Production of large resonant plasma volumes in microwave electron cyclotron resonance ion sources

Microwave injection methods for enhancing the performance of existing electron cyclotron resonance (ECR) ion sources. The methods are based on the use of high-power diverse frequency microwaves, including variable-frequency, multiple-discrete-frequency, and broadband microwaves. The methods effect large resonant "volume" ECR regions in the ion sources. The creation of these large ECR plasma volumes permits coupling of more microwave power into the plasma, resulting in the heating of a much larger electron population to higher energies, the effect of which is to produce higher charge state distributions and much higher intensities within a particular charge state than possible in present ECR ion sources.

FIELD OF THE INVENTION 
The present invention relates to the field of microwave electron cyclotron 
resonance (ECR) ion sources utilizing a B-minimum magnetic mirror 
confinement geometry. More specifically, it relates to the use of 
microwave radiation in a variety of forms to increase the physical sizes 
of the ECR zones in such sources. 
BACKGROUND OF THE INVENTION 
A conventional single-frequency electron cyclotron resonance (ECR) ion 
source is shown in FIG. 1. These ion sources have played major roles in 
the advancement of accelerator technology since their inception. This is 
because of their capability for generating useful high-intensity, 
high-charge state ion beams required for many applications. Such ion 
sources offer a number of major advantages over more conventional 
hot-cathode ion sources including long source lifetime due to the 
nonfilamentary cathode structure, operational stability when chemically 
reactive feed materials are used, and stable operation over a wide 
dynamical pressure range. 
ECR ion sources are designed in accordance with the well-known B-minimum 
magnetic field confinement principle. In the representative ion source 10 
illustrated in FIG. 1, a multimode cavity 11 serves as the plasma 
confinement vessel. A single-frequency microwave source 17 injects 
microwave power through a microwave input waveguide 12 into the cavity 11. 
The feed material of interest is introduced in gaseous or solid form 
through a feed port 13. Solenoids 8 and 15 act in concert to provide a 
solenoidal magnetic field for confining the plasma 19 in the axial 
direction, and a multipole magnet structure 16 provides a multicusp 
magnetic field for confining the plasma 19 in the radial direction. The 
two magnetic field distributions are designed so as to effect a minimum in 
the magnetic field (the B-minimum geometry) as required for optimum plasma 
confinement. A high potential difference maintained between the source 10 
and an electrode system 14 draws the high charge state ion beam 9 from the 
ion source. The microwave radiation used to generate and maintain the 
plasma in these sources is typically injected from a single-frequency 
microwave power supply with a frequency in the range of 2.45 to 14 GHz. 
The bandwidths of these power supplies are usually quite narrow, typically 
20 MHz. Since ion-neutral collisional recombination processes tend to 
lower the charge state distribution at higher operating pressures, low 
pressures of about 10.sup.-6 Torr are typically maintained in the plasma 
chamber 11 of the source. 
Electron cyclotron resonance ion sources as shown in FIG. 1 are 
particularly characterized by the production of a thin three-dimensional 
electron heating region 18 within the plasma 19 where electrons in the 
region are accelerated to high energies and then ionize neutral atoms or 
other ions in the plasma during collisions. The region 18 is usually a 
thin, fluted, ellipsoidal "surface" which intersects the axis of symmetry 
of the ion source 10 at two positions, as illustrated in FIG. 1. The ECR 
region 18 is known by various names including electron cyclotron resonance 
(ECR) zone, resonant plasma volume, ECR heating zone, ECR surface, etc. 
The number density of electrons, the electron energy (temperature), and 
energy distribution of the electron population are three of the 
fundamental properties which govern the performance of ECR ion sources in 
terms of degree of ionization, ion beam intensity, and multiple ionization 
capabilities. The maximum electron temperature is affected by several 
processes including the ability of the plasma to adsorb microwaves, the 
time required to produce "hot" (or energetic) electrons, the time for 
thermalization of the "hot" electrons and the ability of the source to 
confine the "hot" electrons. The magnetic field geometry and magnetic 
field strength determine the confinement attributes of the ECR ion source. 
