U.S. patent number 3,660,715 [Application Number 05/064,720] was granted by the patent office on 1972-05-02 for ion source with mosaic ion extraction means.
Invention is credited to Richard F. Post.
United States Patent |
3,660,715 |
Post |
May 2, 1972 |
ION SOURCE WITH MOSAIC ION EXTRACTION MEANS
Abstract
An ion source including means for producing a directed stream of
ionized gas or plasma wherein a polycellular channeled mosaic
element is positioned to transect the path traversed by the plasma
so that a potential sheath is established in the plasma. The
channel diameter is made less than the Debye length of the ions in
the plasma wherefor the ions are accelerated out of the plasma,
pass through the channels of the mosaic element and emerge as an
ion beam on the other side of the matrix.
Inventors: |
Post; Richard F. (Walnut Creek,
CA) |
Assignee: |
|
Family
ID: |
22057848 |
Appl.
No.: |
05/064,720 |
Filed: |
August 18, 1970 |
Current U.S.
Class: |
315/111.51;
313/230 |
Current CPC
Class: |
H05H
1/34 (20130101); H05H 1/24 (20130101); H05H
1/3452 (20210501); H01J 27/022 (20130101) |
Current International
Class: |
H01J
27/02 (20060101); H05H 1/24 (20060101); H05h
001/00 () |
Field of
Search: |
;315/111
;313/63,231,230 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Demeo; Palmer C.
Claims
What I claim is:
1. An ion source comprising:
plasma source means including a stack of interleaved metallic
annular and insulating material washers defining an axial channel
therein, said metallic washers having a material selected from the
group consisting of hydrogen, deuterium and tritium occluded
thereon, and means for creating an electrical discharge between
portions of said washer members within said channel so as to
release, ionize and heat said occluded gas to generate a heated
plasma stream containing electrons and ions which is ejected
axially from said channel;
funnel guide element means including walls converging toward a
central aperture which is axially aligned with said channel and
arranged to collect and direct said plasma stream toward said
central aperture; and
means including a substantially planar mosaic disk element disposed
transversely across said aperture, said mosaic element defining a
plurality of straight line channels substantially in alignment with
the axil flow vector of said plasma stream through said aperture
with the diameter of said channels being less then the Debye length
of said plasma stream so that a plasma sheath potential is
developed in the plasma in proximity to said mosaic element whereby
electrons therein are decelerated and ions are accelerated
therefrom at an energy above their initial kinetic energy in the
plasma stream to pass through the channels in said mosaic element
to form an energetic ion beam emergent therefrom.
2. Apparatus as defined in claim 1 wherein said plasma has a
density of at least about 10.sup.12 particles/cc providing a Debye
length below about 25 microns and wherein said channels have a
diameter less than about 25 microns in magnitude.
3. Apparatus as defined in claim 2 wherein said plasma source means
further includes a cylindrical housing disposed in insulated
relation about said stack of interleaved washers said housing
having a conical cup portion defining an orifice axially aligned
with said channel through which said plasma stream is directed and
wherein said metallic washers are formed of a material selected
from the group consisting of titanium and zirconium.
4. Apparatus as defined in claim 3 further including a vacuum
housing defining an evacuable chamber said housing including a
central section to one end of which is attached to cover plate
centrally apertured to receive and support said plasma source
housing and to the opposite end of which is attached a base plate
centrally apertured to receive and support said guide element means
with the aperture in axial alignment with the washer stack channel
as well as to support said means including said mosaic element with
the channels thereof in axial alignment with the aperture of the
guide element.
5. Apparatus as defined in claim 4 wherein said mosaic element
comprises a disk of glass material, said glass material being
disposed in a honeycomb configuration with thin wall portions
defining said plurality of straight line channels, said straight
line channels being in substantially parallel alignment.
6. Apparatus as defined in claim 4 wherein a charging voltage in
the range of about 2 kilovolts to about 15 kilovolts is applied
across said washer stack by said means for creating a discharge
between said washer members, wherefor the plasma ions have an
energy in the range of about 50 to 150 electron volts, wherein the
channels of said mosaic elements have a diameter in the range of
about 25 microns to about 10 microns and a thickness in the range
of about 0.025 inches to about 0.010 inches.
7. Apparatus as defined in claim 6 wherein said mosaic disk element
is provided with a metallic plating or at least one side thereof.
