract
Apparatus for and a method of producing controlled discharges
substantially throughout a large volume of a gaseous medium by
generating in an enclosure a controlled density of free electrons
in the medium and controlling the electron temperature of the free
electrons to a level preventing a substantial increase in their
density by a self-regenerative ionization process so that for a
wide range of uniformity of both the density and temperature of the
medium, a stable and controlled discharge is produced in the medium
suitable for the intended use of the medium. Apparatus for and the
method of producing a discharge in accordance with the invention is
useful for the production of lasting action, electrically
conductive ionized gas for use in magnetohydrodynamic (MHD) devices
and the like, or to produce or facilitate carrying out chemical
processes such as, for example, the generation of ozone and the
like.
Inventors: |
Daugherty; Jack D. (Winchester,
MA), Douglas-Hamilton; Diarmaid H. (Boston, MA), Patrick;
Richard M. (Winchester, MA), Pugh; Evan R. (Lexington,
MA) |
Assignee: |
Avco Corporation (Cincinnati,
OH)
|
Family
ID: |
22110985 |
Appl.
No.: |
05/072,982 |
Filed: |
September 17, 1970 |
Current U.S.
Class: |
372/74; 310/11;
315/111.01; 372/58; 422/186.07; 422/906; 204/176; 313/420; 372/33;
372/85 |
Current CPC
Class: |
H01S
3/09707 (20130101); H01J 17/00 (20130101); H02K
44/08 (20130101); Y10S 422/906 (20130101); H01J
2893/006 (20130101) |
Current International
Class: |
H02K
44/00 (20060101); H01J 17/00 (20060101); H02K
44/08 (20060101); H01S 3/097 (20060101); H01s
003/00 () |
Field of
Search: |
;331/94.5 ;313/74
;315/111 ;204/176,313,316 ;310/11 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dumanchin et al.: Comptes Rendus, vol. 269, November, 1969, pp.
916-917. .
Beaulieu: DREV Memorandum M-2005/70 January, 1970..
|
Primary Examiner: Wibert; Ronald L.
Assistant Examiner: Bauer; Edward S.
Claims
We claim:
1. In the method of producing a spatially uniform controlled
discharge substantially throughout a gaseous working medium in a
working region, the steps comprising:
a. providing a gaseous working medium at a pressure in a working
region disposed in a cavity having imperforate walls for confining
the gaseous working medium that upon the production of secondary
electrons in said medium said medium has ambipolar and thermal
diffusion rates incapable of damping local increases in secondary
electron density in said medium;
b. generating ionizing radiation externally of said cavity;
c. introducing said ionizing radiation into said cavity through one
of said walls to produce substantially throughout said working
region a substantially spatially uniform predetermined density of
secondary electrons in said medium by ionizing said medium, said
one wall being impervious to gases and pervious to said ionizing
radiation; and
d. providing a sustainer field for providing substantially
uniformly throughout said working region a predetermined electron
temperature effective to increase the average energy of said
secondary electrons without substantially increasing said
predetermined electron density by self-regenerative ionization,
said electron temperature producing said controlled discharge
substantially uniformly throughout said working region at a
predetermined level.
2. The method as defined in claim 1 wherein said electron
temperature is controlled at least in part by flowing said medium
through said cavity.
3. The method as defined in claim 2 wherein said density and
temperature are maintained at said values less than that which will
produce uncontrolled arcing for times less than the characteristic
time of said discharge.
4. The method as defined in claim 1 wherein the density of said
secondary electrons is controlled at least in part by flowing said
medium through said cavity.
5. The method as defined in claim 1 wherein said working medium is
passed through said cavity.
6. In the method of producing a spatially uniform controlled
discharge substantially throughout a gaseous working medium in a
working region, the steps comprising:
a. passing a gaseous working medium at a pressure through a working
region disposed in a cavity having imperforate walls for confining
the gaseous working medium that upon the production of secondary
electrons in said medium said medium has ambipolar and thermal
diffusion rates incapable of damping local increases in secondary
electron density in said medium;
b. generating ionizing radiation externally of said cavity;
c. introducing said ionizing radiation into said cavity through one
of said walls to produce substantially throughout said working
region a substantially spatially uniform predetermined density of
secondary electrons in said medium by ionizing said medium, said
one wall being impervious to gases and pervious to said ionizing
radiation;
d. providing a sustainer field for providing substantially
uniformly throughout said working region a predetermined electron
temperature effective to increase the average energy of said
secondary electrons without substantially increasing said
predetermined electron density by self-regenerative ionization;
and
e. providing a pressure and velocity of said medium in said working
region to produce said controlled discharge substantially uniformly
throughout said working region at a predetermined level.
7. In apparatus for producing a controlled discharge for providing
molecular excitation of a gaseous working medium, the combination
comprising:
a. means defining a cavity having a working region disposed
therein, said cavity having imperforate walls for confining a
gaseous working medium and defining a predetermined cross section
and volume;
b. a working medium in said cavity and working region at a pressure
that upon the production of free electrons in said medium at said
pressure said medium has ambipolar and thermal diffusion rates
incapable of damping local increases in electron density in said
medium;
c. first means for generating ionizing radiation externally of said
cavity;
d. second means for introducing said ionizing radiation into said
cavity through one of said walls and producing substantially
throughout said working region a substantially uniform
predetermined density of secondary electrons in said medium by
ionizing said medium, said one wall being impervious to gases and
pervious to said ionizing radiation; and
e. third means for providing a sustainer field for providing
substantially throughout said working region a predetermined
electron temperature of said secondary electrons effective to
increase the average energy of said secondary electrons without
substantially increasing said predetermined electron density by
self-regenerative ionization, said electron temperature producing
said controlled discharge substantially uniformly throughout said
working region at a predetermined level.
8. The combination as defined in claim 7 wherein said cavity
includes gas inlet and gas outlet means, and additionally including
fourth means coupled to said gas inlet for flowing said medium
through said cavity.
9. The combination as defined in claim 8 wherein said fourth means
includes further means for providing a predetermined pressure and
velocity of said medium in said working region.
10. The combination as defined in claim 8 and additionally
including diaphragm means separating said first means and said
cavity, said ionizing radiation being introduced into said medium
through said diaphragm intermediate said gas inlet and said gas
outlet and normal to the direction of flow of said medium through
said cavity.
11. The combination as defined in claim 10 wherein said third means
includes electrode means for providing a sustainer electric field
in said cavity normal to the direction of flow of said medium, said
electrode means comprising a first electrode adjacent the wall
through which said radiation is introduced and through which said
radiation passes, and a second electrode oppositely disposed to
said first electrode adjacent the opposite wall of said cavity.
12. The combination as defined in claim 11 wherein said second
means includes a perforate plate member and a thin diaphragm
covering and carried by said plate member, said diaphragm being
disposed between said plate member and said medium.
13. The combination as defined in claim 7 wherein:
a. said medium has an upper and lower laser state;
b. said first and second means provides a density of secondary
electrons in said medium sufficient to support a population
inversion; and
c. said third means increases the average energy of said secondary
electrons to a level to produce a population inversion in said
medium in said cavity.
14. The combination as defined in claim 13 and additionally
including:
a. means for passing said medium through said cavity in the form of
pulses; and
b. means for actuating said first, second and third means
intermediate said pulses to produce said population inversion
intermediate said pulses.
15. The combination as defined in claim 13 wherein said medium is
continuously passed through said cavity.
16. In the method of light generation by stimulated emission of
radiation substantially throughout a gaseous active medium in a
working region, the steps comprising:
a. providing a gaseous active medium at a pressure in a working
region disposed in a cavity having imperforate walls for confining
the gaseous working medium that upon the production of secondary
electrons in said medium said medium has ambipolar and thermal
diffusion rates incapable of damping local increases in secondary
electron density in said medium, said medium having an upper and
lower laser state;
b. generating externally of said cavity a broad area electron beam
having a cross sectional area conforming substantially to that of
said working region;
c. introducing said electron beam into said cavity through one of
said walls to produce substantially throughout said working region
a substantially spatially uniform predetermined density of
secondary electrons in said medium having an average energy
insufficient to produce a population inversion in said medium, said
one wall being impervious to gases and pervious to said electron
beam; and
d. providing a sustainer field for providing substantially
uniformly throughout said working region a predetermined electron
temperature effective to increase the average energy of said
secondary electrons without substantially increasing said
predetermined electron density by self-regenerative ionization,
said electron temperature producing an average energy level
sufficient to support a population inversion in said medium.
