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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ion generating apparatus. In particular
the present invention relates to ion generating apparatus utilizing a thin
disk shaped plasma.
2. Prior Art
Recent ion engine research has focused on the use of time-varying (wave)
electric energy or radio frequency waves, particularly microwave and UHF
waves, for electric propulsion engines or other ion generating apparatus.
Specific research projects presently being conducted involve utilizing rf
(Divergilio, W., MSU/NASA Workshop, East Lansing, Mich. Feb. 24-25, 1982)
Loeb, H. W., "State of the Art and Recent Developments of the Radio
Frequency Ion Motors," AIAA Paper 69-285 March 1969; Noriyuki Sakudo,
Katsumi Tokiguchi, Hidemi Koike, Ichiro Kanomata, "Microwave Ion Source",
Review of Scientific Inst. 48, July 7, 1977; VHF (Nakanishi, S., MSU/NASA
Workshop, East Lansing, Mich., Feb. 24-25, 1982) and microwave
(Divergilio, W., MSU/NASA Workshop on Advanced Propulsion Concepts Using
Time Varying Electromagnetic Fields, East Lansing, Mich., Feb. 24-25,
1982) electric energy sources. These engine concepts possess certain
advantages over the more conventional well-developed direct current ion
source apparatus. For example, time varying electric ion sources appear to
(1) improve overall system efficiency (low eV/ion); (2) allow high ion
beam densities when accelerated; (3) yield longer life engines due to the
absence of metal cathodes in the discharge region; and (4) result in
simplified overall system design. The same apparatus can also used for
free radical or ion irradiation of various materials rather than as ion
engines. In each instance the radio frequency wave apparatus are complex.
Objects
It is therefore an object of the present invention to provide a large
diameter ion beam generating or ion with a minimal plasma volume source
apparatus wherein a plasma is formed as a thin disk adjacent an ion
attracting means, such as a metallic negatively charged accelerator grid
alone or in combination with a screen grid. It is further an object of the
present invention to provide efficient microwave or rf power coupling even
without a magnetic field. It is further an object of the present invention
to provide an ion or free radical generating apparatus which is relatively
simple and inexpensive to construct. These and other objects will become
increasingly apparent by reference to the following description and to the
drawings.
IN THE DRAWINGS
FIG. 1 is a schematic isometric view of the preferred ion engine of the
present invention.
FIG. 2 is a front cross-sectional view of FIG. 1, particularly illustrating
the construction of the ion engine using a chamber (15) for forming the
disk shaped plasma.
FIGS. 3, 3a, 4, 4a, 5 and 5a are front cross-sectional and plan
cross-sectional views of various TM or TE empty cavity (11) microwave
modes used in the apparatus of the present invention.
FIG. 6 is a schematic view of the attached vacuum system (102, 103) used
with the ion generating apparatus shown in FIG. 2.
FIG. 7 is a schematic view of the preferred argon gas flow system to the
chamber (15) in the apparatus of FIG. 2.
FIG. 8 is a diagram of the microwave circuitry for supplying the microwaves
to cavity (11) in the apparatus of FIG. 2.
FIG. 9 is a diagram of the ion extraction circuit of the present invention
particularly showing the use of a screen grid (17a) and an accelerator
grid (17b) as shown in FIG. 2. FIG. 9a shows an ion neutralizer (20)
circuit which generates electrons from a metal wire (501).
FIGS. 10a and 10b show graphs of TE field patterns with a plasma.
FIG. 11 shows characteristics of the ion source operated in the TE.sub.211
and TM.sub.011 cavity modes.
FIG. 12 schematically shows an alternate open chamber (25), fixed length
cavity TM.sub.010 mode (rather than closed chamber (15) as in FIGS. 2 and
12a) of the ion generating apparatus of the present invention.
FIGS. 13 and 14 show front and back schematic views, respectively of the
ion source apparatus of the present invention, wherein bar magnets (26,
27) are used to form magnetic cusps to aid in confining the plasma ions in
the chamber (15 or 25).
FIGS. 15 and 16 are schematic front cross-sectional views showing ion
generating apparatus for ion and free radical plasma etching of various
materials (32).
