U.S. patent number 3,679,897 [Application Number 04/853,818] was granted by the patent office on 1972-07-25 for laser bombardment of microparticle beam for producing atomic particles in the form of a beam or an expanding cloud.
This patent grant is currently assigned to TRW Inc.. Invention is credited to Saul Altshuler, William Bernstein, Bernard Hamermesh, David O. Hansen.
United States Patent |
3,679,897 |
Hansen , et al. |
July 25, 1972 |
LASER BOMBARDMENT OF MICROPARTICLE BEAM FOR PRODUCING ATOMIC
PARTICLES IN THE FORM OF A BEAM OR AN EXPANDING CLOUD
Abstract
A beam of accelerated microparticles of substantially uniform
velocity is passed transversely through an intense pulsed laser
beam. In traversing the laser beam, the microparticles are
thermally evaporated by the energy of the laser beam to produce
either a beam or an expanding cloud of atomic particles.
Inventors: |
Hansen; David O. (Westminster,
CA), Altshuler; Saul (Manhattan Beach, CA), Bernstein;
William (Los Angeles, CA), Hamermesh; Bernard (Shaker
Heights, OH) |
Assignee: |
TRW Inc. (Redondo Beach,
CA)
|
Family
ID: |
25316987 |
Appl.
No.: |
04/853,818 |
Filed: |
August 28, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
599163 |
Dec 5, 1966 |
|
|
|
|
Current U.S.
Class: |
250/251; 313/230;
376/121; 315/500; 313/161; 376/101; 376/122 |
Current CPC
Class: |
H05H
3/02 (20130101) |
Current International
Class: |
H05H
3/00 (20060101); H05H 3/02 (20060101); G01h
021/00 (); H01s 003/00 () |
Field of
Search: |
;250/41.3,41.9SE,42,84
;313/63,230,161,153 ;331/94.5 ;328/227 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Birch; Anthony L.
Parent Case Text
CROSS REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No.
599,163, filed Dec. 5, 1966, entitled "Neutral Atomic Beam
Generating Device Employing a Giant Pulse Laser for Forming the
Atomic Beam," now abandoned.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. In combination:
means for providing and accelerating a stream of charged
microparticles along a beam path;
laser beam means for generating a pulsed laser beam directed
transversely of said beam path;
particle detection means for detecting the transit of selected ones
of said charged microparticles during a first time interval;
and
means responsive to the output from said particle detection means
for actuating said laser beam means to cause a laser beam pulse to
irradiate said selected microparticles during a subsequent time
interval to convert said microparticles into atomic particles;
said atomic particles forming a beam when said microparticles have
relatively high longitudinal velocities in excess of 5 kilometers
per second, and said microparticles forming a radially expanding
cloud when said microparticles have relatively low longitudinal
velocities below 100 meters per second.
2. The invention according to claim 1, wherein said detection means
includes means responding to microparticles of selected
velocity.
3. The invention according to claim 1, wherein said laser beam
means comprises a giant pulse laser.
4. The invention according to claim 3, and further including first
means for sensing the transit of selected microparticles during a
first time interval and for initiating pumping of said giant pulse
laser; and
second means for sensing the transit of said selected
microparticles during a second time interval and for pulsing said
giant pulse laser during a third time interval coincident with the
passage of said selected microparticles through the path of said
laser beam.
5. The invention according to claim 4, wherein said first and
second sensing means comprise particle detectors spaced along the
particle beam path for detecting microparticles of selected
velocity.
6. In combination:
means for providing and accelerating a stream of microparticles
along a beam path;
magnetic confinement means in said beam path;
means for irradiating said microparticles within said magnetic
confinement means with a laser beam directed transversely of said
beam path and of sufficient intensity to convert said
microparticles into charged atomic particles;
and means for injecting a stream of energetic neutral particles
within said magnetic confinement means so as to collide with said
atomic particles and ionize said energetic particles;
said ionized energetic particles being thereby trapped within said
magnetic confinement means
7. The invention according to claim 6, wherein said microparticles
have longitudinal velocities sufficiently below 100 meters per
second as to form a radially expanding plasma cloud.
8. The invention according to claim 7, wherein said microparticles
are lithium droplets.
9. The invention according to claim 7, wherein said energetic
particles are neutral hydrogen atoms having energies in excess of
1,000 electron volts.