Description
BACKGROUND OF THE INVENTION
The invention disclosed herein was made under or in the course of
Contract No. W-7405-ENG-48 with the United States Atomic Energy
Commission.
Ion sources for producing beams of ions used, e.g., in various
types of energetic particle devices such as particle accelerators,
ion bombardment devices, controlled fusion reactors, etc.,
generally employ electrically biased extractor electrodes for
separating ions from a plasma to form the ion beam. Occluded gas
ion sources of this type are described in "Controlled Thermonuclear
Reactions," Glasstone and Lovberg, D. Van Nostrand Company, Inc.,
1960, beginning at page 145. In these as well as in other types of
ion sources the ions are extracted from relatively large orifices
or surfaces of a plasma body of relatively large dimension and
extraction is obtained by electrostatic attraction which often
requires the application of relatively high potentials.
Difficulties ensue due to electrical discharges between the
electrodes, disruption of the plasma and the structures are
complicated by the geometries required.
SUMMARY OF THE INVENTION
The invention relates, in general, to the production of ion beams
for use in various charged particle devices and, more particularly,
to an ion source wherein extraction of ions from a plasma body is
accomplished by means of a passive mosaic element, defining a
plurality of critically dimensioned cellular channels, disposed in
proximity to the plasma body.
An ion source suitable for practice of the invention includes a
means for producing a defined body of plasma generally in accord
with various means well-known in the art. Such means usually
include a means for supplying a suitable gas together with means
for ionizing and heating the gas to provide the plasma body. One
such suitable plasma producing means is that utilized in the
above-referenced occluded gas ion source, i.e., an arrangement of
hydrided metallic elements across which an electrical discharge is
applied to release and ionize hydrogen gas to form the plasma.
Other suitable devices may produce a plasma by instituting a
continuous or pulsed electrical discharge in a suitable gas, for
example, between electrodes positioned therein, by means of radio
frequency energy applied thereto, or the like. Usually the plasma
will be produced in a chamber defined by enclosure wall surfaces.
Moreover, the plasma body may be constrained within a magnetic
field as discussed more fully hereinafter.
For extraction of ions from such a plasma body, in accordance with
the invention, a porous mosaic element formed of either insulating
or conductive material is placed in proximity to the plasma, for
example, as a portion of the enclosure wall, at a location from
which extraction of the ions is desired. For effective extraction
to occur, certain critical conditions must be satisfied. The mosaic
element should define a large plurality of reasonably uniform
diameter capillary channels passing in substantially straight line
relation therethrough. Various polycellular or honeycomb type
materials are satisfactorily provided that the channel diameter is
smaller than the Debye length of the plasma. Furthermore, the
plasma body dimensions must be at least one order of magnitude and
preferably at least several orders of magnitude greater in extent
that the Debye length thereof.
With these conditions satisfied a potential sheath is produced in
the plasma as if the porous mosaic element was a solid wall similar
to the remainder of the enclosure. The potential sheath is of a
polarity and the potential thereacross is of a magnitude sufficient
to accelerate positively charged ions out of the plasma body. The
ions having a velocity vector sufficiently parallel to the axis of
the mosaic channels pass therethrough to emerge on the opposite
side of the mosaic element as an ion current or ion beam.
If the first condition noted above is not satisfied, i.e., if the
channel diameter exceeds the Debye length the potential sheath
necessary to effect extraction of the ions will not form as if the
capillary channels were not present. If the second condition is not
satisfied a true and appropriate plasma body will not be
present.
Accordingly it is an object of the invention to provide an ion
source in which extraction of ions from a plasma body is effected
without the use of electrically energized extraction
electrodes.
Another object of the invention is to provide an ion source
including means for producing a confined plasma body wherein
extraction of ions from the plasma body is effected by means of a
passive channeled mosaic element disposed in proximity to the
plasma.
Still another object of the invention is to provide an ion source
including means for producing a plasma body wherein a mosaic
element is disposed with one surface thereof in proximity to the
plasma body, said mosaic element having a plurality of straight
line capillary channels formed therein, which channels have a
diameter less than the Debye length of the plasma so that a
potential sheath is developed in the plasma effective to accelerate
and extract ions therefrom to pass through said channels to provide
an ion beam emerging at a second surface of said element.