GENERAL DESCRIPTION
The present invention relates to an ion generating apparatus including a
plasma source employing a radio frequency wave coupler, particularly a
cavity (11), which is excited in one or more of its TE or TM modes of
resonance and optionally including a magnetic field which aids in
confining the ions in the coupler and including ion attracting means (17b)
for attracting the ions from said plasma, preferably by means of a
suitable voltage potential which accelerates the ions, the improvement
which comprises:
(a) an electrically insulated plasma defining chamber (15) mounted in
closely spaced relationship to an area (16) of a metallic radio frequency
coupler;
(b) at least one metallic ion attracting means (17b) mounted adjacent to
and electrically insulated from the insulated chamber which forms part of
the radio frequency waver coupler; and
(c) gas supply means (18, 19) for providing a gas which is ionized to form
the plasma in the plasma defining chamber, wherein the radio frequency
wave resonance applied to the coupler and plasma maintains the plasma in
the shape of an elongate, thin disk adjacent to the ion attracting means
during steady state operation of the apparatus.
The present invention also relates to the method which comprises providing
an ion generating apparatus including a plasma source employing a radio
frequency wave coupler, particularly a cavity (11), which is excited in
one or more of its TE or TM modes of resonance and optionally including a
magnetic field which aids in confining the ions in the coupler and
including ion attracting means (17b) for attracting the ions from said
plasma by means of a suitable voltage potential which accelerates the
ions, wherein the plasma, and ion attracting means are maintained at a
reduced pressure in operation, wherein the ion source apparatus includes
the improvements of: (1) an electrically insulated plasma defining chamber
(15) mounted in a closely spaced relationship to an area (16) of a
metallic radio frequency wave coupler; (2) at least one ion attracting
means (17b) mounted adjacent to and insulated from the insulated chamber
which forms part of the radio frequency wave coupler; and (3) gas supply
means (18, 19) for providing a gas which is conveyed to form the plasma in
the insulated chamber;
(b) forming the plasma in the chamber with the radio frequency wave at
elevated gas pressures below atmospheric pressures and at elevated gas
flow rates from the supply means;
(c) reducing the gas pressure and the flow rate of the gas into the chamber
to provide a plasma disk adjacent the ion attracting means; and
(d) attracting ions with the ion attracting means.
The present invention is particularly concerned with microwaves in the
range of about 1000-3000 megahertz and UHF down to about 300 megahertz.
VHF can also be used below 300 megahertz to about 200 megahertz. In the
following description the reference will be to microwaves which are
preferred.
The plasma disk, ion generation apparatus of the present invention includes
several important improvements to microwave plasma systems. They are: (1)
generation of a resonantly sustained microwave discharge inside an area or
part of microwave/plasma coupler or cavity; (2) probe and length tuning of
the microwave/plasma coupler to efficiently create the plasma in the area;
and (3) minimization of plasma volume in the cavity. The microwave
discharge fills only part of the coupler. The plasma and the external
metallic coupler, usually a microwave cavity, combine to form a resonant
circuit. Probe and length tuning of the microwave/plasma coupler provide
the variable impedance transformation (Asmussen, J., R. Mallavarpu, J. R.
Hamann, and H. C. Park, Proc. IEEE 62(1), 109-117 (1974); Fredericks, R.
M. and J. Asmussen, J. Appl. Phys. 42, 3647 (1971)) required to adjust for
zero reflected power and achieve variable operation over a wide range of
pressure and microwave input powers.
The discharges generated in microwave cavities at low pressure are often
called resonantly sustained discharges (Messiaen, A. M. and P. E.
Vandenplas, Appl. Phys. Letters, 15, 30 (1969), Fredericks, R. M. and J.
Asmussen, Appl. Phys. Letters 19, 508 (1971)) or plasmoids because they
are similar to the lower frequency rf plasmoids studied by Wood (Wood, R.