10. Atomic beam generating apparatus, comprising:
means for providing and accelerating a stream of charged
microparticles along a beam path;
means for irradiating said microparticles with a laser beam
directed transversely of said beam path and of sufficient intensity
to produce a stream of atoms of substantially the same velocity as
said microparticles;
and means for removing charged atoms from said stream to render
them electrically neutral.
11. The invention according to claim 10, and further including
means for detecting the transit of selected microparticles during a
first interval of time;
and means responsive to the output from said detecting means for
causing said laser beam to irradiate said selected microparticles
during a subsequent time interval.
12. The invention according to claim 11, wherein said detecting
means includes means responding to microparticles of selected
velocity.
13. The invention according to claim 10, wherein said laser beam
means comprises a giant pulse laser.
14. The invention according to claim 13, and further including
first means for sensing the transit of selected microparticles
during a first time interval and for initiating pumping of said
giant pulse laser;
and second means for sensing the transit of said selected
microparticles during a second time interval and for pulsing said
giant pulse laser during a third time interval coincident with the
passage of said selected microparticles through the path of said
laser beam.
15. The invention according to claim 14, wherein said first and
second sensing means comprise particle detectors spaced along the
particle beam path for detecting microparticles of selected
velocity.
16. Atomic beam generating apparatus, comprising:
means for providing and accelerating a stream of charged
microparticles along a beam path;
means for irradiating said microparticles with a pulse of coherent
radiation directed normal of said beam path and of sufficient
intensity and duration to cause vaporization of microparticles in
said stream and produce therefrom a stream of atoms of
substantially the same velocity as said microparticles;
and means for removing charged atoms from said stream to render
them electrically neutral.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the generation of atomic particles, and
more particularly to the interaction of accelerated microparticles
with an intense laser beam for producing atomic particles in the
form of a beam or a radially expanding cloud.
2. Description of the Prior Art
Heretofore, plasmas have been produced by irradiating with a laser
beam such targets as gases, solid surfaces and thin foils. These
methods have all suffered from the presence of neutral atoms either
in the original gas target or being evolved from the solid
surfaces. Also the presence of a solid surface will compromise any
possible magnetic containment system. Another method has utilized
an electrostatic suspension system for small lithium hydride
pellets of about 10 microns in diameter. This method has eliminated
the effects of neutral gas and has been combined with a magnetic
containment system. Still another known method involves the use of
a thin quartz fiber suspension system. All of these methods have
the drawback that they are limited to a single event and are not
repetitive. Furthermore, they do not permit any freedom in the
selection of various laser beam-target interaction regions, such as
may be required in certain systems utilizing an expanding cloud of
atomic particles.
SUMMARY OF THE INVENTION
Means are provided for generating and accelerating a stream of
microparticles along a beam path. A laser beam is provided for
irradiating the microparticles transversely of the beam path. The
laser beam has sufficient intensity to convert the microparticles
into atomic particles having substantially the same longitudinal
velocity as the microparticles. The atomic particles can be either
neutral atoms or ions, depending upon whether the laser beam has a
low or a high power density, respectively. Depending upon whether
the microparticles have a high or a low longitudinal velocity, the
atomic particles are formed into either a beam or a radially
expanding cloud, respectively.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view of apparatus for producing a neutral
atomic beam according to the invention;
FIG. 2 is a diagrammatic view showing the interaction of a low
velocity microparticle beam and a low power density laser beam for
producing a radially expanding puff of neutral atoms;
FIG. 3 is a diagrammatic view showing the interaction of a high
velocity microparticle beam and a high power density laser beam for
producing a plasma jet;
FIG. 4 is a diagrammatic view showing the interaction of a low
velocity microparticle beam and a high power density laser beam for
producing a radially expanding plasma cloud; and
FIG. 5 is a schematic view of apparatus employing a magnetic
confinement means into which laser-bombarded microparticles and
high energy particles are introduced to create a high temperature
plasma.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In each of the embodiments described below, a laser beam is used to
irradiate a beam of discrete, electrostatically accelerated solid
or liquid microparticles. The embodiments differ essentially as to
the power of the laser beam and/or as to the velocity of the
microparticles. As used herein, the term "low velocity" as applied
to the microparticles means a velocity < 10.sup.4 centimeters
per second; the term "high velocity" as applied to the
microparticles means a velocity > 5 .times. 10.sup.5 centimeters
per second; the term "low laser power density" is defined as
approximately 10.sup.8 watts per square centimeter; and the term
"high power density" is defined as approximately 10.sup.11
-10.sup.12 watts per square centimeter.