17. The method as defined in claim 16 wherein said medium is at
least sequentially passed through said cavity at a pressure and
velocity to produce substantially uniformly throughout said working
region a population inversion in said medium.
18. The method as defined in claim 17 wherein the electron
temperature is controlled by providing a sustainer electric field
in said medium and said pressure and velocity are provided to
produce substantially maximum population inversion in said working
region.
19. The method as defined in claim 16 wherein said population
inversion is serially provided in the form of pulses and the energy
added to the medium by the introduction of said free electrons is
less than the energy added to the medium by said sustainer
field.
20. In high powered laser apparatus the combination comprising:
a. gas supply means for producing a flow of a gaseous medium having
a predetermined velocity and pressure and an upper and lower laser
state;
b. means defining a cavity including a working region for receiving
said medium from said gas supply means and through which said flow
passes;
c. first means for generating externally of said cavity a broad
area electron beam having a cross sectional area conforming
substantially to that of said working region, said means defining
said cavity including walls for confining said medium, one of said
walls including a diaphragm impervious to said medium and pervious
to said electron beam;
d. second means for introducing said electron beam into said cavity
through said diaphragm forming a part of said one of said walls of
said cavity and produce a substantially uniform spatial
distribution of secondary electrons in said medium in said working
region by ionizing said medium, said secondary electrons having an
average energy insufficient to produce a population inversion in
said medium; and
e. third means for providing a sustainer field for controlling the
electron temperature of said secondary electrons in said medium to
substantially uniformly throughout said working region increase
their average energy without substantially increasing the density
thereof by self-regenerative ionization at said velocity and
pressure and produce a population inversion in said medium in said
working region.
21. The combination as defined in claim 20 wherein said third means
includes means for generating a sustainer electric field in said
cavity.
22. The combination as defined in claim 21 wherein said means for
generating said sustainer electric field includes first and second
electrode means in said cavity.
23. The combination as defined in claim 22 wherein said second
electrode means is comprised of a perforate member and disposed in
spaced relationship over said diaphragm.
24. The combination as defined in claim 23 wherein said working
region includes means for passing a light beam through said working
region.
25. The combination as defined in claim 23 wherein said cavity
includes means defining an optical cavity in said working
region.
26. The combination as defined in claim 25 and additionally
including control means for actuating said second and third means
in the pulsed mode.
27. The combination as defined in claim 25 and additionally
including control means for actuating said second and third means
in the continuous mode.
28. The combination as defined in claim 20 wherein said first means
adds energy to said medium in an amount that is less than that of
said third means.
29. The combination as defined in claim 20 and additionally
including:
a. A perforate plate member carried by said means defining said
cavity and covered by said diaphragm, said diaphragm being disposed
over said plate member and between it and said medium in said
cavity.
Description
The present invention in its broadest sense is directed to the
production of and apparatus for providing useful controlled
discharges in a gas at pressure levels and volumes such that
discharge stabilization by electron pair diffusion to confining
walls is negligible, that is, the discharge is not wall
dominated.
In one embodiment, the invention may comprise means for increasing
if not providing the desired electrical conductivity of the gaseous
working medium in MHD devices such as generators and accelerators.
It is equally applicable to other devices and the like that require
or use electrically conductive or ionized gas.
In another embodiment, the invention comprises means for producing
ozone wherein the working medium may comprise oxygen or air which
is passed through a discharge comprising an independent source of
electrons and an electric field in accordance with the invention.
Since the electric field is decoupled from the production of
electrons optimum conditions for ozone formation are attainable
without severe requirements on ballasting as present in the use of
a Townsend discharge, or on electrode geometry as present in the
use of corona discharge. Because uniform conditions are provided in
the positive column, the overall energy efficiency is increased and
heat dissipation involved in the process is reduced.
In a still further and preferred embodiment, the invention
comprises a high-power gas laser which is volumetric in character
and that can be scaled in all three characteristic dimensions as
well as in pressure level. A controlled discharge is created where
electron-ion diffusion to the walls is negligible.
While the preferred embodiment of the present invention will be
described in connection with an electrically excited nitrogen
(N.sub.2), carbon dioxide (CO.sub.2) and helium (He) laser, it may,
as noted above by way of example, be applied to other systems where
a conducting ionized gas is required or useful and including, but
not restricted to, gas constituents other than N.sub.2, CO.sub.2
and He as well as other lasing systems. Since the discharge
produced by this invention does not require ionization by the
discharge electrons, in a lasing environment, a discharge in
accordance with the invention can be adjusted to the correct
electron temperature for most efficient laser operation. Moreover,
a laser in accordance with the invention is volumetric in the sense
that the proper gas temperature and lower laser state
concentrations are maintained by means other than diffusion through
the gas to cooled side walls. Further, apparatus in accordance with
the invention may be operated in the flowing gas as well as the
static pulse mode.
Light amplification by stimulated emission of radiation (laser) has
extended the range of controlled electromagnetic radiation to the
infrared and visible light spectrum. A laser produces a beam of
coherent electromagnetic radiation having a particular well-defined
frequency in that region of the spectrum broadly described as
optical. This range includes the near ultraviolet, the visible and
the infrared. The coherence of the beam is particularly important
because it is that property which distinguishes laser radiation
from ordinary optical beams. On account of its coherence, a laser
beam has remarkable properties which set it apart from ordinary
light which is incoherent. While the maser (microwave amplification
by stimulated emission of radiation) and the laser are based on the
same principles of statistical and quantum mechanics, the problems
and the physical embodiments for achieving laser action are
completely different from those for masers.
Coherence, the essential property of lasers is of two kinds:
spatial and temporal. A wave is spatially coherent over a time
interval if there exists a surface over which the phase of the wave
is the same (or is correlated) at all points. A wave is
time-coherent at an infinitesimal area on a receiving surface if
there exists a periodic relationship between its amplitude at any
one instant and its amplitude at later instants of time. Perfect
time coherence is an ideal since it implies perfect
monochromaticity, something which is forbidden by the uncertainty
principle.
Laser beams have a number of remarkable properties. Because of
their spatial coherence, they generally have an extremely small
divergence and are therefore highly directional. For example, a
ruby laser beam one inch in diameter at the source will be about
four feet across on a surface ten miles away. The very best that
could be accomplished over the same distance with an incoherent
source, such as an arc lamp at the focus of a six-foot parabolic
mirror, would be a beam spread over an area more than one-third of
a mile across. Another important feature of lasers is the enormous
power that can be generated in a very narrow wave length range.
Under certain operating conditions, nearly monochromatic bursts of
millions of watts can be produced. To get comparable radiation
intensity from a black body, it would have to be raised to a
temperature of hundreds of millions of degrees--a condition not
practically achievable. A laser beam, because it possesses space
coherence can be focused to form a spot whose diameter is of the
order of one wave length of the laser light itself. Enormous power
densities are thus attainable. For example, the focused output of a
50-kilowatt infrared burst from a laser can have a radiant power
density of the order of 10.sup. 12 watts/cm.sup.2 ; this is about
100 million times the power density at the surface of the sun.
Extraordinarily high temperatures, orders of magnitude greater than
that at the sun, can be generated at the small area which absorbs
this concentrated radiation. Furthermore, since the electric field
strength of an electromagnetic wave is proportional to the square
root of its intensity, the field at the focus of the laser beam can
be millions of volts per centimeter. A promising potential of
lasers comes from time coherence. It is this property which
permitted prior art exploitation of radio and microwaves for
communications. However, laser frequencies are millions of times
higher than radio frequencies, and hence are capable of carrying up
to millions of times more information. In fact, one single laser
beam has in principle more information-carrying capacity than all
the combined radio and microwave frequencies in use at the present
time.
Accordingly, systems applications of lasers are useful for
communication in space, on earth, and under sea. Military
surveillance and weapons systems, mapping, medical, mining,
manufacturing, and computer technology may also include lasers.