W., Phys. Rev. 35, 673 (1930)) and Tallet (Tallet, J., "Plasmoides a'haute
Frequence et Descharges Resonnantes," Rapport CEA-R. 2502 Centre des
Etudes Muele'aires de Sac-lay, Gifsur-Yvette, France (1964)). Microwave
discharges created in this manner have densities greater than the critical
density. For example, electron densities of 10.sup.12 /cm.sup.3 or more
can be achieved with exciting frequencies of 2.45-30 GHz (Fredericks, R.
M. and J. Asmussen, Appl. Phys. Letters 19, 508 (1971); Mallavarpu, R., J.
Asmussen and M. C. Hawley, IEEE Trans. on Plasma Science PS-6, 341
(December 1978); J. Rogers and J. Asmussen, 1982 IEEE international
conference on Plasma Science, Ottawa, Canada, May 17- 19, 1982; J. Rogers,
Ph.D. Dissertation, Michigan State University, 1982). At reduced pressures
(<50 microns) of mercury where the mean free path of ions is comparable to
the size of the discharge vessel, measurements have shown that the
effective collision frequency, .nu..sub.e, is (1) higher than similar
microwave discharges in larger vessels and (2) higher than predicted by
simple electron neutral collisions. These "high" collision frequencies
together with high plasma densities provide the power coupling mechanism
to sustain the discharge at the low pressures (1.times.10.sup.-4 torr)
required for the ion source application.
Specific Description
FIGS. 1 and 2 show the preferred ion source of the present invention. The
principle components of the ion source are displayed in the
cross-sectional view of FIG. 2. The system consists of a 17.8 cm inside
diameter brass cylinder walls 10 forming the microwave cavity 11. A
sliding short 12, with brushes 13 contacts cylinder 10 and the adjustable
excitation probe 14, provide the impedance tuning required to minimize
reflected power. The sliding short 12 can be moved back and forth along
the longitudinal axis of the cavity 11 to adjust its electrical length
while the radial penetration of the excitation probe 14 into the cylinder
walls 10 varies the cavity 11 mode coupling. A quartz dish 15 is shaped
like a petri dish and keeps the working gas in region 16 while allowing
the microwave power to produce a disk-like plasma adjacent to the
extraction grids 17a and 17b. The working gas is introduced into region 16
by means of an annular ring 18, supplied by gas feed tube 19. The ion
attracting means 17 are conventional 8 cm ion extraction grids 17a and 17b
which extract the ions from the microwave plasma. The first grid 17a (the
screen or microwave coupling grid) is electrically connected to the cavity
end plate 10a while the second grid 17b (accelerating grid) is placed
about 1 mm from the screen grid 17a. A heated tungsten filament
neutralizer 20 is placed downstream from the grids 17a and 17b to provide
the neutralizing electrons for the ion beam. A conduit 21 is provided as a
microwave inlet port to the cavity 11 for probe 14. The short 12 is
adjusted by means of rods 22 supported by plate 23 outside the cylinder
walls 10 controlled by threaded post 24 and nut 25.
The ion source minimizes the plasma volume by creating a thin disk-like
plasma adjacent to the extraction grids 17a and 17b. The ions in the disk
plasma and the grids 17a and 17b form part of a resonant microwave cavity.
The plasma is created by exciting the cavity in one of several modes. For
example, the TM.sub.010, TM.sub.011, TE.sub.111 and TE.sub.211 cavity
modes create and maintain a plasma disk. The field patterns for these
modes are displayed in FIGS. 3, 3a, 4, 4a, 5 and 5a. Note that the
TE.sub.111 and TE.sub.211 modes have fields that are tangential to the
flat disk plasma surface, while the TM modes produce a field that is
perpendicular to and a maximum at the flat disk surface. Thus, the TM
modes appear to be the most promising, efficient low pressure modes.