Other objects and advantageous features of the invention will be
apparent in the following description and accompanying drawing of
which:
FIG. 1 is a longitudinal cross-sectional view of an ion source,
utilizing a channeled mosaic element for extracting an ion beam in
accordance with the invention; and
FIG. 2 is a schematic illustration of a second embodiment of the
ion source of the invention.
DETAILED DESCRIPTION OF THE INVENTION
A plasma body such as that produced in the ion source of the
invention is an electrically neutral collection of charged
particles, i.e., a collective mass of electrons and positive ions
in which the ions and electrons generally have a Maxwellian
distribution in velocity space but in which the ion and electron
temperature may be different. The ion temperature is generally less
than or equal to the electron temperature, usually less. With a
hydrogen plasma the random ion current is at most about
one-fortieth of the random electron current [(mass electron/mass
proton).sup.1/2 = 1/42.]
However, if such a plasma is confined within a container or
enclosure with solid walls, the electron and ion currents reaching
the boundary wall must be equal otherwise an absurdly high positive
charge would be continuously built up in the plasma. The phenomenon
that equalizes the ion and electron currents escaping from the
plasma is termed "the plasma sheath." When the plasma is formed in
the container more electrons than ions are initially lost resulting
in the accummulation of a positive charge in the plasma which slows
escape of electrons and accelerates loss of positive ions until the
escape currents are equal. The main effect is to reduce the
electron current.
The magnitude of the potential difference between the plasma body
and the wall, "the sheath potential drop" is of the order of a few
times kTe/e. This potential drop occurs over a characteristic
distance which is of the order of the Debye length which is defined
as follows:
.lambda..sup.D = (kTe/4.pi.ne.sup.2).sup.1/2 (cgs units) wherein Te
is the electron temperature in .degree.K, ne is the electron
density, k is the Boltzman Constant and e is the electron charge.
Substituting numerical values:
.lambda..sub.D = 6.9
(.sup. /ne).sup.1/2 cm The Debye length is a measure of the
distance into the plasma wherein variations in the potential can
occur. Now it may be noted that, if a section of wall having
openings therein extending normal to the plasma surface is
substituted for a solid section of the plasma container wall, a
potential sheath forms in the plasma in proximity thereto as long
as the openings or pores in the wall section are smaller than the
Debye length of the plasma. Ions are accelerated out of the plasma
by such potential and impinge upon the wall and enter the openings
therein. Provided that the openings are in the form of straight
channels extending through the wall section, ions having a
sufficiently large velocity vector parallel to the axis of said
channels will pass therethrough and appear as an ion beam on the
opposite side thereof. With a plasma having a density of the order
of 10.sup.12 particles/cc, the requisite channel diameter is of the
order of 10 microns. With lower density plasmas, the Debye length
is larger, and a larger channel size can be used. Generally
speaking, channel sizes of less than about 50 microns can be used
and the porous wall section can be conveniently provided as a
mosaic element.
The transmitted particle beam comprises accelerated ions and
decelerated electrons. A important advantage to be gained by use of
a porous wall section, i.e., the mosaic element for extracting the
ions is evidenced by the following consideration. Any device that
is used to a accelerate charged particles incurs the limitations
imposed by the Child-Langmuir Law (c.f. "Electric Phenomena in
Gases" Papoular, R. Iliffe Books, Ltd., London 1965, p. 102) which
gives the maximum current density, j.sub.m, that can be accelerated
by a potential difference, V, between two grids a distance, d,
apart, as follows:
where m = mass of the ion.
By using the mosaic element to create a sheath potential for
accelerating ions out of the plasma body, the distance over which
acceleration occurs is of the order of the Debye length which is a
very small distance, of the order of up to a few tens of microns in
a relatively dense plasma, i.e., with a density of the order of
10.sup.12 particles/cc. With denser plasmas the distance, d, is
even shorter. Accordingly, an ion beam of high current density can
be produced even with a sheath potential of a few volts using such
a mosaic element. The limit would be the ion current reaching the
walls of the container and the transmissivity of the matrix
element. A preponderance of ions pass through the element. The
energy thereof will depend on the sheath potential as well as the
initial kinetic energy of the particle in the plasma. Various means
are known in the art for increasing such a sheath potential and
accordingly the ion energy of the emergent ion beam may range
upwardly from a few electron volts determined by the kinetic energy
(temperature) of the ions in the plasma added to the energy
supplied by the sheath potential. The ion current emerging from the
mosaic element may reach densities at which charged particle
repulsion causes the beam to "blow up" in which case low energy
electrons may be introduced for space charge neutralization to
offset the repulsive forces. Current densities of the order of
about 0.1 to above about 1.0 amps sq cm may easily be obtained.