Two conditions must be fullfilled in order to bring about laser
action: (1) population inversion must be achieved and (2) an
avalanche process of photon amplification must be established in a
suitable cavity such as, for example, an optical cavity. Population
inversion can, for example, be accomplished if (1) the atomic
system has at least three levels (one ground and at least two
excited levels) which can be involved in the excitation and
emission processes and (2) the lifetime of one of the most
energetic of the excited states is much longer than that of the
other or others.
When a system is in a condition where light (photon) amplification
is possible, laser action can be achieved by providing (1) means
for stimulating photon emission from the long-lived state, and (2)
means for causing photon amplification to build up to extremely
high values. In the usual embodiment, this is accomplished by
fashioning the medium containing the active atoms into a cylinder
with perfectly (as far as possible) parallel ends polished so
highly that the surface roughness is measured in terms of the wave
length of the laser. The ends may be simply polished metal or they
may be silvered or dielectric coated so that they behave as mirrors
which reflect photons coming toward them from the interior of the
cylinder. Such a structure, whether the mirrors are within or
outside the container, is called an optical cavity. If now pumping
means, such as for example, an electric discharge acts on the
medium and brings about population inversion of the long-lived
state with respect to another lower energy excited state even
though the long-lived state is only relatively long-lived, in a
small fraction of a second there will be spontaneous emission of
photons. Most of these photons will be lost to the medium but some
of them will travel perpendicular to the ends and be reflected back
and forth many times by the mirrors. As these photons traverse the
active medium, they stimulate emission of photons from all atoms in
the long-lived state which they encounter. In this way the degree
of light amplification in the medium increases extraordinarily and
because the photons produced by stimulated emission have the same
direction and phase as those which stimulate them and assuming the
optical quality of the laser media is suitable, the electromagnetic
radiation field inside the cylinder or cavity is coherent. In order
to extract a useful beam of this coherent light from the cavity,
one (or both) of the mirrors is made slightly transmissive. A
portion of the highly intense beam leaks through the mirror, and
emerges with regularly spaced wave fronts. This is the laser
beam.
Parallelism of the mirrors is a rigorous geometrical requirement in
low gain lasers. Thus, in low gain lasers, if the mirrors are not
precisely parallel, the light rays that build up in the cavity will
tend to digress further and further toward the edges of the mirrors
as they are reflected back and forth between the mirrors, and
finally they will be directed out of the cavity altogether. It is
essential that any deviation from parallelism be so small that the
coherent photon streams will reflect back and forth a very large
number of times to build up the required intensity for laser
action.
In all diffraction limited optical configurations such as those
discussed above, coherent wave fronts appear to originate from a
common center and so they can, by use of a lens, be made
plane-parallel and hence, except for diffraction effects,
non-divergent. In high gain lasers other optical configurations
such as oscillator amplifier configurations and unstable resonators
are used. A characteristic of these devices is that the photons
only make a small number of passes through the laser medium. In
present operational lasers, the photon is reflected about only two
or three times.
By way of example, a continuously operating gas laser is disclosed
in an article, "Population Inversion and Continuous Optical Maser
Oscillation in a Gas Discharge Containing He-Ne Mixture," Physical
Review Letter, 6, page 106, 1961. In the usual embodiment of static
gas, prior art gas lasers, the gas is statically contained in a
tube about 100 centimeters long. The mirrors which form the ends of
the optical cavity are disposed either inside the tube or external
to it. Pumping is accomplished in this system by electrical
excitation (either radio frequency or direct current).
In addition to the helium-neon gas laser system, other gas laser
systems have been achieved with helium, neon, argon, krypton,
xenon, oxygen, and cesium (the last optically pumped in the gaseous
state) as emitting atoms.
Other systems include carbon dioxide, helium, and nitrogen. For a
more complete discussion of the high-power flowing system including
carbon dioxide, helium and nitrogen reference is made to patent
application of C. K. N. Patel, Ser. No. 495,844, filed Oct. 14,
1965 abandoned in favor of continuation-in-part application, Ser.
No. 814,510 filed Mar. 28, 1969, now U.S. Pat. No. 3,596,202, and
assigned to Bell Telephone Laboratories, Inc. Such a high-power
laser typically includes two reflectors forming a suitable
resonator or cavity, a tube forming the side walls of the laser,
suitable pumping apparatus including a cathode, anode and
direct-current sources connected in appropriate polarity between
the anode and the cathode; inlet apparatus; a source of carbon
dioxide, helium, and nitrogen connected to the inlet apparatus; and
equipment for exhausting the spent gases from the laser or for
cooling and separating them for reuse.
As indicated hereinabove, a laser output may be generated in
various media (i.e., crystals, semiconductors and gases) by pumping
or introducing energy to create an inversion where a large number
of the atoms are in high energy levels to support photon emission.
In prior art gas lasers, whether flowing or static, the lasers were
pumped or excited by using a diffusion controlled electrical
discharge in a small tube maintained at a low pressure. Typically,
in such gas discharge tubes (typically of the order of 1 centimeter
in diameter) operating at low pressures (about 1-10 torr) there is
a loss of electron-ion pairs from the center of the plasma to the
side walls of the tube by radial diffusion (so-called ambipolar
diffusion of ion-electron pairs). For a steady state operation of
the discharge, this loss must be made up by a net ionization rate
in the plasma which exactly balances the diffusion loss rate. This
required ionization rate dictates what temperature the electrons
must have to sustain the discharge, and hence what applied E/N is
needed to give the electrons that temperature. For long tubes E/N
is defined by the applied voltage divided by the tube length and
gas density.
In such situations, the discharge can be said to be "ballasted" by
the tube walls, i.e., since radial diffusion of the electron-ion
pairs is fast, any small local increase in electron density is
reduced by diffusion. This fact makes such discharge radially and
axially uniform as well as quite reliable and simple to
produce.
The plasma (neutral gas plus electron-ion pairs) contained inside
the electric discharge tube tends to remain radially smooth as long
as the time required for the electron-ion pairs to diffuse to the
surrounding walls is equal to the ionization time such as, for
example, the time required to double the electron density. Since
the ambipolar diffusion time is generally proportional to the
product of the gas pressure and the tube diameter squared for large
diameters, this ambipolar diffusion time can, under some
circumstances, become long compared to the ionization time in the
tube, especially for high ionization rates, large diameter tubes
and high pressures. In this latter situation, the discharge is no
longer "ballasted" by the presence of the tube walls, i.e., local
increases in the electron density are not immediately diffused to
the walls where they are reduced by wall recombination, etc.
Accordingly, local non-uniformities can be produced by these higher
electron densities and the fast-growing non-uniformities can become
worse. Often the result is that the previously uniform glow
discharge turns into arcs, streamers or current spokes. This latter
condition often is a plasma that is very inefficient, and often
useless for certain purposes.
From the above, it will be seen that in high-pressure, large
diameter discharge tubes the tendency is for any local increase in
electron density not to be damped by diffusion to the confining
walls. Upon occurrence of such disturbances, one can reduce their
tendency to grow by reducing the ionization rate which means a
lower electron temperature since the local ionization rate is a
function of the local electron temperature. A lower electron
temperature, however, requires that a lower electric field must be
applied. The proper balance is a critical one: too high an electric
field can allow the high pressure large diameter discharge to
"spoke," but if too low an electric field is applied, the discharge
cannot be started in the first place. Further, at high pressures,
it is generally found that an applied voltage or electric field
large enough to start a discharge is also large enough to cause the
discharge to be radially non-uniform and, for example, "spoke." For
the preceding reasons, it will be seen that if a discharge tube or
cavity has a sufficiently large volume, maintenance of a controlled
discharge therein by diffusion to the walls is not possible. As
used herein the term "controlled discharge" means in a gaseous
medium a discharge having predetermined properties which although
such properties may vary in space and time, they remain at least
within desired limits for the time that the discharge exists. Such
properties include but are not limited to the electronic and
molecular states of the gaseous medium as well as the optical,
electrical and chemical qualities of the medium, and its heating,
ionization, dissociation, and recombination rates. A controlled
discharge provided in accordance with the invention has a
"characteristic time" which is substantially the duration of the
time that sustainer current flows in the gaseous medium as a result
of the motion of secondary electrons generated in the gaseous
medium under the influence of an electric field termed herein a
sustainer electric field more fully described hereinafter. For the
case of a flowing medium wherein flow time through the cavity or
working region is less than the duration of sustainer current in
the gaseous medium, the "characteristic time" is the flow time
through the cavity.
