U.S. patent number 3,778,656 [Application Number 05/275,304] was granted by the patent office on 1973-12-11 for ion source employing a microwave resonant cavity.
This patent grant is currently assigned to Commissariat A L'Energie Atomique. Invention is credited to Charles Fremiot, Jean Grando.
United States Patent |
3,778,656 |
Fremiot , et al. |
December 11, 1973 |
ION SOURCE EMPLOYING A MICROWAVE RESONANT CAVITY
Abstract
The ion source comprises a plasma source employing a microwave
cavity which is excited in one of its modes of resonance and a
static magnetic field adjusted to the electron cyclotron resonance,
means for extracting the ions from the plasma and comprising an
expansion cup pierced by an aperture which communicates with the
cavity and ion-extraction electrodes brought to suitable
potentials. The density of the plasma is of maximum value in the
vicinity of the aperture and the static magnetic field is
substantially zero in the vicinity of the extraction
electrodes.
Inventors: |
Fremiot; Charles (Grenoble,
FR), Grando; Jean (Sassenage, FR) |
Assignee: |
Commissariat A L'Energie
Atomique (Paris, FR)
|
Family
ID: |
9081130 |
Appl.
No.: |
05/275,304 |
Filed: |
July 26, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Jul 29, 1971 [FR] |
|
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7127812 |
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Current U.S.
Class: |
313/363.1 |
Current CPC
Class: |
H01J
27/18 (20130101) |
Current International
Class: |
H01J
27/16 (20060101); H01J 27/18 (20060101); H05h
013/00 () |
Field of
Search: |
;313/63 ;315/111 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Hostetter; Darwin R.
Claims
What we claim is :
1. An ion source comprising :
a plasma source employing a microwave cavity which is excited
according to one of its modes of resonance and a static magnetic
field which is adjusted to the electron cyclotron resonance,
means for extracting the ions from said plasma, comprising an
expansion cup pierced by an aperture which communicates with said
cavity and ion-extraction electrodes brought to suitable
potentials,
wherein the density of said plasma is of maximum value in the
vicinity of said aperture and wherein said static magnetic field is
of substantially zero value in the vicinity of said extraction
electrodes.
2. An ion source according to claim 1, wherein said static magnetic
field assumes the value corresponding to cyclotron resonance only
in the vicinity of said expansion cup aperture.
3. An ion source according to claim 1, wherein a high static
magnetic field gradient exists in the vicinity of said
aperture.
4. An ion source according to claim 3, wherein said expansion cup
is fabricated from a metal having low reluctance and constitutes a
magnetic shield for said static magnetic field.
5. An ion source according to claim 4, wherein said expansion cup
is of soft iron.
6. An ion source according to claim 1, wherein said microwave
cavity is a cylinder of revolution, said mode of resonance is
transverse-electric, said static magnetic field is axial and
wherein the plasma is confined within a leaktight dielectric
cylinder which is coaxial with said cavity and has a small radius
compared with the radius of said cavity, said expansion cup being
disposed in the axis of said dielectric cylinder.
7. An ion source according to claim 6, wherein the mode of
resonance of the cavity is the TE.sub.111 mode.
8. An ion source according to claim 6, wherein said mode of
resonance is a mode in which the electric field has zero amplitude
in certain internal zones of the cavity, the dielectric cylinder
for the injection of gas being located in the vicinity of said
zones.
9. An ion source according to claim 8, wherein the cavity is
excited in accordance with the TE.sub.022 mode.
10. An ion source according to claim 8, wherein the cavity is
excited in accordance with the TE.sub.122 mode.
11. An ion source according to claim 1, wherein said microwave
cavity is formed of a cylindrical cavity along the axis of which is
disposed a hollow metallic cylinder having low reluctance which is
in contact with the top wall of said cylindrical cavity and opens
into said cavity in the proximity of said expansion cup aperture,
said cylinder being part of the magnetic circuit which guides said
static magnetic field.
Description
This invention relates to an ion source which makes use of a
microwave resonant cavity and finds an application in the equipment
of particle accelerators.
Ion sources for accelerators must have a certain number of
qualities and among these can be mentioned:
FLEXIBILITY OF ADJUSTMENTS BOTH IN ENERGY AND IN INTENSITY,
REPRODUCIBILITY OF RESULTS,
A LONG LIFETIME,
THE POSSIBILITY OF OBTAINING HIGH ION DENSITIES.