Associated Systems
Schematic diagrams of the vacuum/input gas flow and microwave/electrical
systems are shown in FIGS. 6 and 7 respectively. As shown in FIG. 6, the
cylinder walls 10 are positioned with the chamber 15 located on top of and
adjacent to a 45.7 cm radius by 45.7 cm cylindrical bell jar 100 by means
of a plexiglass plate 104. The jar 100 can pumped down to 10.sup.-6 Torr
by the diffusion pump 102 and roughing pump 103. A thermocouple 105,
pressure gauge 106 and ion gauge 107 were provided as is conventional. The
jar 100 was mounted on a metal tank 108. A gate valve 109 was provided for
the diffusion pump 102 in a known manner. Argon gas flow was regulated
from the input feed tank 200 with a pressure regulator 201 and control
valves 202 and was measured with a calibrated rotometer 203. Argon gas
entered the plasma disk in the chamber 15 through holes in the annular
ring 18. This ring 18 was connected to the feed rotometer 201 via copper
tubing 204 and a short section of quartz tubing 205 which allowed
electrical isolation from ground potential. Thermocouple pressure gauge
206 provided pressure measurement. Back pressure was measured with gauge
207. Argon gas flow rate from tank 200 and vacuum chamber pressure from
diffusion pump 102 and roughing pump 103 apparatus were not independent in
this system since argon gas was not bled into the vacuum system below the
ion source outside of chamber 15.
The microwave system, shown in FIG. 8, consisted of a continuously variable
0-150 W, CW, 2.45 GHz oscillator 300, three port circulator 301, matched
load 302, and directional coupler 303 and power meters 304 to measure
incident, P.sub.i, and reflected power P.sub.r. A radial choke 305 was
placed on the outside of the coaxial cavity 11 which provided the the dc
isolation from ground potential required for the application of grid 17b
voltages. As shown in FIG. 9, the dc biasing circuits consisted of the
usual screen 17a and accelerating grid 17b connected to dc power supplies
400 and 401. FIG. 10 shows a hot tungsten wire 501 and circuit 500 for the
neutralizer 20. In order to insure discharge stability at low pressures
the oscillator (300) may be frequency stabilized with a phase-locked loop
in a known manner.
To ignite the plasma it was necessary to bring the environmental pressure
in the bell jar 100 up to approximately 300 to 400 microns of Hg. The
incident microwave power was set in the neighborhood of 60 W. The cavity
11 was then tuned to a given cavity 11 mode by adjusting the short 12
length and the probe 14 depth until a match was attained, i.e., the
reflected power was zero. The plasma generally goes on when there is a
match, however, occasionally a Tesla coil was used to ignite the plasma.
To reach the operational pressures and flow rates for a given use the
plasma was ignited by the procedure outlined above; then the pressure was
reduced by cutting back on the flow rate from the tank 200. This was done
in steps so that the cavity 11 could be retuned to a match for the
particular cavity 11 mode and chamber 15 pressure. In this manner, the
environmental pressure in chamber 15 was lowered to the operating pressure
while maintaining a match between the microwave/cavity 11 system and the
microwave power system.
Results
Preliminary experiments were performed to develop an understanding of the
microwave ion source behavior. All experiments utilized a quartz disk
chamber 15 of 9.42 cm inside diameter and 5.1 mm depth forming a plasma of
equal diameter and depth of 1.47 cm. The results of these experiments are
summarized below:
1. Electromagnetic mode excitation. A microwave plasma disk was ignited and
maintained with the TE.sub.211 and TM.sub.011 modes. Both modes have
demonstrated matched (P.sub.r (reflected power) approximately equal to
0.05 P.sub.i (incident power)) operation in argon gas down to vacuum
chamber pressures of about 1.times.10.sup.-4 Torr and flow rates of about
20 sccm argon. In each of these modes, the plasma fills the disk 15 and is
in contact with the grids 17a and 17b at all pressure below 100 microns
when absorbed powers are 40-80 watts. At higher pressure or lower absorbed
power levels, the disk plasma partially fills the chamber 15 taking on
shapes that are related to the electromagnetic exciting fields.
Usually the plasma was ignited at a high flow rate and vacuum chamber
pressures of about 300 microns of mercury. After the ignition, the vacuum
system was gradually pumped by pumps 120 and 103 down to lower pressures
while small changes in cavity 11 length using short 12 and probe 14 tuning
were made to minimize the reflected power. Reflected power could always be
adjusted to less than 5% of the incident power. The lower pressure limit
of about 1.times.10.sup.-4 Torr for the system operation was determined by
experimental vacuum valve (v) sensitivity. Lower flow rates and pressures
can be obtained with new valves.