This invention is an improvement over that disclosed in pending
patent application Ser. No. 859,424 filed Sept. 19, 1969 by James
P. Reilly, abandoned in favor of continuation-in-part application
Ser. No. 50,933, filed June 29, 1970, to which reference is made,
and assigned to the same Assignee as this patent application.
It is an object of the invention to provide apparatus for and a
method of producing controlled discharges in a gaseous medium.
It is another object of the invention to provide a controlled
discharge in a gaseous medium in a controlled manner with
predetermined effect on background temperature, density and
pressure of the medium.
A still further object of the invention is to provide apparatus for
and a method of producing controlled, large, volumetric discharges
without the inherent ionization instability that occurs when the
discharge current itself produces the ionization.
A further object of the invention is to provide apparatus for and a
method of producing spatially uniform discharges in a gaseous
medium that can be used, for example, to provide a lasing medium
chemical reaction processes, mediums for MHD devices and the like
and other applications where a conducting gaseous medium is
necessary or useful to achieve a desired result.
Another object of the present invention is to provide apparatus for
and a method of producing a population inversion suitable for use
in a gas laser oscillator or amplifier.
It is another object of the present invention to provide apparatus
for and a method of producing laser action in a flowing gas by
electrical excitation.
A still further object of the invention is to provide a method of
and apparatus for controlling the gas temperature in a gaseous
laser by proper choice of gas flow velocity and input power to
increase the efficiency of the lasing of the gaseous laser.
A still further object of the present invention is to provide a
method of and apparatus for producing laser action in a flowing gas
by generating free electrons, and an electrical discharge to
maintain the optimum electron environment to produce the lasing
action.
A still further object of the present invention is to provide a
method of and apparatus for producing laser action in a flowing gas
by electrical excitation comprising an electron beam to create
electrons and a DC voltage to produce a discharge which maintains
the optimum electron environment to produce lasing action.
Due to the ability to control the distribution of a discharge in a
lasing medium, a still further object of the invention is to permit
in an electrically excited gas laser an arrangement of electrical
excitation means resulting in optimum optical qualities.
The novel features that are considered characteristic of the
invention are set forth in the appended claims. The invention
itself, however, both as to its organization and method of
operation, together with additional objects and advantages thereof,
will best be understood from the following description when read in
conjunction with the accompanying drawings in which:
FIG. 1 is a perspective view with parts broken away of the
apparatus in accordance with the invention;
FIG. 2 is a sectional view taken on lines 2--2 of the apparatus
shown in FIG. 1;
FIG. 3 is a sectional end view taken on lines 3--3 of the apparatus
shown in FIG. 2;
FIG. 4 is a perspective view with parts broken away showing details
of the electron source;
FIG. 5 is a perspective diagrammatic view illustrating method of
operation and coordinates associated with electron generation, gas
flow, and lasing activity; and
FIG. 6 is a block diagram of the circuitry associated with the
electron gun and sustainer electrodes.
Attention is now directed to FIGS. 1-6 which show various details
of laser apparatus incorporating the invention. Apparatus is shown
in these figures wherein a gaseous medium capable of producing
lasing action such as, for example, a mixture comprising 16%
CO.sub.2, 34%N.sub.2 and 50 % He is supplied from a suitable
conventional source (which may comprise a plenum chamber and
diffuser - not shown) to suitable means defining a cavity or
working region 10 of laser apparatus 12 via gas inlet means 11. The
cavity or working region 10 of the laser apparatus (generally
designated by the numeral 12) is shown by way of example as being
generally rectangular in configuration. The term "cavity" as used
herein means not only one that is defined by walls, but also one
that is not defined by walls or the like since in certain cases
such are not essential to carrying and/or using the invention. As
best shown in FIGS. 2 and 3, the rectangular working region 10
comprises oppositely disposed top and bottom walls 14 and 15
adapted to receive and support respectively mirror holder and
adjustment assemblies 21 and 22 more fully described hereinafter.
Carried on the inner surfaces, the top and bottom walls 14 and 15
are oppositely disposed arcuate flow members 16 and 17 which are
arranged and adapted to function to provide a smooth laminar flow
through the working region 10. The mirror assemblies 21 and 22 are
each disposed in members 16 and 17 and recessed to provide minimum
disruption of flow and minimize spurious arcing. Oppositely
disposed side walls 18 and 19 are sealably attached to the top and
bottom walls 14 and 15, side wall 18 being provided with a circular
opening to sealably receive electron gun apparatus 25 more fully
shown and described hereinafter. Oppositely disposed to the
aforementioned circular opening in side wall 18 is a recess in side
wall 19 for receiving a flush mounted electrically conductive
electrode plate 52 more fully described hereinafter. The
aforementioned components other than electrode 52 defining the
working region 10 are preferably comprised of an electrical
nonconductive material, such as, for example, Lucite, Melamine,
Fiberglass - Epoxy, and the like.
As shown in FIGS. 1 and 4, the electron gun generally designated by
the numeral 25 includes a rectangular electron source comprising an
electrically conductive enclosure 26 constructed of stainless steel
or the like and open at one end. Within enclosure 26, electrons are
generated in conventional manner by thermionic emission from a
plurality of spaced filaments 27 which are supported within and
near the rear portion of enclosure 26 by a plate 28 comprised of
electrically nonconductive material. Filaments 27 are supported by
electrically conductive stand-offs 29 which are coupled to a source
of filament current 30. The filaments are heated in conventional
manner by source 30 to produce the thermionic emission. The
enclosure 26 is mechanically supported within and insulated from
the outer cylindrical wall 36 by supports 33 and 34 which also
provide electrical connection to the pulse circuit 40. Supports 33
and 34 permit application to enclosure 26 the potential necessary
to control the amount of electrons generated therein. One form of
control may be provided as shown via a reticulated screen grid 35
electrically and mechanically connected to the enclosure 26 and
covering the open end thereof.
A conventional pulse circuit 40 (see FIG. 6) coupled to grid 35 via
supports 33 and 34 and enclosure 26 provides the necessary
potential to control the amount of high energy electrons released
by the electron gun. The pulse circuit 40 is triggered or actuated
by a timing circuit 41. Actuation and control of the electron gun
is more fully described hereinafter. Broadly, the electron emitter
or gun provides an abundance of high energy electrons which are
defocused and directed toward the working region 10 through the
screen grid 35 (see FIG. 5).
The volume surrounding the electron gun within wall 36 is evacuated
by a vacuum pump (not shown) in conventional manner and the
electron gun maintained at a low pressure to provide an optimal
environment for the free electrons generated therein to pass
unhampered through screen grid 35 and be attracted and accelerated
toward a reticulated electrical conducting plate 45. Plate 45, made
of stainless steel or the like, is maintained at a potential high
compared to that of screen grid 35. Electrons generated at the
filaments 27 are strongly accelerated toward plate 45 and a portion
pass through the plurality of holes 46 provided in plate 45. A thin
sheet of material or diaphragm 47 is disposed between the working
region 10 and the electron gun to permit the existence of separate
pressure regimes. Diaphragm 47, which may be at least in part
supported by plate 45, must possess adequate structural stability
to withstand any required pressure differential (the vacuum in the
electron gun 25 and the pressurized gas flow in the working region
10) and composed of a material arranged and adapted to transmit the
maximum number of electrons without absorbing an excessive portion
of their energy which can reduce efficiency and/or result in
failure of the diaphragm. While preferably composed of metal, the
diaphragm 47 may be composed of either nonconductive or conductive
material.