Data accumulated over the years clearly indicate that sources with higher 
confinement fields generate higher charge state ion beams. 
The ECR zone or zones in any ECR source are limited to regions of the 
ionization volume where the magnetic field meets the resonance condition, 
given by: 
EQU .omega..sub.cc =Be/m=.omega..sub.rf ( 1) 
where .omega..sub.cc is the electron-cyclotron resonant frequency, 
.omega..sub.rf is the resonant frequency of the microwave source, B is the 
resonant magnetic field strength, e is the electron charge, and m is the 
mass of the electron. Whenever the microwave frequency is tuned to the 
electron-cyclotron frequency, electrons are resonantly excited and thereby 
given sufficient energy to cause ionization within an evacuated volume. At 
low collision frequencies (low ambient pressures), some of the electrons 
are stochastically heated to very high energies which are capable of 
removing tightly bound electrons, and therefore are responsible for 
producing multiply charged ions. 
For a particular microwave frequency, microwave power can no longer be 
coupled into the plasma whenever the plasma density reaches a certain 
value, referred to as the critical density n.sub.c. The critical density 
n.sub.c occurs whenever the microwave frequency .omega..sub.p is equal to 
the plasma frequency .omega..sub.rf or .omega..sub.rf =.omega..sub.p. The 
relation between the critical density n.sub.c and the plasma frequency 
.omega..sub.p is given by the following expression: 
EQU n.sub.c =.omega..sub.p.sup.2 .epsilon..sub.0 m/e.sup.2 ( 2) 
where .epsilon..sub.0 is the permittivity of free space. 
Since the ECR condition is met whenever .omega..sub.cc =.omega..sub.rf (Eq. 
1), the critical density increases quadratically with the resonant 
magnetic field strength B. For example, the critical density for 2.45 GHz 
is 7.45.times.10.sup.10 /cm.sup.3 while the critical density for a 14 GHz 
excitation frequency is 2.43.times.10.sup.12 /cm.sup.3. Under certain 
conditions, right-hand circularly polarized electromagnetic waves can 
propagate in a magnetized plasma in the so-called whistler mode, even if 
the plasma density is above the critical density n.sub.c, provided that 
the microwave frequency is less than the corresponding electron cyclotron 
frequency and that the waves propagate along the direction of the magnetic 
field. 
The 2.45 GHz ECR microwave source requires a magnetic field strength of 875 
Gauss to meet resonance conditions, while the 14 GHz source requires 5000 
Gauss fields for resonance to occur. Thus, while low frequency microwave 
sources have lower cutoff densities and require physically larger plasma 
chambers for coupling the microwave power, they benefit in terms of 
emittance degradation by the lower magnetic fields in the extraction 
region of the ECR ion source. High frequency microwave sources, on the 
other hand, have the decided benefit of higher cutoff densities and small 
diameter plasma chambers, but suffer in terms of emittance degradation as 
a consequence of the strong magnetic fields that exist in the extraction 
region of the ECR ion source. The emittance is degraded in direct 
proportion to the magnitude of the field strength in the extraction region 
of the ECR ion source. 
In conventional single frequency ECR ion sources (FIG. 1), the shapes, 
physical sizes, and locations of the ECR zones 18 are determined by the 
frequency and bandwidth of the microwave power and the magnitude and 
distribution of the magnetic field which meets the ECR condition (Eq. 1). 
The magnetic field distribution used in traditional ECR ion sources 10 
confines the plasma effectively, but severely restricts the physical sizes 
of the ECR or "hot" zones in relation to the total size of the ionization 
volume. Because these ECR "surfaces" are small in relation to the physical 
size of the ionization chamber, the ECR zones constitute a small fraction 
of the ionization volume. Thus, the absorptivity of microwaves by the 
plasma is determined not by the physical size of the plasma volume but by 
the size of the ECR zone 18 in the ion source 10. Electrons can only be 
accelerated in the ECR zone; those which leave the ECR zone through 
scattering have a reduced probability for further stochastic acceleration 
because of the reduced probability of returning to the zone, and therefore 
the probability for further acceleration is reduced. Traditional ECR ion 
sources 10 with small ECR zones may be more susceptible to these 
scattering effects which may limit the high energy electron population and 
thereby limit the ionization rate of the ion source. The remedy of the 
problem may be found by increasing the relative size of the ECR zone. The 
creation of a large ECR plasma "volume" would permit coupling of more 
power into the plasma, resulting in the heating of a much larger electron 
population to higher energies. As a consequence, the performance of the 
ECR ion source may be enhanced in terms of charge state and intensity 
within a charge state over conventional ECR ion sources. 