Depending upon the velocity of the microparticles and the laser
power, the following four configurations and their uses are
obtained:
1. The use of a high microparticle velocity and low laser power
will result primarily in a beam of neutral atoms with a directed
velocity equal to the microparticle velocity. This neutral atom
beam can be used for laboratory studies of a variety of atom-atom
collision phenomena, atom-surface collisions, and chemical
reactions. The method offers the major advantage of much greater
instantaneous atomic fluxes in the energy region 1-1,000 ev than
can be obtained with more conventional means.
2. The use of a low microparticle velocity together with low laser
power will result in the production of an expanding cloud of
neutral atoms. An application of this technique could occur in
controlled fusion research where it is often desirable to introduce
a controlled, localized, and well-known puff of unionized gas into
the interior of an energetic plasma for diagnostic purposes.
3. The use of a high microparticle velocity with high laser power
will result in the production of an isolated plasma jet or
plasmoid. This technique is also relevant to the problem of plasma
injection into magnetic containment systems and to laboratory
studies of the interaction of such plasmoids with electric and
magnetic fields.
4. The use of a low directed microparticle velocity with high laser
power will result in a spherically expanding plasma with a
negligible drift velocity. Because of the very small charge to mass
ratio of the microparticles, this technique provides a useful
method for plasma generation within magnetic confining field
geometries without either any substantial modification of the field
geometry or the introduction of large amounts of neutral gas or
other impurities. This plasma may be used for study of the
confinement properties of the containment system. This laser plasma
may serve as a unique seed plasma for the subsequent generation of
a plasma of fusion interest by energetic neutral injection
techniques.
NEUTRAL ATOMIC BEAM
There will now be described an apparatus for generating a neutral
atomic beam. This apparatus employs high microparticle velocity and
low laser power density as defined above. Referring to FIG. 1, an
elongated vacuum chamber 10 contains at one end thereof a particle
accelerator 12 capable of accelerating microscopic particles to
hypervelocities up to 45 kilometers (4.5 .times. 10.sup.6 cm) per
second. The particle accelerator 12 may be one of the kind
disclosed in the following publications: J. F. Friichtenicht,
"Two-Million-Volt Electrostatic Accelerator for Hypervelocity
Research," Review of Scientific Instruments, Vol. 33, page 209
(February 1962), and H. Shelton, C. D. Hendricks, Jr., and R. F.
Wuerker, "Electrostatic Acceleration of Microparticles to
Hypervelocity," Journal of Applied Physics, Vol. 31, page 1243
(July 1960). The particle accelerator 12 is capable of accelerating
particles ranging in size from 0.03 to 2 microns in radius.
Examples of materials that have been investigated are particles of
iron, carbon, and aluminum.
An alternative liquid droplet generator has been described by J. M.
Schneider and C. D. Hendricks in Review of Scientific Instruments,
Vol. 35, page 1349 (1964). This system generates uniform 50 to
2,000 micron diameter charged droplets of various liquids at a
repetition rate up to several thousand per second. In a paper
written by M. M. Hoffman entitled "Formation of Uniformly Charged
Conducting Droplets and Possible Propulsion Applications," issued
in Los Alamos Scientific Laboratory Report LA2549, Apr. 1961, there
is described a Gallium droplet generator, which is applicable to
other low melting temperature metals, such as lithium.
The particles are ejected from the particle accelerator 12 in the
form of a particle beam, indicated by the arrow 14, which may be
0.5 millimeters in radius, and at a rate of 10 particles per
second, for example. The accelerated particles may have an
electrostatic surface charge of the order of 5 .times. 10.sup.-16
coulombs or greater when leaving the particle accelerator 12, the
surface charge having been previously induced on the particles to
promote the required acceleration.
The particle beam 14 passes successively through two particle
detectors 16 and 18 spaced longitudinally along the axis of the
particle beam 14. Each of the detectors 16 and 18 may be one of the
kind disclosed in the article by H. Shelton, C. D. Hendricks, Jr.,
and R. F. Wuerker, published in the Journal of Applied Physics,
Vol. 31, page 1243, dated July 1960, and entitled "Electrostatic
Acceleration of Microparticles to Hypervelocity." The detectors 16
and 18 sense the velocity of the particles by measuring their time
of flight between the detectors. When a particle of predetermined
velocity passes through both detectors 16 and 18 in succession, the
detectors 16 and 18 generate two electrical trigger pulses 20 and
22 which are spaced by a time interval equal to the transit time of
the particle.