After electrons from screen grid 35 pass first through the holes 46
in the plate 45 and then the diaphragm 47, they enter the working
region 10 by passing through a reticulated cathode 50 which may be
constructed of a wire mesh and insulated, if desired, from the
electron gun 25 by a ring of non-conducting material 51. In the
working region 10, electron energy is maintained by a sustainer
electric field between oppositely disposed anode plate 52 and
previously mentioned cathode 50 which are coupled to the sustainer
circuit 53. Cathode 50 which may be comprised of a wire mesh grid
as previously noted prevents, for the electron beam and sustainer
electric field arrangements as shown, damage to the diaphragm 47
from spurious arcs which may be otherwise inadvertently struck
between the anode 52 and/or cathode 50 and diaphragm 47. A high
voltage direct current potential is typically maintained between
anode 52 and cathode 50 by a conventional sustainer circuit 53
which may comprise capactive discharge means charged by power
supply 54 and triggered by timing circuit 41 for pulsed operation.
The example hereinbefore given is for a shower head type electron
beam which covers a broad area, however, the same result may be
accomplished by the provision of a rapidly swept beam of electrons
over a broad area.
The production of a volumetric controlled discharge, which for the
embodiment illustrated in FIGS. 1-6 comprises the excitation and
inversion of a gaseous medium in the working region 10 between the
sustainer anode 52 and cathode 50 is provided in accordance with
the invention in two steps. A "discharge" as used herein is, in an
ionized medium, the flow of current under the influence of a
sustainer electric field or fields. While primarily described
herein is the use of DC voltages with inter-cavity electrodes, to
provide a sustainer field the invention described herein includes
the use of radio frequency electromagnetic fields, inductive
electrode structures, capactive electrode structures, movement of
an electrically conductive medium in the presence of an applied
magnetic field, and the introduction of laser energy into the
working cavity to provide the sustaining electric fields. For a
more complete discussion of the basic process here involved,
reference is made to the aforementioned patent application Ser. No.
859,424 filed Sept. 19, 1969. The present invention comprises an
improvement over the aforementioned patent application in the
provision of ionizing radiation, such as, for example, the
provision of highly efficient ionizing radiation through the use of
electron gun means rather than high voltage discharge means or the
like disclosed in the aforementioned application. The ionizing
radiation provided in accordance with the invention provides a
source of secondary electrons at very low temperatures and
increased efficiencies heretofore unobtainable since theory
indicates that the only way comparable conditions in high power,
high pressure devices can be duplicated is to operate the pulser
circuit of the aforementioned patent application at levels of the
order of one million volts and/or high repetition pulse rates -- a
result not easy if not impossible, of practical attainment.
As taught in the aforementioned patent application, a principal
feature in providing a volumetrically scalable discharge is the
control of gas temperature and discharge uniformity wherein an
electrical discharge or the like produces free electrons and
ionization of the working medium in a sustainer electric field.
Electron temperature, which is a function of E/N in any gas
mixture, is controlled by adjustment of the sustainer electric
field E and control of the gas density N. In flow applications,
proper design determines the allowable temperature rises
(.DELTA..tau.) in the gas and the corresponding density (.DELTA. N)
in the gas. In pulsed applications, the heat capacity of the gas,
the pulse width and the effect of pressure waves must be considered
in the proper control of .DELTA. N. If the electron temperature is
kept sufficiently low so that the ionization due to the sustainer
field is small compared to ionization due to the aforementioned
free and preferably high energy electrons, the volumetric discharge
can be maintained in a controlled manner to high pressures. For
example, controlled discharges in accordance with the invention of
up to one atmosphere have been established.
The aforementioned patent application disclosed in detail the
provision of a short, high voltage pulse substantially inductively
spread throughout the volume of the working medium. This is
accomplished by the provision of a plurality of electrodes and a
short pulse. The discharge is uniformly provided or spread
throughout the cavity containing the working medium because the
volumetric discharge through all of the electrodes offers the least
inductive impedance and thereby makes the current in the short
pulse flow reasonably uniform throughout the volume of the cavity
containing the working medium. An important criterion for this
arrangement to be practically effective is that the inductive
impedance of the short pulse discharge circuit be comparable to the
resistive impedance of the discharge. This has been accomplished in
accordance with the aforementioned patent application and produced
uniform ionizations over large volumes in times less than about
10.sup..sup.-6 seconds with minimum disturbances of the working
medium. In one case, the working medium was a mixture of N.sub.2,
CO.sub.2 and He which was used to produce a lasing medium.
An important feature of volumetric ionization in accordance with
the aforementioned patent application as well as the present
invention is to stabilize the discharge by suppressing the arcing.
Much of the arcing which occurs in systems in accordance with the
aforementioned patent application occurs due to the electrode
configuration and various electrode configurations have been tested
in conjunction with that invention and in all cases ionization in
accordance with the present invention creates a stabilizing effect
which allows the operation in areas that heretofore would have
created arcing and breakdown.
In accordance with the present invention, an electron beam is
provided to produce free electrons and ionize the working medium.
The electron beam which replaces the short high-voltage pulse of
the aforementioned patent application, is, among other things, more
efficient in producing ionization of the working medium. For
example, a 50 kv electron passing through air produces the order of
1000 secondary electrons along its path before losing its energy.
The effective ionization potential of a gas mixture of N.sub.2,
CO.sub.2 and He is approximately 30 volts, with half the primary
electron energy loss going into ionization.
When a laser application, for example, requires very high power,
there is an advantage in working with relatively high gas pressure
(such as, for example, up to one atmosphere or more) and large
transverse dimensions (up to 30 centimeters or more). Such
conditions would require voltage levels in excess of 1,000,000
volts in the pulser circuit of the aforementioned patent
application. The present invention eliminates this high voltage
requirement. An electron beam ionizer in accordance with the
invention need be provided, for example, with only a voltage of the
order of 150 kv to achieve useful ionization for such distances and
pressures. Further the provision of electron beam ionization in
accordance with this invention permits continuous ionization
through such large volumes thereby eliminating the necessity for
repetitive pulse ionization in, for example, a laser application.
In addition to the preceding, electron beam ionization can be
simply and conveniently controlled by controlling the potential on
a grid disposed in front of the electron emitting means. Thus,
ionization level and, for example, laser output for laser
applications may be simply and economically controlled by
controlling the grid voltage which may comprise part of a low
powered, easily controlled circuit. This feature of control along
with the ability of an electron beam to ionize in a truly
continuous fashion makes apparatus in accordance with the invention
highly attractive for ionizing a working medium in any application
where it is desired or convenient to separate ionization from
maintenance of a discharge.
In apparatus in accordance with the invention, at least one wall of
means defining a working region must transmit or provide high
energy electrons which deliver their kinetic energy to the working
medium in the form of ionization with a high efficiency. The
electron beam voltage, i.e., the energy of the electrons in the
beam providing the aforementioned high energy electrons must be
sufficiently high that the electrons will enter the working region
by, for example, penetration of a diaphragm or foil disposed in a
wall of the container before passing through and ionizing the
working medium. The intensity of the electron beam current is
broadly determined by the ionization level requirements such that
the volumetric recombination (or attachment) rate equals the
production rate of ionization in the electron beam for a particular
application. Increasing the intensity of the electron beam leads to
increasing the level of ionization with a corresponding higher
volumetric recombination rate. The diaphragm or diaphragms through
which the high energy electrons enter the working region need be
only such that they transmit the necessary number of electrons and
are adequately supported and cooled during transmission of the high
energy electrons. The support requirements are such that the
diaphragm must withstand the pressure differences between the
working gas and the vacuum region where provided on the other side
of the diaphragm where the high energy electrons are created and
accelerated toward the diaphragm. Typically, a suitable geometry is
one where there is a high vacuum region exterior of one or more of
the walls of the cavity defining the working region. Electrons are
generated in the vacuum region by any suitable method such as, for
example, plasma emission, thermionic emission, photo emission,
electron bombardment and the like. Upon generation of the
electrons, they are in conventional manner accelerated through a
suitable electrostatic or electromagnetic structure and caused to
pass through the diaphragm into the working region.
Irrespective of the method of generating electrons, they may be
typically coupled to the working region through the diaphragm. The
diaphragm may be disposed over a reticulated member and in certain
pulsed applications the foil temperature rise may be limited simply
by its intrinsic heat capacity and may be cooled in any suitable
manner such as by gas flow or conduction and may be comprised of
Al, Be, T.sub.i, C, and the like. Since the function of the
diaphragm is to separate the working medium in the working region
from the vacuum in the electron gun, it typically should be capable
of withstanding a pressure difference of one atmosphere. Since the
diaphragm is heated by absorbing energy from transmitted electrons
in C. W. or numerous rapid pulse applications, it must be cooled.