DESCRIPTION OF AN EMBODIMENT
An ion source utilizing an occluded gas source means for producing
a plasma from which ions are extracted to form an ion beam in
accordance with the teachings of the invention is illustrated in
the FIG. 1 of the drawing. As illustrated therein the ion source is
constructed within a generally cylindrical housing 11 defined an
evacuable chamber 12. Housing 11 may be constructed with a
cylindrical section 13 of stainless steel, glass, ceramic or other
tubing material suitable for vacuum service and with a hermetically
sealed cover plate 14 attached, e.g., by flanges or the like to one
end of section 13. The other end of cylindrical section 13 may be
provided with an annular base plate 16, i.e., centrally apertured
plate 16, which can serve as means for attaching the ion source in
vacuum tight relation to a part of a vacuum housing 17 of a device,
e.g., particle accelerator, fusion reactor, etc., in which the ion
beam is to be employed. While such source mounting plate might be
attached directly to housing 17, in order to facilitate making
certain measurements, an electrically insulated mounting may be
used. For this purpose, an annular member 18, of insulating
material, e.g., polymerized acrylic resin or the like, is
interposed between plate 16 and housing 17 and the plate 16 is
attached thereto by means of flange bolts 19 insulated from plate
16 by means of a washer 21 and an insulating sleeve 22.
The occluded gas plasma source 23 used herein, is generally
constructed with an assembly comprising a plurality of annular
metallic washers 24 interleaved with and separated by means of thin
insulating annular washers 26, typically of mica and of larger
diameter, stacked and arranged concentrically along an axial
passage 27. With this arrangement a series of gaps exist between
the inner margins of the washers extending across the inner margin
of the intervening mica washer. The aforesaid stack assembly may be
disposed within a mounting housing including a metallic cylindrical
body portion 28 and a conical nozzle cap portion 29 affixed
thereto, e.g., by a threaded joint 31. Conical cap portion 29 is
provided with an orifice 32 axially aligned with channel 27 of the
washer assembly and is in direct electrical contact with a washer
24 at one end of said assembly. Support for the opposite end of
said washer assembly is provided by means of a tubular conductor
33, retained concentrically within the cylindrical body portion 28,
by an annular insulating member 34 fitted into a stepped portion of
the interior wall of body portion 28. The interior end of conductor
33 abuts in direct contact with a washer 24 at the second end of
the stacked washer assembly while the second end of conductor 33
projects beyond the free end of cylindrical body 28. An insulating
sealed cap 36 affixed to the free end of body 28 may be used to
support the conductor 33 thereat.
The metallic washers 24 comprise a metal such as titanium or
zirconium which have been treated to occlude a large amount of a
gas such as hydrogen, deuterium, tritium, etc., from which it is
desired to produce a suitable ion beam. More specifically, the
titanium washers may be heated in high vacuum to outgas impurities
therein and then contacted, e.g., with H.sub.2, D.sub.2, etc., at
an elevated temperature to initiate a hydriding reaction therewith.
Details of such a preparation are disclosed, for example, in Report
No. UCRL-4496, issued by the University of California Radiation
Laboratory, abstracted in Nuclear Scientific Abstracts, Aug. 31,
1955.
The plasma source 23 also includes a trigger electrode 37 disposed
in insulated sealed relation within conductor 33 having an inner
end disposed in proximity to the inner end of conductor 33 and the
washer 24 abutting therewith and defining a spark gap 38
therebetween. A pulse power source is connected across the stacked
washer assembly to furnish power for generating the plasma. Such a
power source may comprise a pulse line 41 comprising a series of
capacitors connected to a tapped inductor and including damping and
current limiting resistors as in conventional practice. A direct
current power supply 42 is connected to one end of the pulse line
while the other end of the pulse line is connected between
conductor 33 and the grounded mounted housing of the plasma source
to establish a suitable potential across the stacked washer
assembly. A means for generating a spark discharge across spark gap
38 is provided to initiate a discharge of pulse line 41 along the
series of gaps between the washers in axial passage 27. Such means
may comprise a pulse transformer 20 having the secondary connected
between conductor 33 and electrode 37. A trigger pulse applied from
a trigger generator (not shown) to the primary of said pulse
transformer will then produce a spark discharge across gap 38 which
discharge will initiate discharge of the pulse line energy across
the washer gaps as noted above. Such a discharge causes gas to be
released from the metallic washers 24 to be ionized and heated and
be ejected from passage 27 through orifice 32 axially along housing
section 13, to form plasma body 43 therein. The plasma of body 43,
with a hydride source will generally comprise H.sup..sup.+ ions,
electrons and a substantial proportion, e.g., 50 percent of
Ti.sup..sup.+ ions. Flow of pulse current from pulse line 41 can be
terminated, for example, by short circuiting the output therefrom
as by means of a triggered ignitron 44 or other switch connected
across the output of the pulse line. A substantially square wave
pulse of selected duration can then be applied to such source.