The principal processes which limit high-charge-state ion production in the 
ECR ion source are through charge exchange, wall recombination, ion 
residence time in the plasma, the bombarding electron current, the 
confinement time of the hot electrons, and the electron temperature. 
Charge-exchange collisions between ions and neutral atoms reduce the 
degree of ionization and high-charge-state population of the ion source. 
High-charge-state particles, ionized in the ECR zones of the ion source, 
must necessarily pass through extended regions of unheated plasma before 
extraction. As a necessary consequence of the requirement of neutrality in 
the plasma, there is a dynamical charge balance between electron and ion 
loss processes. Most of the ions recombine at the radial walls of the 
vacuum chamber and re-enter the plasma as neutrals while most of the 
electrons are lost at the ends of the plasma chamber. 
Because of the thin ECR surfaces in conventional ECR ion sources, the 
probability for ionizing a neutral atom during passage through the ECR 
zone and re-entry into the interior of the plasma volume is believed to be 
low. Therefore, the population of neutrals is postulated to be greater in 
the interior of the source than if the ECR zone was of sufficient 
thickness to ionize the particles with high probability during passage to 
the interior of the plasma volume. As a consequence, the average charge 
state of the ion distribution in the plasma would be lowered through 
charge exchange collisions between the neutrals and multiply-charged ions 
within the plasma. The ability to quickly ionize a large fraction of the 
neutral population that results from recombination of the multiply-charged 
ions which strike the walls of the vacuum chamber effectively reduces the 
rate of resonant charge exchange. This increases the residence time of an 
ion in a given charge state, and thereby increases the probability for 
subsequent and further ionization. If the colliding partners are 
positively ionized, the long-range forces and relatively low energies 
reduce the likelihood of charge transfer in these collisions. The ability 
to eliminate or drastically reduce the charge exchange process increases 
the lifetime of a charged particle within a particular charge state, thus 
increasing the probability of further ionization. Therefore, the advantage 
of having a thick ECR zone between the walls of the chamber and the 
interior of the plasma where the multiply charged ions are extracted will 
improve reionization efficiency of neutrals returning from the walls, 
thereby reducing charge exchange recombination processes within the 
central plasma region of the ion source. 
Large ECR volumes result in significantly greater interaction of the 
microwaves with the plasma electrons, both in terms of total power 
absorptivity and in a more uniform spatial distribution of the 
absorptivity. The presence of a large ECR zone as well as the additional 
probability of accelerating larger electron populations to higher average 
energies increases the charge state distributions and ion beam intensities 
within a particular charge state. 
Prior attempts to increase the size of the resonant plasma volume in an ECR 
ion source have been by 1) tailoring the magnetic field and 2) by heating 
with two microwave frequencies. Z. Q. Xie and C. M. Lyneis used 10 GHz and 
14 GHz microwave frequencies simultaneously to excite the plasma in their 
Advanced ECR (AECR) ion source at the Lawrence Berkeley Laboratory (see 
"Improvements on the LBL AECR Source", Z. Q. Xie and C. M. Lyneis, 
Proceedings of Twelfth International Workshop on ECR Ion Sources, edited 
by M. Sekiguchi and T. Nakagawa, The Institute of Physical and Chemical 
Research (RIKEN) Apr. 25-27, 1995, Wakoshi, Japan, INS-J-1821995, p. 