However, any suitable cooling means may be used.
While a "shower-head" type electron beam arrangement is shown and
described herein for irradiating a large area by a relatively low
energy electron beam of the order of 50-150 kv, it is to be
understood that the invention is not so limited and that other
arrangements such as, for example, one or a plurality of small
electron guns of the type used in electron beam welders and the
like may be used where appropriate to the application. Further, if
the use of a diaphragm is undesirable, a series of small holes in a
plurality of plates defining a plurality of serially disposed
chambers which are differentially pumped may be employed to provide
separation of the electron gun from the working region without
requiring the electrons to pass through a solid member. In this
case, the electrons pass directly through one or more of a series
of aligned holes in the plates and the gas in the working region
will not diffuse rapidly enough through the hole adjacent the
electron gun to substantially affect the generation of electrons.
Suitable voltages may be applied to the space between plates to
obtain maximum focusing of the electrons and the pressure between
plates successively decreased in the direction of the electron
gun.
Electron beam current and ionization level required in a given
working medium are determined by the application. Thus, many
N.sub.2 --CO.sub.2 laser applications require only a relatively low
level of ionization and low volumetric beam current. Further, in
this particular application, the cooling requirements of the
diaphragm are modest and can be satisfactorily met by heat
conduction to cooled support members. However, for MHD generator
and accelerator applications, for example, higher ionization levels
and higher volumetric beam currents are necessary for practical
devices. Accordingly, a greater cooling of the diaphragm will be
required for this type of application than with, for example, a
laser.
The quality of the electron beam, i.e., the spread, energy and
uniformity of the electron beam throughout the working medium are
determined by the application. Thus, for many of the laser
applications, the intensity of the electron beam must be
substantially uniform (with variations not exceeding about a few
percent) in order to produce a working medium with the
substantially uniform ionization necessary to provide uniform gain
and optical properties in the lasing medium.
While the provision of an electron beam is preferred for the
embodiment disclosed by reason of the electron beam being a highly
efficient method of producing volumetric ionization, it is to be
understood that other applications may require ionizing radiation
in the form of photons, alpha particles, x-rays, and the like and
such are included within the scope of this invention.
As may be seen from the preceding discussion, the level of
ionization that can be obtained using high energy electrons is
determined by balancing the production rate of secondary electrons
with the loss rate due to either recombination attachment or flow.
Accordingly, it is important to understand the limits of high
energy electron current density and energy to understand the
relevant loss process discussed herein below.
In an embodiment actually reduced to practice, to produce laser
action, an electron gun produced a stream of electrons which were
directed at a thin metallic foil diaphragm supported by a
perforated plate with 470 1/16 inch holes in an area 2 .times. 4
inches. The limiting condition on the electron beam current was
that the thin metallic foil used not be heated to a temperature at
which its structural strength was significantly reduced, since its
function is to withstand the pressure difference between the
working region and the gun and still transmit electrons. This
temperature was arbitrarily set as 200.degree. K, the foil being
aluminum having a thickness of .apprxeq.10.sup.-.sup.3 cm. Other
materials of other thicknesses may be used, and the foil may be
cooled by a variety of means, including conduction to cooled
supports, or, for example, forced convection with gas blown across
its surface in a pulse mode of operation.
In the pulse operation, we may assume that all the energy is
deposited in the foil, a lower limit on the incident current
density in terms of the incident beam energy E (volts) and the
pulse length t (sec) is approximately
E I t<0.5 joule/cm.sup.2 (1)
If E=50 kv, t=20 .mu. sec. I<0.5 amp/cm.sup.2, approximately 20
percent of the incident electrons will be transmitted with a mean
energy reduction of about 10 kv and it is these transmitted
electrons which are available for pulsed ionization of the gas. As
will be seen later, the above limit represents a current density
far in excess of that required for ionization of the gas system
selected for lasing operation.
Consider now the processes of ionization and recombination in the
gas used which is essential to proper operation of the laser:
The production rate, p, of ions in a gas per cm.sup.3 is
where
.rho. = gas density, gm cm.sup.-.sup.3
I = emergent electron beam current density, amp/cm.sup.-.sup.2
<.delta. E/.delta. m< = mean energy loss rate, volt/cm.sup.2
/gm.sup.- .sup.1
E.sub.i = mean energy loss per ionization, volt
e = charge on electron; coulomb
When two body recombination dominates as in the application being
considered, the electron density n.sub.e is given by:
dn.sub.e /dt = .alpha.n.sub.e.sup.2 + p
Where .alpha. is the effective recombination coefficient and p is
the production rate, it follows that in equilibrium, that is, for
dn.sub.e /dt = 0, we have:
where
P = gas pressure, dynes cm.sup.-.sup.2
T = gas temperature, .degree.K,
.alpha. = effective recombination coefficient, cm.sup.3
/sec.sup.-.sup.1,
M = molecular weight of gas,
m.sub.p = proton mass, gm,
k = Boltzman's constant, erg/.degree.K.
The approximate maximum values of n.sub.e for a typical electron
beam and current density 1 mA/cm.sup.2, in a mixture of Helium,
Nitrogen and CO.sub.2 in the proportions 3:2:1 are given
immediately below, with characteristic decay time .tau. =
1/.alpha.n.sub.e and range R at E = 50 kv, using E.sub.i = 50
volts.
3:2:1
He:N.sub.2 :CO.sub.2
n.sub.e R .tau. P.sub.Torr cm.sup..sup.-3 cm .mu.sec 30 4 .times.
10.sup.11 122 12 300 10.sup.12 12 4 760 2 .times. 10.sup.12 4.8
2.4
To better understand the invention, the process involved in the
creation of lasing power by the use of an electron beam and a
sustainer discharge in accordance with the invention will now be
discussed.
Thermionic electrons from a tungsten filament were modulated by a
grid whose potential could be varied with respect to the filament
and the electrons were accelerated through a potential V.sub.O. The
value of V.sub.O was chosen by optimizing the ionization produced
in the gas. For higher energies Aluminum foil is more transparent
and more electrons are transmitted, but the ionization density
produced is lower. Accordingly, in Eq. (2) it may be shown that
< .delta. E/.delta. m > .apprxeq. C 1n E/E, where C is a
constant,
and < .delta. E/.delta. m > decreases as E increases.
The optimum value of V.sub.O used was approximately 50 kv and the
electron gun was maintained at a vacuum (p < 0.1 micron) and
separated from the laser chamber by a thin foil of aluminum of
thickness 10.sup..sup.-3 cm. Aluminum was chosen simply because of
its ready availability. The laser chamber may be at any pressure
from below 1.mu. up to about one atmosphere or more.
After passing through the foil, the electron beam entered the
working region through a relatively wide mesh grid of stainless
steel. This grid constituted a cathode and a gold plated disk
constituted an anode, between which a sustainer voltage V
(<10kv) was applied. The grid was provided to prevent damage to
the foil, and the gold plate on the anode served to reflect a
proportion of the incident primary electrons, thus increasing the
ionization of the gas. The filaments in the electron gun were
maintained at -50 kv with respect to the foil (which was at or near
ground potential) by a 5 micro farad capacitor which supplied the
pulsed electron beam current. The filaments were pulsed negative
500 V with respect to the grid. Many other schemes for projecting a
beam of electrons into a gas are possible such as photoelectric,
field emission, electron bombardment and ion bombardment.
The sustainer current was supplied by a 250 .mu. F capacitor at
voltages up to about 10 kv. Either the anode or the cathode can be
grounded. The velocity of gas comprising the working medium which
flowed through the laser chamber normal to the electron beam can be
varied up to about Mach 1. In preliminary tests velocities of about
one meter per second were used in order to ensure that the gas was
uncontaminated through leaks.
The existence of uniformity of the electron beam in the working
region and the low intensity variations of the electron beam was
corroborated by replacing the anode wall with a lucite wall coated
with sodium salicylate, a substance that is flourescent when
excited by high energy electrons.