2. Electron density. Electron density measurements were not performed in
the present experimental system. However, the general operation of the
plasma vs. power is similar to that observed in other microwave
experiments (Mallavarpu, R., J. Asmussen, and M. C. Hawley, IEEE Trans. on
Plasma Science PS-6, 341 (December 1978); where density measurements were
performed. Thus it is expected that plasma electron densities in the
plasma disk are similar to those measured earlier i.e, the densities are
equal to or greater than 10.sup.11 /cm.sup.3 required for ion source
operation.
3. Coupling Efficiency and Loaded Cavity Q
Cavity 11 electromagnetic fields were measured by inserting small
diagnostic coaxial probes (not shown) into the cavity. The TE.sub.211 mode
fields were measured by inserting coaxial probes (not shown) into a number
of holes equally spaced along the cavity 11 axis and equally spaced along
the cylindrical cavity 11 azimuthal angle. These probes were connected to
a power meter (not shown) providing a measurement of the square of the
radial electric field adjacent to the cavity 11 walls 10. Measurements
were made with and without the plasma. A typical measured TE.sub.211 field
pattern with a plasma present is shown in FIG. 10.
The coaxial probes also allow the following measurements and calculations
to be performed. (1) the coupling efficiency to the discharge, (2) cavity
Q with the plasma present, (3) the power lost to the walls 10, and (4)
absorbed microwave power density <P>, in the plasma disk. Table 1 shows
measured and calculated characteristics for the TE.sub.211 mode together
with the important plasma/cavity definitions.
TABLE 1
______________________________________
Characteristics of the Plasma Cavity System:
TE.sub.211 mode, P.sub.t = 75 W, Q.sub.u = 1240,
chamber pressure 4.2 .times. 10.sup.-4 Torr, flow rate
26 sccm of Ar
Coupling
P.sub.t P.sub.c Eff Q.sub.u
<P>
______________________________________
75 W 13.5 W .82 223 0.6 W/cm.sup.3
______________________________________
Definitions
P.sub.t = total measured power absorbed in the plasma/cavity P.sub.i
-P.sub.r
P.sub.c = power lost in cavity walls
P.sub.p = P.sub.t - P.sub.c = power absorbed in the plasma volume
V = plasma volume
Q.sub.u = unloaded Q of the cavity system with the plasma present
Q.sub.uo = unloaded Q of the cavity system without the plasma
##STR1##
##STR2##
The measurements indicate that 13.5 W of the 75 W coupled into the
cavity 11 is lost in the conducting walls 10 and the quartz disk chamber
15 yielding a 82% coupling efficiency. Over sixty watts of power is
absorbed by the plasma resulting in a discharge power density of 0.6
W/cm.sup.3. Coupling efficiency can be improved 5-10% with lower loss
quartz materials and improved cavity construction.
4. I-V Characteristics and Efficiencies
Common measures of the efficiency of D.C. ion engines are the ion
production cost measured in W/(beam amp) and the mass utilization which is
a measure of the fraction of the incoming gas that is used in the ion
beam. The ion production cost will be given two ways. First by:
e.sub.t =P.sub.t /I.sub.g (1200 v),
where I.sub.g (1200 v) is the extrapolated grid current. Thus, e.sub.t is a
measure of the power going into the cavity 11 to produce the extrapolated
grid current at 1200 v. Secondly, by,
e.sub.p =p.sub.p /I.sub.g (1200 v).
This is a measure of the power going into the plasma to produce the
extrapolated grid current at 1200 v.
The mass utilization efficiency is given by
##EQU1##
where f is the flow rate of the argon gas in units of sccm Ar.
The extrapolated grid 17b currents at 1200 v were obtained by measuring the
I-V characteristics of the ion source out to a screen grid 17a voltage of
80 v or more where the I-V curve saturates with a constant slope. The
measurement of the I-V characteristics were taken by increasing the screen
grid 17a voltage while holding the grid 17b voltage at zero (see FIG. 10)
and measuring the ion current to the acceleration grid 17b. Typical I-V
characteristics for this ion source are shown in FIG. 11 for operation in
both the TE.sub.211 and TM | | |