The trigger pulses 20 and 22 are fed to a velocity selector and
pulse generator 24, which generates an output signal pulse 26 only
when the trigger pulses 20 and 22 from the detectors 16 and 18 are
spaced by an interval that corresponds to the predetermined
particle velocity. For example, assume a predetermined particle
velocity of 5 kilometers per second and a spacing of the detectors
16 and 18 of 1 centimeter. The transit time of a 5 kilometer per
second particle through a distance of 1 centimeter is 2
microseconds. Thus, the velocity selector and pulse generator 24
produces an output signal only when the two successive trigger
pulses 20 and 22 are spaced 2 microseconds apart. The velocity
selector and pulse generator 24 may be one of the kind disclosed by
J. F. Friichtenicht in NASA Contract Report CR-263, entitled
"Particle Parameter Selector System for an Electrostatic Particle
Accelerator," published July 1965 and available from the
Clearinghouse for Federal Scientific and Technical Information,
Springfield, Virginia, 22151.
The output signal pulse 26 is fed to the pumping circuit 28 of a
giant pulse laser 30. The laser 30 may be disposed within the
chamber 10, or it may be positioned externally thereof, as shown.
The giant pulse laser 30 may be one of the kind disclosed in the
article by F. R. Marshall, D. L. Roberts, and R. F. Wuerker,
published in the Bulletin of the American Physical Society, Series
II, Vol. 7, No. 7, page 445, dated Aug. 27, 1962, and entitled
"Energy Storage and Radiation Emission from Kerr-Cell-Controlled
Lasers." Briefly, the giant pulse laser 30 is initially pumped
while a Q-switch 32 in the laser cavity is operated in a mode which
inhibits regenerative action in the cavity, thereby preventing the
laser 30 from generating an output light signal from the output
circuit 34 thereof. The Q-switch may comprise the combination of a
Kerr cell and a Glan-Thompson polarizer. At a predetermined time,
the Q-switch 32 is switched to a different mode which allows
regenerative action to take place in the cavity and cause
stimulated emission of light therefrom. The light output signal is
a high intensity light pulse of short duration, which is
illustrated as a laser beam 35 exiting from the output circuit
34.
The sequence of operation of the giant pulse laser 30 must be such
as to cause the laser beam 35 to irradiate the particle beam 14 for
a sufficient time to vaporize the particles. Thus, taking into
account the velocity of the particles, the presence of the
particles of desired velocity must first be sensed, the laser 30
must be pumped for a predetermined time, and then the Q-switch 32
must be actuated between the two modes to generate a laser beam
that irradiates the moving particles.
As indicated previously, the output signal pulse 26 from the
velocity selector and pulse generator 24 is fed to the pumping
circuit of the giant pulse laser 30 to initiate the pumping
sequence. The laser 30 must be pumped for about 60 microseconds
prior to actuating the Q-switch 32. It will be understood that the
pumping circuit 28 may include switching means responsive to the
output signal pulse 26 for energizing the means for pumping the
laser 30.
During the initial pumping period, a 5 kilometer per second
particle will have travelled a distance of about 0.5 meters beyond
the second detector 18. Thereafter it is desired to initiate
operations for actuating the Q-switch 32 in the manner described
above. The position of a particle is again sensed by means of two
additional particle detectors 36 and 38. The third detector 36 is
located about 2.5 meters beyond the second detector 18, and the
fourth detector 28 is about 1 centimeter beyond the third detector
36.
The third and fourth detectors 36 and 38 are similar to the first
and second detectors 16 and 18. In the example cited for
illustration, the third and fourth detectors 36 and 38 sense the
passage of a 5 kilometer per second particle therebetween and
develop two trigger pulses 40 and 42, spaced 2 microseconds apart,
which are fed to a delay generator 44.