Two mirrors in the laser chamber, whose axis was normal to both the
gas flow and the electron beam, were positioned vertically in the
apparatus. One mirror was copper and concave and the other one was
IRtran 98 percent reflecting at a wavelength of 10.6.mu.. The
mirrors were spaced 18 cm apart and were supported in a tube whose
orientation could be adjusted by means of screws. The mirrors were
aligned using standard techniques and the output from laser action
between the mirrors passed through a 10.6.mu. filter into a
germanium crystal infrared detector, the output of which was fed
into an oscilloscope triggered by the electron beam current. The
sustainer current was measured as well as the infrared detector
signal and the infrared detector was calibrated with a thermocouple
calorimeter. It was found that laser gain became sufficient to
begin lasing action only some time after the electron beam pulse
reaches its maximum. This time lag represents the time required to
achieve a population inversion by pumping the CO.sub.2 molecules
into their upper state and is sensitive to the temperature
dependence of the electron pumping rate. Increasing the sustainer
voltage, and, therefore, the electron temperature in the lasing
medium decreased this time lag.
A useful level of ionization was achieved with a pulse of 20.mu.
sec. duration. After the pulse, the gases ionized by the pulse
recombine and it is in this recombining debris stage of the cycle
that laser action occurs. In another experiment laser action was
accomplished with an essentially C. W. (i.e., ionization compared
to flow and cooling times) E-beam. Thus, the electron beam pulse
length may be varied from infinite to continuous to very slow
thereby creating either a truly cw laser, an effectively cw laser,
or a pulsed laser. For high power operation, the working medium may
be provided in the form of pulses and the E-beam and sustainer
circuits actuated substantially between pulses. Such operation
permits substantial heat removal while still providing a
substantially homogeneous medium in the working region.
As discussed in the aforementioned patent application, various
gases and gas mixtures may be employed to support laser action
although a 3:2:1 ratio of He:N.sub.2 CO.sub.2 is discussed herein,
any gas or combination of gases such as CO, H.sub.2 O, SO.sub.2,
HCN, NO, H.sub.2, Ar, NO.sub.2, N.sub.2 O, HF and the like may be
handled in the manner discussed hereinabove and other gases may be
added if required or desired.
The present invention is applicable to substantially any useful
laser gas mixture, the principal advantage of the invention being
that it is applicable to suitable gas mixtures at high pressures,
producing a controllable volumetrically scalable gas discharge over
a wide range of operating conditions and electrode configurations.
The present invention permits the production of a stable and
controlled discharge when the gas mixture constituents and electron
temperature T.sub.e are selected so that the rate of one or more of
the variety of available recombination process (atom recombination,
molecular recombination, attachment, etc.) exceeds the rate of
ionization. When this is established, the discharge will not be
self-sustaining, i.e., it will not run without the ionizing means
being actuated and it is this feature that permits the ionizing
means to control the discharge characteristics. If
(T.sub.e).sub.max is defined as the condition for a specific gas
mixture wherein ionization equals recombination, viable laser
apparatus will be provided if an inversion is produced by
electronic excitation (and/or appropriate gas kinetic de-activation
of states related to the laser transition) at some electron
temperature or temperatures T.sub.e, such that T.sub.e
<(T.sub.e).sub.max. A specific example is the N.sub.2 --CO.sub.2
laser mixture. Ionizations become significant when T.sub.e is of
the order of 1.5 ev or higher. However, a net preferential
excitation (producing an inversion) can be made to occur for
electron temperatures of less than 1.5 ev in both N.sub.2 and
CO.sub.2. The prior art teaches a large number of atoms and
molecules that can be excited electronically by a discharge. Any
lasing species which may be inverted by direct electronic
excitation or by excitation via an auxiliary species (as in the
N.sub.2 --CO.sub.2 system) at T.sub.e <(T.sub.e).sub.max may be
expected to be susceptible to the ionizer-sustainer concept and
especially the electron beam ionizer-sustainer concept of the
present invention.
Further the present ionizer-sustainer concept may be expected to be
applicable to use of a gas mixture containing a gas which has a
high net attachment rate (producing an effective recombination) at
high electron temperatures which occur, for example, in O.sub.2 for
values of T.sub.e up to about 3 ev. Use of such a gas mixture may
be expected to permit operation at higher than usual electron
temperatures wherein significant ionization of one of the lasing
mixture constituents occurs. This may make lasing transitions
acceptable which are not otherwise stably available
(C.W.N.sub.2).
TABLE I
Output wavelength 10.6.mu. Output coupling .apprxeq.1% Peak pulsed
output power 3 watts EB pulse width .apprxeq.100 .mu.sec Sustainer
pulse width .ltoreq.800 .mu. sec Laser pulse width .ltoreq.600 .mu.
sec Gas 16% CO.sub.2, 34% N.sub.2, 50% He Gas pressure 30 Torr
Input velocity 1 m/sec. quasistatic Laser cavity size Diameter:
2.54 cm Length: 18 cm Electrodes Stainless steel mesh grid cathode
Laser discharge chamber size Grid length: 10 cm, Grid width: 5 cm,
Chamber depth: 5 cm EB circuit HV power supply: 150 kv at 5 mA
Energy storage capacitor: 5 .mu. F Floating filament-heating
Isolation transformer turning circuit: ratio 11:1 Sustainer power
supply: 50 kv at 5 mA Sustainer capacitor: C = 250 .mu. F
broadly, as may now be seen, the electron beam creates a desired
electron density uniformly using only a small amount of energy
while the sustainer discharge provides a voltage to give these
electrons a desired temperature sufficiently high for laser action
for example, but not high enough to generate any appreciable
increase in electron density. The sustainer discharge deposits the
dominant amount of energy in the gas directly where it is desired.
In the case of an N.sub.2 --CO.sub.2 laser, the energy is put into
the upper laser state of CO.sub.2 and into Nitrogen vibration, the
optimum electron temperature assuring optimum laser efficiency.
Upon creation by the electron beam of a uniform electron-ion cloud,
the cloud stays uniform during the time of the electric field
provided by the sustainer voltage as long as the sustainer voltage
does not result in a rapid creation of electrons. If the level of
the sustainer voltage or field is raised to the point where it too
produces a rapid ionization, then discharge non-uniformities may be
created. However, provision of a sustainer field selected to create
negligible electrons results in maintenance of stable, uniform and
controlled discharge for several flow times through the working
region.
As will now be apparent, the present invention permits the
provision in a flowing gas laser of a spatially uniform discharge
at the optimum electron temperature required for efficient laser
operation at arbitrary pressure levels and physical sizes. While
the invention is not so limited, this may be accomplished by
utilization of the aforementioned two-step process comprising
preferably, first an electron beam which creates in the gas a
non-spoking predetermined spatial distribution (preferably uniform)
electron density or ionization which would ordinarily, if left on
its own, disappear by volumetric processes and/or flowing out of
the channel and be incapable of producing efficient high power
laser action. However, the second step or sustainer discharge is
provided which gives the electrons produced by the first step the
necessary electron temperature for preferably optimum laser (or
other) excitation, with no significant increase in electron
density.
It is to be understood that the invention is not limited to the
apparatus shown and described and that, for example, other methods
of an apparatus for creating the initial electron density can be
used such as ultraviolet radiation, electrical discharge, protons
and the like provided by electron beam means for introducing one or
more electron beams to produce ionization of the gaseous medium as
and for the purposes set forth hereinabove. Irrespective of whether
the electrons are generated in the above described manner or any
other suitable manner, they must be heated to the correct electron
temperature by the E/N applied by the sustainer discharge.
Reference has previously been made to the fact that the present
invention may be used to produce or facilitate carrying out
chemical processes such as, for example, the generation of
ozone.
Heretofore, for industrial applications ozone has been principally
produced by the use of the well-known Townsend or silent discharge.
Recently a second process based on the use of a corona or high
pressure glow discharge has begun to be used in commercial
applications.
The Townsend discharge process is characterized by two significant
operating characteristics that are at least somewhat interdependent
-- the requirement of a low current density in the discharge and a
low overall energy efficiency. The production of ozone requires
high levels of activation energy even with low conversion rates of
the order of one mole percent of the working medium; hence cooling
is essential if an undesirable change in chemical kinetics due to
temperature rise is to be prevented. The aforementioned low current
density and low overall energy efficiency characteristic of the
Townsend process has not only resulted in high production costs but
has severely limited the application of processes incorporating the
Townsend discharge.