Voltages in the range of a few KV to at least 20 KV can be employed
with such a pulse line.
For purpose of extracting ions from plasma body 43 a mosaic element
46 is disposed to intercept or otherwise be disposed to be in
closed proximity to plasma body 43. The matrix element 46 may be
formed of any material suitable for vacuum service and it may be
either conductive or non-conductive. Suitable materials include
glass, ceramic, metals such as stainless steel, nickel, etc. The
element may take the form of a planar disk plate, as shown, or it
may be a curved section corresponding to a cylindrical or spherical
plasma body, or any other suitable shape. The element must be
provided with a large plurality of very small channels, preferably,
substantially parallel straight line channels transpiercing the
mosaic element. The element is arranged preferably with such
channels aligned with the path of the ions leaving plasma body
43.
In the event that magnetic field, supplied, e.g., by solenoids
wound about vessel section 13, is used to combine or guide the
plasma, the channels must also be aligned with the magnetic field
line guiding centers for effective extraction to occur. Generally
speaking the channel diameter must be below about 50 microns and
usually below about 25-30 microns dependent, as indicated above, on
the Debye length of the particular plasma. With particle densities
of the order of 10.sup.12 particles/cc, channel diameters of the
order of 10-15 microns or smaller yield satisfactory or superior
results and are preferred. Such element extracts ions from the
plasma body 43 to form a beam 45, e.g., of H.sup.+ ions emergent
from the opposite side to enter housing 17.
One highly satisfactory mosaic element formed of a material,
commercially available, comprises a glass Pyrex) element which on
microscopic examination can be seen to be a polycellular or
honeycomb mosaic in which the capillary channel walls have a
hexagonal thin wall configuration leaving at least about 50 percent
open channel space. Disks of such material having channel sizes
such as 10 and 25 microns diameter are available. Versions having a
metallic plating on one or both sides are also available and are
suitable. Similarly suitable metallic polycellular mosaic materials
can also be made as disclosed in U.S. Pat. No. 3,222,144 issued
Dec. 7, 1965 to Donald E. Davenport. A thin matrix element is
generally preferred since the shorter the channel length the higher
the ion beam transmissivity, due to improved geometry, yielding
larger ion currents. The current output can also be increased by
increasing the ratio of open channel area present, by increasing
the incident particle energy and by reducing the radial energy of
the particles, i.e., the ratio of rotational energy to
translational energy. The probability (Po) of passage through the
channel of a particle at the center of the channel is P(o) = 1/2
(Vz/Vr) .sup.2 (R/L).sup.2 where Vz = translational velocity along
capillary axis, Vr = velocity transverse to axis, L= length of
channel and R = radius of channel. Matrix elements of the order of
0.025 inch thick for a 25 micron hole size and 0.010 inch thick for
a 10 micron hole size or less are satisfactory.
The mosaic element 46 may be mounted, for example, by means of a
cup-shaped alumina mounting holder 47 secured to the base plate 16,
transverse to the axis of vessel 13 within the aperture defined by
the annular base plate 16. The bottom 49 of means 47 is apertured
centrally and mosaic element is disposed to cover said aperture in
bottom 49 of means 47. The mosaic element 46 may be retained in
position by a funnel-shaped guide element 51 secured to plate 16 by
a mounting ring 52, or by any other suitable retaining means. The
walls of element 51 coverage on an axial aperture 53 therein which
defines an area of mosaic 46 toward which the plasma body
approaches or is in proximity to.