24-28.). This resulted in moving the charge states to higher values by 3 
to 4 units for bismuth and uranium. The Xie and Lyneis approach is 
illustrated in FIG. 1 by the additional single-frequency microwave source 
17' and waveguide 12'. The waveguide 12' is shown as a different size than 
waveguide 12 to illustrate that the two rectangular waveguides 12 and 12' 
are oriented 90 degrees with respect to each other. When operated with the 
two different frequencies, the ECR interaction surface areas were 
increased by about a factor of two. As a consequence, the absorptivity of 
microwave power by the plasma was also increased, making more electrons 
available for acceleration by the respective RF fields. Electrons which 
scatter out of a particular ECR zone 18 and cross into the second zone 18' 
can also be further accelerated. The outer (14 GHz) surface may serve to 
ionize neutrals which result during charged particle recombination at the 
walls of the chamber and thus reduce the population of neutrals which 
would otherwise lower the charge state distribution created in the 
interior region of the ion source by the action of the 10 GHz ECR surface. 
The results of Xie and Lyneis' experiments with the AECR serve to point 
out the importance of the physical sizes of the ECR zones on the 
performance of ECR ion sources. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a system for increasing 
the physical sizes of ECR zones in conventional B-minimum ECR sources by 
the utilization of diverse frequency microwave power to achieve large ECR 
volumes within the ECR ion sources rather than by reliance on magnetic 
field improvements. 
Another object of the present invention is to enhance the performance of 
existing ECR ion sources by generating higher charge state heavy-ion beams 
with higher intensities within a particular charge state compared to 
present single- or two-frequency ECR ion sources. 
In accordance with a first preferred embodiment of the present invention, 
there is provided an electron cyclotron resonance (ECR) ion source for 
producing high charge state high-intensity ion beams, the ion source 
having a B-minimum magnetic field for confining a plasma, and wherein the 
plasma is excited by microwaves from a microwave source inputted to the 
ion source through a waveguide, and also wherein the ion source is 
operable to produce an electron cyclotron resonance zone of heated plasma 
electrons within the plasma by absorption of microwaves from the microwave 
source; the improvement wherein the microwave source is a diverse 
frequency microwave source for increasing the physical size of the 
electron cyclotron resonance zone within the plasma, and the waveguide is 
a broadband waveguide having a bandwidth chosen to fall within the 
resonant frequency distribution of the magnetic field of the ion source. 
In accordance with another preferred embodiment of the present invention, 
the diverse frequency microwave source comprises a plurality of signal 
generator and microwave amplifier pairs, each of the signal generator and 
microwave amplifier pairs operable to produce a discrete microwave 
frequency and inject same into the broadband waveguide. 
In accordance with another preferred embodiment of the present invention, 
the diverse frequency microwave source comprises a microwave amplifier and 
a variable frequency signal generator; the microwave amplifier operable to 
produce microwaves and inject them into the broadband waveguide; and the 
variable frequency signal generator operable to input a continuously 
varying frequency signal to the microwave amplifier such that a large 
volume ECR zone is produced within the plasma. 
In accordance with another first preferred embodiment of the present 
invention, the diverse frequency microwave source comprises a microwave 
amplifier and a broadband signal generator, the microwave amplifier 
operable to produce microwaves and inject them into the broadband 
waveguide; and the broadband signal generator operable to input a 
broadband frequency signal to the microwave amplifier such that microwaves 
produced by the microwave amplifier are distributed across the bandwidth 
of the waveguide. 
In accordance with yet another preferred embodiment of the present 
invention, the diverse frequency microwave source comprises a plasma wave 
tube (PWT) amplifier, the plasma wave tube amplifier operable to produce 
microwaves over a wide bandwidth and inject them into the broadband 
waveguide such that a large volume ECR zone is produced within the plasma 
resulting in an ion beam with higher charge state ions and higher beam 
intensities within a particular charge state from the ECR ion source.

DETAILED DESCRIPTION OF THE INVENTION 
The ECR ion source improvements described hereinbelow are based on the use 
of high-power diverse frequency microwaves, especially variable-frequency, 
multiple-discrete-frequency, and broadband microwaves derived from 
traveling wave tube (TWT), magnetron, klystron, gyrotron, or plasma wave 
tube (PWT) technologies, to create large ECR "volumes" in conventional 
B-minimum ECR ion sources 10. 