The delay generator 44 may be one of the kind disclosed in the
article by J. Bell and J. H. Green in Nuclear Instruments and
Methods, Vol. 36, page 320, dated 1965, and entitled "A Wide Range
High Resolution Time to Amplitude Converter," or in the article by
A. E. Blaugrund and Z. Vager in Nuclear Instruments and Methods,
Vol. 29, page 131, dated 1964, and entitled "A Time-to-Pulse-Height
Converter With Simultaneous Random Coincidence Subtraction." The
delay generator 44 generates a rectangular delay pulse 46 that has
a delay, relative to the fourth trigger pulse 42, that is
proportional to the time interval between the third and fourth
trigger pulses 40 and 42. The time delay may be equal to the
trigger pulse interval or it may be some multiple thereof. For
example, suppose that the time delay is set to be equal to the
trigger pulse interval. The delay pulse 46 will appear 2
microseconds after the fourth trigger pulse 42 occurs, and at a
time when the 5 kilometer per second particle has travelled 1
centimeter beyond the fourth detector 38 and arrived at a location
or interaction region identified in the drawing by the numeral
48.
The delay pulse 46 is fed to the Q-switch 32 of the laser 30 to
switch the laser 30 to its regenerative mode. Within 100
nanoseconds after application of the delay pulse 46, the laser beam
35 issues from the output circuit 34 and enters the chamber 10
through a window 50.
Typically, the laser beam 35 is about 2 millimeters in diameter and
has a duration of 60 nanoseconds, with a total power output of
about 3 joules. Thus, if the laser beam 35 is directed along an
axis which coincides with the location 48 occupied by the particle
when the delay pulse 46 first appears, and if the largest particle
of 2 microns diameter is assumed, the particle will be within the
laser beam 35, since the particle will have moved only 0.005
millimeter during the 100 nanosecond delay between switching of the
laser 30 and the appearance of the laser beam 35. The particle will
remain within the laser beam 35 for 60 nanoseconds, which is
sufficient time to vaporize a 2 micron particle and produce a burst
of atoms therefrom.
The limitation in the repetition rate of the apparatus above
described is primarily determined by the power dissipation in the
active material of the laser 30. A repetition rate of 1,000 per
second for a period of 10-100 milliseconds can be achieved with a
single laser. By using a multiple laser system, the total period
can be extended to several seconds without any other substantial
change in the system. Because the sensing system comprising the
particle detectors 16 and 18 and their associated electronics which
controls the firing of the laser 30 operates on individual
particles, a high precision in the particle repetition rate is not
required.
The atomic composition of the original charged particle consists
essentially of neutral atoms, with a small fraction, no greater
than 10.sup.-.sup.4, of ions which constitute the surface charge of
the solid particle. The effect of the laser beam is to break all of
the atomic and ionic bonds in the solid particle. Following laser
impact, a burst of free atoms and free ions results, all traveling
with the same velocity as the original charged particle. Because of
the high particle velocity and low laser power density, the
vaporized solid particle will consist primarily of neutral atoms
with a radial expansion velocity small compared to its
longitudinally directed velocity--a beam of neutral atoms. Before
this beam can be used, it is necessary that all possible charged
particles, which can significantly influence cross-section values
etc., be removed as by passing the beam through an electric field,
as shown. The atoms pass undeflected through the electric field
provided by a pair of spaced electrodes 52 and 54 while the
positive ions are attracted to the more negative electrode 54,
shown grounded. The electrodes 52 and 54 may be in the from of
rectangular plates about 1 centimeter wide and 4 centimeters long
in the axial direction of the particle beam and spaced 2 to 3
centimeters apart. The d.c. voltage between the plates may be about
1 kilovolt. Thus a neutral atomic beam, indicated by the arrow 56,
issues from the electrodes 52 and 54.
RADIALLY EXPANDING PUFF OF NEUTRAL ATOMS
In this embodiment, the apparatus is the same as that shown in FIG.
1, with the exception that the deflection plates 52 and 54 are
eliminated. The particle beam 14 is of low directed velocity and
the laser beam 35 has a low power density. As shown schematically
in FIG. 2, the low velocity particle beam 14 and low power density
laser beam 35 intersect in the interaction region 48 to produce a
radially expanding puff of neutral gas, typically containing
10.sup.16 atoms. The radial velocity of the neutral atoms, which is
indicated by the arrows 60 is high as compared with the
longitudinal velocity of the particle beam 14, and typically is 10
times as great.
The ability to inject such a puff of gas of known density and
species into a specific location in a controlled thermonuclear
fusion containment geometry at a selected time represents a major
improvement in plasma diagnostic technique. Present techniques
employ fast mechanical valves which cannot provide the desired
localization, timing and quantitative injection.