Some of the qualities just mentioned are combined in ion sources of
known types but there is no source at the present time which
exhibits all these properties together. Thus, sources of the
"duoplasmatron" type do not have a very long lifetime by reason of
the presence of electrodes which are placed in the plasma, the
lifetime of these electrodes -- in particular the cathode -- being
limited as a result of the intense bombardment to which they are
subjected.
High-frequency discharge sources operating between 20 and 100
Mc/sec avoid this major obstacle but the electron density of the
plasma which is created remains of low value.
Plasma sources of the type designed for the use of a cavity which
is resonant at microwave frequencies are also known. In some of
these devices, the interaction between microwave and plasma is
considerably increased by the application of a static magnetic
field such that the conditions of electron cyclotron resonance are
satisfied.
These plasma sources of the microwave cavity and cyclotron
resonance type result in the creation of plasmas having high
density but are not equipped with adequate ion-extraction
means.
It is the precise object of the present invention to provide an ion
source which employs on the one hand a plasma source having
microwave-frequency excitation at cyclotron resonance and, on the
other hand, extraction means of the type employed in the
duoplasmatron but specially adapted to the plasma source employed.
The ion source obtained in accordance with the invention
accordingly has all the properties noted in the foregoing and thus
represents a substantial technical improvement upon known
sources.
More precisely, the invention is concerned with an ion source
comprising:
A plasma source employing a microwave cavity which is excited
according to one of it modes of resonance and a static magnetic
field which is adjusted to the electron cyclotron resonance,
Means for extracting the ions from said plasma, comprising an
expansion cup pierced by an aperture which communicates with said
cavity and ion-extraction electrodes which are brought to suitable
potentials,
Wherein the density of said plasma is of maximum value in the
vicinity of said aperture, and wherein said static magnetic field
is of substantially zero value in the vicinity of said extraction
electrodes.
In a preferred embodiment, said static magnetic field assumes the
value corresponding to cyclotron resonance only in the vicinity of
said expansion cup aperture.
In another advantageous embodiment, a high static magnetic field
gradient exists in the vicinity of said aperture.
Finally, in another alternative embodiment, the expansion cup has a
low reluctance and constitutes a shield for the static magnetic
field.
Further properties and advantages of the invention will become
apparent from the following description of two exemplified
embodiments which are given by way of explanation and not in any
limiting sense, reference being made to the accompanying drawings,
wherein :
FIG. 1 is a diagram of a microwave circuit for the excitation of
the ion source ;
FIG. 2 illustrates an ion source comprising a cylindrical cavity of
revolution in which the plasma is confined within a dielectric
cylinder ;
FIG. 3 shows the curves representing the relationship between the
height and the radius of the cylindrical cavity in respect of
different modes of excitation ;
FIG. 4 shows the lines of electric force and the distribution of
the amplitude of the field for the TE.sub.111 and TE.sub.122 modes
;
FIG. 5 describes the TE.sub.011 and TE.sub.022 modes ;
FIG. 6 shows an alternative embodiment of the invention employing a
truncated coaxial cavity which is excited in the TEM mode.
In FIG. 1, a microwave generator 1 excites an ion source 2 in
accordance with the invention by means of a microwave circuit
comprising a variable attenuator 3, a directional coupler 4, a
matching element 5, a piston 6, a coaxial line 7 and an excitation
antenna 9.
Pumping means (not illustrated) serve to create a vacuum within the
cavity by means of the duct 21.
The operation of said microwave-frequency excitation circuit is as
follows :
The generator 1 contains a microwave-frequency source such as a
magnetron, for example, which produces the electromagnetic field.
The attenuator 3 serves to vary the power of the wave which is
injected into the system. The positions of the piston 6 and of the
matching element 5 are adjusted so as to have the best possible
matching of the cavity of the source 2. Mismatch is measured by
means of the directional coupler 4 which draws-off a part of the
wave reflected by the cavity towards the generator.