Details of the formation of ozone by a Townsend discharge are
wellknown. Thus, while high positive column energy efficiencies
have been consistently measured, a low overall efficiency results
because of the severe potential drop at the electrodes. While the
dielectric layer used to stabilize the discharge is principally
responsible for the aforementioned electrode drop, without
stabilization provided by this dielectric layer, the Townsend
discharge does not function satisfactorily as an industrial
process.
The glow discharge process is not subject to the two basic
deficiencies of the Townsend discharge of low current density and
high cathode drop. In the glow discharge process the current
density is about 2-3 orders of magnitude higher than that of the
Townsend discharge and the cathode drop is generally less than
about 1,000 volts. However, compared to the Townsend discharge, the
positive column energy efficiency of the glow discharge process is
significantly lower.
Glow discharge processes are generally conducted under low pressure
and with walls cooled to liquid air temperature. On the other hand,
the high pressure discharge, or corona discharge, is subject to a
more limited stability range than the low pressure glow discharge.
Further, with the exception of a high frequency electrodeless
discharge, electrode geometry is generally a critical factor in
achieving stabilization of a corona discharge.
Due to the necessity of stabilization by a dielectric layer or
specific electrode geometry, the above-described processes are
essentially surface processes. In such processes the kinetics in
the active volume of the working medium is not homogeneous and
optimum or near optimum conditions cannot be uniformly
established.
The basic problem in the above-described processes is believed to
exist because of close coupling between the emission of electrons
from the electrodes and the electric field in the active volume.
Where such coupling exists, it is very difficult to maintain a
steady and uniform discharge without arcing.
Ozone may be more easily and efficiently produced than heretofore
by utilizing, in accordance with the present invention, an
independent source of electrodes in the form of an electron beam
or, alternately, repeated short electron beam pulses superimposed
on a sustaining electric field as and for the purposes hereinbefore
described. As in a laser application, in this case the electric
field in the active volume is also decoupled from the requirement
of the electron emmission, whereby optimum conditions for ozone
formation may be provided without for example, severe requirements
on ballasting as required for a Townsend discharge or severe
requirements on electrode geometry as in the case of a corona
discharge. The production of ozone in accordance with the present
invention is a truly volumetric process; hence for large scale
applications, not only are scaling problems simpler than with prior
art processes, but overall equipment size can be drastically
reduced. Furthermore, the uniform conditions provided in the active
column in accordance with the present invention provides an
improvement in the overall energy efficiency and minimizes heat
dissipation involved in the process as compared to that of the
prior art. Accordingly, ozone may be produced in accordance with
the present invention in higher concentrations than that heretofore
available without the necessity of cooling.
Ozone may be produced with apparatus substantially as shown and
described hereinabove with the exception that the mirror means
defining the optical cavity are not required. The working medium
for the production of ozone may be air or preferably pure oxygen.
Electrons are generated by the electron gun in the manner
previously described, enter the working region, and collide with
oxygen molecules to form secondary electrons and ions. The electron
temperature in the working region must be maintained at a level
which is favorable to ozone production.
In the working region, electrons are generated through ionization
by primary and secondary electrons and are lost by attachment to
the molecules of oxygen. Both the ionization rate of secondary
electrons and the electron attachment rate to oxygen molecules are
strongly influenced by the electron temperature. Since, in
accordance with the invention, there is an excess of electrons due
to ionization by the primary electrons, a spatially uniform current
can be maintained with an ionization rate of secondary electrons
less than the attachment rate. Accordingly, the stability of the
discharge process in the working region is substantially greater
than that in conventional discharge processes where net ionization
by secondary electrons is essential to sustain the discharge
process.
If a secondary electron temperature in the range of approximately
2-3 electron volts is provided, a large percentage of the energy
lost in elastic collision goes into dissociation of oxygen which is
essential to the production of ozone in high concentrations. The
electron temperature range suitable for the production of ozone is
higher by about a factor of 2 than the range necessary to produce
laser action. Further, since a low ambient temperature may be
easily provided in the working region, this permits the production
of higher concentration of ozone with much greater efficiency than
heretofore possible.
Another application of the present invention is to
magnetohydrodynamic power generation (MHD). Electric power can be
extracted from an electrically conductive stream of plasma by
passing the plasma through a magnetic field transverse to the
direction of flow. The magnetic field creates an electric field
perpendicular to the magnetic field and to the direction of flow,
and suitably constructed electrodes arranged parallel to the
electric field permit the kinetic energy of the plasma to be
coupled out as electrical energy. For a more complete discussion of
MHD devices, reference is made to U. S. Pat. No. 3,264,501.
In this type of application, an electron beam in accordance with
the invention is injected into the plasma to maintain the required
level of ionization independent of the electron temperature. In
this manner, a stable plasma discharge may be usefully produced in
the plasma wherein the ionization is volumetric and stabilized not
by ambipolar diffusion of ion pairs to the walls as in a
conventional discharge, but by equilibrium between ion
recombination and ion production by the electron beam. It is to be
emphasized that the parameters set forth below and their numerical
values are given only by way of illustration.
Requirements on Recombination Coefficient - MHD Generator
Gas: Helium Flow Velocity .apprxeq.1.5 .times. 10.sup.3 m/sec Flow
time: .apprxeq.10.sup.-.sup.3 sec Energy Extracted from plasma:
.apprxeq.0.2 eV/particle Ionization Level: n.sub.e
.apprxeq.10.sup.14 electrons/cm.sup.3 Gas Density: .apprxeq.3
.times. 10.sup.19 cm.sup.-.sup.3 Effective energy per ionization 50
eV Energy required per particle to ionize: 10.sup.14 ions/cm.sup.3
.apprxeq. 5/3 .times. 10.sup.-.sup.4 eV/cc Assuming 100 ionizations
per flow time, energy required to maintain ionization .apprxeq.5/3
.times. 10.sup.-.sup.2 eV/cc ##SPC1##
This must be 1/100 of the flow time, giving a requirement for the
recombination coefficient: .alpha.2 <10.sup.5 / n.sub.e
.apprxeq. 10.sup.-.sup.9 cm.sup.3 /sec
Recent experimental results (Berlande et al, Phys Rev Al, 887,
1970) indicate that in the preferred working region, n.sub.e
.apprxeq. 10.sup.14, electron temperature T.sub.e .apprxeq. 3
.times. 10.sup.3 .degree.K, gas temperature Tg.apprxeq.
1300.degree.K, the upper limit on the effective recombination
coefficient is .alpha. < 10.sup.-.sup.9 cm.sup.3 /sec.
The electron beam current density required to maintain an
equilibrium ionization level n.sub.e = 10.sup.14 cm.sup.-.sup.3 is
obtained by equating production rate and recombination rate. Thus:
.alpha.n.sub.e.sup.2 = IE/eE.sub. i R(E), approximately where
.alpha. = effective recombination coefficient
I = eb current density, amp cm.sup.-.sup.2
E = electron energy, volts
R(e) = range of electrons of energy E, cm
E.sub.i = 50 eV per ion pair
e = 5/3 .times. 10.sup.-.sup.19 coulombs
For E = 100 keV, and density N.sub.o = 2.6 .times. 10.sup.19
cm.sup.-.sup.3, then R(E) .apprxeq. 90 cm in helium
I = n.sub.e.sup.2 eE.sub.i R(E)/E
= 7.5 .times. 10.sup.-.sup.3 amps/cm.sup.2 when .alpha. =
10.sup.-.sup.10 cm.sup.3 /sec n.sub.e = 10.sup.14
cm.sup.-.sup.3
An electron beam as set forth above is easily produced as and for
the purposes previously described and may include for example using
a jet of helium to cool the foil or diaphragm, the beam of
electrons being injected into the MHD channel at a suitably chosen
angle relative to the direction of the applied magnetic field.
The various features and advantages of the invention are thought to
be clear from the foregoing description. Various other features and
advantages not specifically enumerated will undoubtedly occur to
those versed in the art, as likewise will many variations and
modifications of the preferred embodiment illustrated, all of which
may be achieved without departing from the spirit and scope of the
invention as defined by the following claims:
* * * * *