PLASMA JET
In this embodiment, the apparatus likewise is the same as that
shown in FIG. 1, except that the deflection plates 52 and 54 are
eliminated. The particle beam 14 is of high directed velocity and
the laser beam 35 has a high power density. As shown schematically
in FIG. 3, the high velocity particle beam 14 and high power
density laser beam 35 intersect in the interaction region 48 to
produce a drifting plasma or jet, indicated by the arrow 62 which
has a high longitudinal velocity compared to its radial expansion
velocity. Typically, the longitudinal velocity is ten times as
great as the expansion velocity.
Plasma jets or plasmoids conventionally are produced by a variety
of discharge guns such as button sources and conical theta pinch.
These suffer particularly from contamination of the plasma with
electrode material and the plasma composition is unknown and
variable. Secondly, because they are produced in a variety of high
current discharges, trapped magnetic fields of unknown magnitude
and orientation can be associated with the plasmoids. Neither of
these drawbacks should be associated with the laser produced
plasmoid. The repetitive operation described above may also provide
major advantages.
RADIALLY EXPANDING PLASMA CLOUD
In this embodiment, the apparatus likewise is the same as that
shown in FIG. 1, except that the deflection plates 52 and 54 are
eliminated. The particle beam 14 is of low directed velocity and
the laser beam 35 has a high power density. As shown schematically
in FIG. 4, the low velocity particle beam 14 and high power density
laser beam 35 intersect in the interaction region 48. The injected
solid or liquid particle will be vaporized and ionized producing a
dense plasma. Because of the high particle temperature, the
expansion velocity, which is indicated by the arrows 64, is large
compared to the directed velocity and typically is ten times as
great; therefore, a plasma expanding about its origin is produced.
Typical temperatures may be 50-100 ev corresponding to an expansion
velocity of 3 .times. 10.sup.6 cm/sec. If 100.mu. diameter lithium
droplets are used as the laser targets, then approximately
10.sup.16 ions will be produced per pulse.
The beam-particle interaction can be produced within a magnetic
field confinement geometry such as a mirror, torus, stellarator,
etc. Because of the large mass to charge ratio of the injected
particle, injection into any magnetic geometry is possible without
perturbation of the trajectory.
Because the particle path and velocity are accurately known, an
electromagnetic suspension system, such as that disclosed by A. F.
Haught and D. H. Polk, Physics of Fluids 9, 2047, 1966, or a
mechanical suspension system such as disclosed by E. W. Sucov, J.
L. Pack, A. V. Phelps, and A. G. Engelhardt, Physics of Fluids 10,
2035, 1967, are not required, and therefore the magnetic
confinement system remains unperturbed. The systems described in
the above cited references are single event configurations, as
contrasted with the high repetition rate of the present system
described herein.
Laser produced plasmas offer the major advantage that transient
plasmas of high temperature and modest density can be produced in a
confinement geometry without any perturbation of the initial vacuum
and without the application of electric fields which may produce
unstable conditions. In accordance with this invention, a
particular application of laser produced plasmas to the problem of
controlled thermonuclear fusion will be described.
One prior art approach to the problem of controlled thermonuclear
fusion has been to separate the functions of containment and plasma
heating to thermonuclear temperatures. This approach is represented
by the PHOENIX Experiment at the Culham Laboratory, Abingdon,
Berks, England, which is reported by L. G. Kuo, E. G. Murphy, M.
Petravic, and D. R. Sweetman, Physics of Fluids 7, 988, 1964, and
the ALICE experiment at the Lawrence Radiation Laboratory,
Livermore, California. The desired high energy protons, deuterons,
and tritons are produced in a conventional 20-40 KV accelerator. In
order that these particles may be trapped in a magnetic confinement
geometry, they must undergo a change in either charge or momentum
during their transit of the containment geometry. The usual
technique is to convert the accelerated 20-40 Kev protons,
deuterons and tritons to neutral atoms in a charge exchange cell. A
small fraction of these beams of neutral atoms will be ionized
during their transit of the magnetic confinement geometry by
Lorentz ionization of those atoms in highly excited states. At 20
Kev, the trapping efficiency is approximately 10.sup.-.sup.4 and
decreases to 10.sup.-.sup.5 at approximately 1 Kev.
Collisional ionization against a seed plasma provides a useful and
more efficient alternative to Lorentz trapping. Such a plasma is
produced according to this invention during each laser-particle
interaction. In this case, it is desirable to use lithium as the
droplet material. It offers the following advantages:
1. Because of its low vapor pressure, it can be cryogenically
removed at the low temperature walls thus preventing a significant
rise in the neutral atom pressure.