The ion source 2 is described in detail in FIG. 2 in one example of
construction. In this figure, a cylinder 11 forms the side wall of
the microwave cavity, said cavity being excited by a coaxial line 7
which is terminated by an antenna 9. A tuning screw 12 serves to
vary the resonant frequencies of the cavity to a slight extent. An
orifice 13 serves to inject the gas to be ionized into a leak-tight
dielectric cylinder 14 which is coaxial with the cylinder 11.
O-ring seals 15 and 16 ensure leak-tightness of the internal
enclosure of the cylinder with respect to the remainder of the
cylinder 11. A cap 42 closes the cavity at the top end and an
expansion cup 17 which is screwed onto the body of the cylinder 11
completes the resonant cavity 50. Said cup defines an expansion
chamber 40 which communicates with the cylinder 14 through the
aperture 41. Only one of the extraction electrodes is illustrated ;
this electrode 18 is negatively polarized with respect to the
expansion cup 17 which can be connected to ground through the
voltage source S. Coils 19 and 20 which are supplied with direct
current produce a static magnetic field which is parallel to the
axis of revolution of the cavity cylinder 11 ; the complete
assembly is mounted on the cylinder 21 ; leak-tightness with
respect to the exterior is ensured by the O-ring seal 22.
The operation of said device is as follows :
The microwave-frequency excitation field propagates within the
coaxial line 7 and excites the resonant cavity 50 by means of the
antenna 9. The shape and position of said antenna are conducive to
excitation of the cavity in a transverse electric mode --
conventionally referred-to below as the TE mode -- characterized in
that the electric field is in a transverse sectional plane of the
cavity. The choice and the nature of the resonance modes will be
explained in detail hereinafter.
The gas to be ionized is injected through the opening 13 ; in
accordance with this alternative embodiment of the invention, the
gas penetrates into the interior of the dielectric cylinder 14 and
is partially ionized under the action of the microwave electric
field in conjunction with the static magnetic field, the amplitude
of which is so adjusted that the cyclotron frequency is in the
vicinity of the frequency of the microwave field. The coils 19 and
20 do not necessarily carry identical currents ; in an advantageous
variant, the coils carry very different currents such that the
static magnetic field corresponds to the cyclotron resonance only
in the lower zone of the cylinder 14 in the vicinity of the
aperture 41 of the expansion cup 17. This aperture is thus located
in the immediate proximity of the zone in which the plasma density
is of maximum value, thereby promoting diffusion of said plasma
within the expansion chamber.
Moreover and in accordance with a further property of the ion
source of the invention, the static magnetic field produced by the
coils 19 and 20 is very weak and even zero in the zone located near
the extraction electrode 18 in order to prevent said field from
disturbing the extraction of ions. This is possible by fabricating
the cup 17 from a metal having low reluctance such as soft iron,
for example, in order to form a magnetic shield. The sudden drop in
amplitude of the static magnetic field across the aperture 41
therefore results in a high magnetic-field gradient at this point,
thereby accelerating the plasma and facilitating the expansion of
this latter within the chamber 40.
This expansion is accompanied by a reduction in density of the
plasma and therefore by a reduction in the space charge which
usually limits the ion currents which can be extracted. The
electrode 18 which is brought to a highly negative potential makes
it possible in accordance with a conventional method to effect a
separation between the electrons and the ions. The shape and
potential of said electrode are such that the ions are guided
towards the enclosure formed by the tube 21, a high vacuum being
maintained by means of pumps (not shown) within said enclosure.
The dimensions of the cylindrical cavity of revolution can be
determined as follows :
In the alternative embodiment of FIG. 2, the magnetic field is
parallel to the axis of the cavity. In order to derive benefit from
the cyclotron resonance which is due to the presence of said field,
it is necessary to ensure that the microwave electric field
contained in the cavity has a component at right angles to the
magnetic field. There is therefore every advantage to be gained by
promoting the transverse-electric (TE) modes of resonance which are
such that the electric field is contained in the transverse section
plane. Strictly speaking, the existence of a component of the
electric field which is parallel to the axis of the cavity as would
be the case for a transverse-magnetic (TM) mode would not make the
system inoperative but would only limit its efficiency. It is
therefore endeavored to excite the cavity in a TE mode which will
be designated more fully and with the conventional notations by
TE.sub.mnp, wherein the indices m, n, p refer to the field
distribution along the usual cylindrical coordiantes .theta., r, z,
the z-axis being the axis of the cavity.