EMBODIMENT 1 
Multiple Discrete Frequencies 
FIG. 2 illustrates a first preferred embodiment of the present invention 
where the ECR plasma volume is increased by exciting the plasma with 
microwave power at multiple discrete frequencies. For this embodiment, a 
plurality of microwave frequencies from individual single-frequency signal 
generators 22, 22', . . . 22.sup.n etc., are inputted to respective 
microwave amplifiers 21, 21', . . . 21.sup.n etc. For example, multiple 
discrete frequencies of 10, 11, 12, 13 and 14 GHz may be selected to fill 
the bandwidth of the broadband microwave input waveguide 27. Whatever the 
set of frequencies used, they are chosen to fit into the resonance 
frequency distribution of the particular ECR ion source 10. This method 
can be readily effected by using traveling wave tube (TWT) amplifiers or 
multiple single frequency amplifiers to provide the microwave power for 
exciting the plasma. 
Other sources of microwave radiation including magnetron, klystron, or 
gyrotron sources could be used just as well for the microwave amplifiers 
21, 21', etc. Multiple klystron, magnetron, gyrotron, or combinations of 
these power sources could also be utilized to achieve the desired larger 
ECR zones within a given ECR ion source. 
The operation of a traveling wave tube is based on the transfer of energy 
between an electron beam and an RF wave. The transfer can only be 
efficient if the electron beam and RF wave are traveling at about the same 
velocity. Since the microwave travels at a velocity about 100 times that 
of the electron in free space, a helical structure is used in the TWT to 
slow the wave down so that the velocities are about the same. By directing 
the electron beam along the axis of the helix, the time-varying electric 
field on the helix causes the electron beam energy to vary according to 
the electric field strength. The resulting velocity modulation causes the 
electron beam to form bunches, and as the bunches move through the helix 
their sizes grow. The helix senses a time-varying electric field from the 
electron bunches which induces an RF wave onto the helix of the same 
frequency as the initial RF wave, but greatly amplified. Power gains up to 
70 db (10,000) can be achieved. A single TWT can deliver several hundred 
watts of RF power. 
In more detail of the multiple, discrete frequency method shown in FIG. 2, 
the signal generators are provided with several discrete frequencies 
chosen to fill the bandwidth of the broadband waveguide 27. For example, 
the bandwidth of a WR90 waveguide is designed to transmit a frequency 
spectrum of 8-12 GHz. A logical choice might be to chose discrete 
frequencies of 8, 9, 10, 11 and 12 GHz for amplification by the TWT 
amplifiers 21, 21', etc. As a consequence, the performance of the ion 
source is improved over that of the single- or two-frequency ECR ion 
source. 
EMBODIMENTS 2 and 3 
Variable Frequency 
In a second preferred embodiment of the invention shown in FIG. 3, a 
variable frequency signal generator 32 is used in combination with a 
microwave amplifier 31. The signal generator 32 generates a continuously 
varying signal which sweeps over a bandwidth matching the bandwidth of the 
waveguide 37 and the frequency distribution within the B-minimum region of 
the magnetic field. The frequency is swept through a selected frequency 
range at a sweep period comparable to or preferably less than the lifetime 
of the ions within the plasma, (less than 1 ms, for example). Since the 
sweep rate is less than the estimated confinement time (a few 
milliseconds) of low charge state particles in an ECR ion source, 
electrons in the ECR region are accelerated to high energies during the 
sweep cycles. 
Further improvements in the ion source performance may be realized by using 
two continuously varying frequencies, phased so that one signal begins its 
sweep from the low side toward the high side of the band and return; while 
another signal is synchronized to begin its sweep simultaneously with the 
former, beginning at the high side of the frequency spectrum and 
decreasing toward the low side and return. Microwave amplifiers 31 and 33, 
together with their respective variable frequency signal generators 32 and 
34, accomplish this. 