2. The low atomic number is desirable to limit possible
bremsstrahlung and excitation losses.
FIG. 5 illustrates one type of arrangement which can be used to
achieve trapping in the magnetic confinement region of injected
energetic neutral atoms by collisional ionization against a seed
plasma. For reasons given above, lithium has been chosen as the
particle material. Lithium can be charged and accelerated in liquid
droplet form as described in the previously cited Los Alamos
Scientific Laboratory Report LA2549. The lithium droplet beam 14a
issuing from a droplet source and accelerator 12a is focused by an
electrostatic lens system 13 at the point 48 at which the
laser-particle collision will occur. This is done in order that the
laser beam may also be well focused. The lithium droplet beam 14a
then passes through the four particle detectors 16, 18, 36, 38 and
then into a magnetic confinement region 70 produced by a magnetic
confinement means 72. The magnetic confinement region 70 may be
produced in conventional manner by a pair of electromagnetic coils
forming a simple magnetic mirror arrangement, for example. The
outputs of the particle detectors are directed to electronics as
previously described to fire the laser 30 at the correct instant to
achieve the laser beam and particle beam collision. The laser beam
35 is directed into the magnetic confinement region 70 so that the
laser beam 35 impacts the particle beam 14a within the region
70.
A 2-joule output laser has ten times the energy required to create
a plasma of 50 electron volts temperature and 1 .times. 10.sup.16
lithium ions and 1 .times. 10.sup.16 electrons. A lithium droplet
of 40 microns radius contains 1 .times. 10.sup.16 lithium atoms.
Therefore the laser beam radius at the collision site 48 may be 120
microns or 0.12 millimeters. These large lithium droplets will have
relatively low linear velocities of the order of tens of meters per
second, if an acceleration voltage of 100,000 volts is used.
Because of this rather low velocity, the timing precision required
is minimal compared to the previously described neutral atomic beam
generation means. Because the energy absorbed by the lithium
droplet is proportional to its surface area, and because the
surface to volume ratio increases as the droplet size is decreased
by 1/radius, any smaller droplet size desired may be used.
An energetic beam 76 of one of the hydrogen isotopes such as
deuterium or tritium neutral atomic particles having an energy in
excess of 1000 electron volts is directed from a source 78 into the
magnetic confinement region 70 at the impact point 48. The
energetic neutral particles pass into the magnetic confinement
region unaffected by the magnetic field. Within the region 70, the
energetic neutral particles collide with the plasma created by the
impact of the laser beam 35 and the lithium droplet beam 14a.
Collision ionization of the energetic neutral particles occurs.
Once ionized, the energetic particles are trapped within the
magnetic confinement region 70 so that they occupy a restricted
volume 80.
It is estimated that the lifetime of the lithium plasma will be
approximately 5-10 .mu.Sec, with an average electron density of 2
.times. 10.sup.13 /cm.sup.3. The trapping efficiency = L N.sub.e
T.sub.i V.sub.e /V.sub.+ = 0.15 where L is the path length taken to
be 5 cm
N.sub.e is the electron density = 2 .times. 10.sup.13
T.sub.i is the ionization cross section = 10.sup.-.sup.16
cm.sup.2
V.sub.e /V.sub.+ is the ratio of the electron to proton velocities
= approximately 4
If the energetic atomic beam 76 is operated in a continuous
fashion, the trapping efficiency will not be very high because of
the low duty cycle. If the beam 76 is pulsed on simultaneously with
the laser-particle interaction for only the 10 .mu.sec duration of
the laser plasma, trapping efficiencies of approximately 0.15 can
be achieved. Secondly, pulsing of the particle accelerator 12a may
permit a substantial increase in the injected neutral beam current
with a consequent increase in the instantaneous trapped plasma
density. The repetition rate of 1/msec is high enough to provide
for accumulation of plasma over many laser-particle pulses. The
initial system pressure should remain unperturbed.
A new method of energetic hydrogen injection employs atom clusters
with a high mass to charge ratio; this injection technique also
requires a trapping plasma of transient nature to provide breakup
and ionization of the clusters.
While the foregoing specific description is based on the use of a
2-joule laser, it is apparent that with the development of higher
powered lasers, much larger lithium droplets than the ones
described can be employed. The consequent increase in plasma
density will increase the trapping efficiency.
* * * * *