In usual applications, evacuation of chamber 12 may be provided by
communication through the channels of the mosaic element to the
region within vacuum housing 17 evacuated with the vacuum pump
means (not shown) as customarily utilized. However, for other
purposes, e.g., when making measurements of performance, etc., the
housing 17 can be a vacuum tank evacuated to below 10.sup..sup.-6
to 10.sup..sup.-7 mm Hg or lower by a vacuum pump (not shown). In
such a case current deposited on the mosaic element 46, guide
element 51 or grid 54 may be measured by a current detecting loop
(, i.e., a bug) 56 disposed about a ground lead 57 connected to
plate 16. Collector grids, double probe detector means or the like
may be provided in housing 17 to measure ion currents and other
effects as in conventional practice. However when utilized as an
ion source, e.g., in an accelerator, etc., plate 16 may merely be
grounded.
Typical constructional details and operating parameters for such an
ion source are set forth in the following illustrative example:
EXAMPLE
Vessel section 13, 2 inches I.D. Pyrex glass tubing, 6 inches
long
Source 10 hydrided titanium washers 1/4 inch O.D. 3/32 inches I.D.
ca. 1 mm thick
Pulse line: six 8.5 microfarad capacitors distributed along a 210
micro henry induction coil
Typical pulse lengths 5-175 microseconds
Charging voltage across pulse line 2 KV to 15 KV
Arc voltage drop across washer stack ca. 100 v. Evacuation pressure
down to 5.times.10.sup..sup.-7 mm Hg.
Source to mosaic spacing ca. 8 cm
Mosaic glass hexagonal channel 25 micron hole diameter 0.025 inch
thick 10 micron hole diameter 0.010 inch thick
(Gold plated bothsides and unplated)
Current density at least about 0.10 to 1.0 amp/cm.sup.2
Plasma ion energy mean ca 1 ev
Ion energy (H.sup.+ ) in ion beam about 50-150 ev
Results indicated that at 5 KV charge, productive of a higher
density plasma, more current passed through the 10 micron mosaic
than through the 25 micron mosaic. This is because the Debye length
of such a plasma is about 25 microns and the 25 micron mosaic tends
to defocus the plasma surface. With a 10 micron mosaic the plasma
sheath forms much more evenly. With a 3 KV charge, the density is
less and the Debye length longer so that defocussing no longer
occurs. Moreover, in the first mentioned case the electron current
is about five times as large as the ion current using the 25 micron
mosaic. With the 10 micron mosaic, ion and electron currents are
about equivalent.
DESCRIPTION OF A SECOND EMBODIMENT
A second embodiment 75 of an ion source in accordance with the
invention as illustrated in FIG. 2 is constructed with a tubular
microwave permeable vacuum vessel 76 constructed, e.g., of ceramic
or glass defining a chamber 77 therein. A mosaic element 49, of the
character described is disposed across the open end of such vessel
76 together with a grid 49, as above. A conduit 79 may be used to
supply a gas, e.g., hydrogen, deuterium, tritium, helium, etc., at
the desired density to chamber 77. The vessel 76 may be disposed
within a waveguide 81 coupled to a microwave energy source (not
shown) for application of microwave energy of a suitable frequency
and at an appropriate level to ionize and heat the gas in chamber
77 to produce a plasma body 82 therein. To regulate the plasma
potential an electrode 83, e.g., of tungsten, etc., may be passed
in sealed relation through the closed end 84 of vessel 76 with the
inner end 86 thereof disposed in plasma body 82. A positive
potential, +, applied from a D.C. power supply (not shown) applied
to electrode 83 will regulate the plasma potential and thereby
control the energy of ions emerging through mosaic 46 in ion beam
87.
Certain relationships may be used to determine the operating
parameters for such an ion source as follows:
The plasma frequency
f.sub.plasma = f.mu..lambda. = 9.times.10.sup.3 .sqroot..sub.ne =
frequency of microwave power to be applied. (Ne = plasma density
particles/cc.) For example, with ne = 10.sup.12 particles/cc,
f.mu..lambda.= 9.times.10.sup.9 and .lambda..congruent. 3 cm
microwaves + may be varied over a considerable range, e.g., up to
at least to several hundred volts giving equivalent ion
energies.
While there have been described in the foregoing what may be
considered to a preferred embodiments of the invention,
modifications may be made therein without departing from the
teachings and scope of the invention and it is intended to cover
all such as fall within the scope of the appended claims.
* * * * *