In order to avoid dimensions which are too large, the choice of
indices m, n and p is limited to the low values 0, 1 or 2. For a
given frequency of the generator, the height h of the cavity is
related to its radius a by relationships which are conventional in
the theory of microwave resonant cavities. In FIG. 3, these
variations are shown in the case of four modes and at a frequency
of 10 gigacycles. Each of these curves has a vertical asymptote
which defines a minimum radius a.sub.min.. By choosing for the
radius of the cavity a value in the vicinity of a.sub.min., the
upper modes will not be excited. In particular, by choosing for a a
value which is close to the lowest value of a.sub.min. (namely that
of the TE.sub.111 mode which has a value of 8.8 mm at 10
gigacycles), it will be possible to excite the TE.sub.111 mode
alone and not the other modes.
The choice of the cavity excitation device is related to the mode
of oscillation which is sought. As shown in FIG. 2, an antenna
which is formed by the extension of the central conductor 9 of the
coaxial 7 excites the modes in which the electric field has a
component at right
The dimensions of the cavity having been defined by means of the
curves of FIG. 3, it can be useful to have a means for fine tuning
of the resonant frequency of said cavity. In FIG. 2, this means is
provided by the tuning screw 12 which is of metal. The depth of
penetration of said screw within the cavity increases the resonant
frequency of this latter and disturbes the lines of force of a mode
of resonance. Different tuning means can be employed such as a
cavity which makes use of a movable end-wall, for example. It is
readily apparent that devices employing tuning means other than
those of the screw type would not constitute any departure from the
scope of this invention.
In FIGS. 4 and 5 there are shown a few examples of distribution of
the electric field in the case of the modes of resonance which are
most commonly employed. It is known that the amplitudes of the
radial and tangential components of the electric field of the mode
TE.sub.m,n,p, are products of sinusoidal function of the polar
angle, of sinusoidal function of the z coordinate, and of Bessel
function of the radius vector.
FIG. 4 describes two modes having an index m = 1. FIG. 4a gives the
direction of the lines of electric force of the TE.sub.111 mode and
FIG. 4b gives those of the TE.sub.122 mode. FIG. 4c gives the
variations in relative amplitude of the electric fields of these
modes for a polar angle .phi. = 0 and FIG. 4d gives those for .phi.
= .pi./2.
FIG. 5 describes two modes having an index m = 0. Nullification of
the first index eliminates variations according to the polar angle,
with the result that the lines of electric force are concentric
circles. Variations in the tangential component of the electric
fields are shown in FIG. 5c.
The sinusoidal variations of the electric fields of these four
modes along the z coordinate which is parallel to the axis are
shown in FIG. 5d.
These different possible distributions result in different
alternative forms of application of the invention :
in the event that the TE.sub.111 mode is chosen, the dielectric
cylinder 14 of FIG. 2 necessarily intersects the lines of electric
force. This cylinder will therefore be the cause of more or less
substantial losses which will result in heating of the material.
However, the choice of this mode does lead to advantages since it
is apparent from the curves of FIG. 3 that, in this mode, the
dimensions of the cavity and therefore the dimensions of the device
are minimized.
The energy dissipated within the dielectric of the cylinder which
limits the plasma can be considerably reduced if the radius of said
cylinder is chosen so as to ensure that the cylinder walls occupy a
zone of the cavity in which the amplitude of the electric field is
small or even zero. In order that the amplitude of a component of
the electric field should be zero at a point of the cavity other
than a point located on the walls, it is necessary to ensure that
the Bessel function which describes the variations in said
amplitude as a function of the radius vector r is cancelled in
respect of values other than the ordinary values r = 0 or r = a, if
a designates the radius of the cavity. In accordance with a known
property of Bessel functions, the index n must in that case be at
least equal to 2.