In a third preferred embodiment of the invention, four or more continuously 
varying signals may be utilized. For example, if four signal generators 
are used, two of the signals would be phased 90 degrees with respect to 
each other, both increasing toward the high frequency side of the sweep 
range and returning. The remaining two frequencies would be phased 90 
degrees with respect to each other and synchronized with the first two 
beginning at the high side of the frequency range and decreasing toward 
the lower side and return. The effect of the above-described variable 
frequency alternatives is to keep the plasma more uniformly ionized and 
homogeneously distributed within the ECR plasma volume, thus reducing 
charge state lowering processes through charge exchange. As a consequence, 
the performance of the ion source is improved over that of the single 
frequency ECR ion source. As is known in the art of TWT's, the signals 
from the signal generators 32, 34 may be enhanced in magnitude with 
voltage controlled pre-amplifiers (now shown) before final amplification 
by the TWT amplifiers 31, 33. 
EMBODIMENT 4 
Broadband Frequency 
In a fourth preferred embodiment of the invention, shown in FIG. 4, a 
broadband signal generator 36 provides input to a microwave amplifier 35. 
The output of microwave amplifier 35 is routed to the vacuum vessel 11 by 
the broadband waveguide 37. For the broadband method, the signal generator 
36 is chosen to be a "noise" generator with a bandwidth compatible to that 
of the broadband waveguide 37 and the ECR frequency distribution of the 
B-minimum magnetic field of the ECR ion source 10. As in the other 
embodiments of the invention, the broadband frequency signal may be 
pre-amplified by a voltage controlled pre-amplifier (not shown) before 
final amplification by the TWT amplifier 35 to enhance the power delivered 
to the B-minimum ECR ion source 10 through the broadband waveguide 37. The 
broadband power keeps the plasma more uniformly ionized and homogeneously 
distributed within the broadened ECR plasma volume 33, thus reducing 
charge-state-lowering processes through charge exchange. As a consequence, 
the performance of the ion source is improved over that of the single- or 
two-frequency ECR ion source. 
EMBODIMENT 5 
Broadband Frequency with a PWT 
A fifth preferred embodiment of the invention, shown in FIG. 5, utilizes 
short-pulse, ultra-broadband RF power generated directly by a plasma wave 
tube, or PWT, 41. The PWT utilizes the interaction between an electron 
beam and a time-varying plasma to generate kilowatt levels (.about.10 kW) 
of power at microwave to millimeter-wave frequencies. The electron beam 
from the PWT 41 first ionizes a feed gas to form the plasma 19, and then 
nonlinearly interacts with the plasma 19 to generate broadband power, for 
example, from 6 to 60 GHz. Slew rates of up to 7 GHz/ms have been measured 
during a single beam pulse. The RF power has a wide instantaneous 
bandwidth, typically 10 GHz or wider. 
The various embodiments of the present invention described hereinabove for 
increasing the sizes of the resonant zones in existing ECR ion sources 
offer the potential of a cost-effective means for enhancing the 
performances of these ECR sources. The presence of large ECR zones as well 
as the additional effect of accelerating much larger electron populations 
to much higher average lo energies causes ECR ion sources to produce 
higher charge states and higher ion beam intensities within a particular 
charge state. Large ECR volumes result in significantly greater 
interaction of the microwave radiation with the plasma electrons, both in 
terms of total power absorptivity and in a more uniform spatial 
distribution of the absorptivity. Since the ECR zones are nearer the axis 
in all of these frequency domain sources, the high charge state population 
is born closer to the axis of extraction and therefore can be extracted 
more efficiently. 
Perhaps the most practical aspect of the diverse frequency methods of my 
invention is that they can be utilized to relatively inexpensively 
transform present "surface" ECR ion sources into "volume" ECR ion sources. 
By use of these methods, traditional ECR ion sources that are based on 
single frequency microwave heating can be converted from resonant 
"surface" ion sources to resonant "volume" ion sources, thus permitting 
the coupling of more microwave power into the plasma resulting in the 
heating of a much larger electron population to higher energies than 
presently possible in ECR ion sources. 
While several preferred embodiments of the improved ECR ion source have 
been shown and described, it will be understood that such descriptions are 
not intended to limit the disclosure, but rather it is intended to cover 
all modifications and alternate methods falling within the spirit and 
scope of the invention as defined in the appended claims or their 
equivalents.