The modes TE.sub.122 and TE.sub.022, the distributions of which are
illustrated in FIGS. 4c, 4d and 5c, possess this property. In the
case of the TE.sub.122 mode, the field is zero at .phi. = 0 and r/a
= 0.35. At .phi. = .pi./2, the field is zero when r/a = 0.72. A
dielectric cylinder having a radius within the range of 0.35a to
0.72a will therefore be subjected to a low microwave-frequency
field. So far as concerns the TE.sub.022 mode, the electric field
is zero when r/a = 0.55, irrespective of the polar angle .phi.. A
thin dielectric cylinder having a radius r = 0.55a and placed
within a coaxial cavity resonant in the TE.sub.022 mode does
therefore not produce any microwave-frequency loss. These
arrangements also make it possible to derive benefit from the
maximum value of electric field for the purpose of creating the
plasma since this maximum value takes place within internal zones
of the cylinder.
The use of modes of resonance having zones in which the field is
either weak or zero within the cavity in order to minimize losses
and heating of the cylinder is clearly not limited solely to the
modes TE.sub.122 and TE.sub.022 which have been described. The use
of any other mode having an index n.gtoreq.2 would constitute only
an alternative embodiment which would not depart from the scope of
the invention.
In the foregoing description in connection with the operation of
the device which is illustrated in FIGS. 1 and 2, no reference has
been made to the disturbances produced as a result of the existence
of a plasma within the interior of the cavity. The two main
disturbances arise from drift of the resonant frequency of the
cavity and from a drop in overvoltage of this latter. If f
designates the no-load frequency of the cavity, creation of the
plasma gives rise to a variation in the resonant frequency which
becomes f.sub.p. Similarly, if the initial overvoltage of the
cavity were Q, the losses within the plasma cause said overvoltage
to change to a value Q.sub.p. If the fixed frequency of the
microwave-frequency generator has been chosen equal to f, the
resonant-frequency drift of the cavity must be of lower value than
the bandwidth of the resonance with plasma in order that the
interaction between the wave derived from the generator and the
plasma of the cavity should still be appreciable. This condition is
therefore written :
.vertline. f - fp .vertline. .ltoreq. f/20.sub.p
For example, if the power absorption by the plasma is such that
Q.sub.p = 1,000, it will not be possible for the resonant frequency
of the cavity to differ from that of the magnetron by an amount
greater than 1.23 megacycles with f = 2,459 megacycles or 5
megacycles with f = 10 gigacycles.
The values of frequency drift and of overvoltage drop depend on the
mode of resonance considered, on the diameter R of the dielectric
cylinder 14, on the static magnetic field employed and on the
electron density n of the plasma :
at low plasma densities (n .ltoreq. 10.sup.14 m.sup..sup.-3 at
2,459 megacycles and at a pressure of 10.sup..sup.-2 Torr) and with
a plasma having small dimensions (R/a .ltoreq. 0.1), the frequency
drift is not significant : it can be considered that the cavity is
resonant at the same frequency as in the case of operation on no
load, that is to say at the frequency of the generator. The drift
decreases in magnitude as R/a is smaller and, for the same value of
R/a, the drift is lower in the TE.sub.onp modes than in the
TE.sub.lnp modes. It is even possible to reduce said drift to zero
by taking as a value of the static magnetic field the value of
cyclotron resonance (875 Gauss at 2,459 megacycles), in which case
the absorption of power by the plasma is of maximum value.
in the case of higher densities (n .apprxeq. 10.sup.17
m.sup..sup.-3 at 2,459 megacycles) in a zero magnetic field, if it
is desired to have an acceptable drift, it is also necessary to
adopt small values of R/a for the TE.sub.111 mode whereas, in the
case of the TE.sub.011 mode, higher values may be adopted ; in the
case of a magnetic field in the vicinity of the value of resonance,
the value of R/a can be increased by maintaining an acceptable
drift for the modes TE.sub.111 and TE.sub.011.
in the case of high values of density (n .apprxeq. 10.sup.19
m.sup..sup.-3 at 2,459 megacycles), the TE.sub.011 mode alone
results in low values of drift in the vicinity of the cyclotron
resonance.
Cylindrical cavities of revolution are not the only structures
which permit the application of the invention In another
alternative embodiment of the invention which is illustrated in
FIG. 6, the microwave cavity has a composite shape. The cavity is
coaxial along the greater part of its length and the remaining
portion is a cylinder of revolution.
In FIG. 6, a cylindrical cavity of revolution 50 is traversed along
its axis by a magnetic metallic cylinder 23 which passes through
the top wall 24 ; the cylinder 23 is pierced by a duct 25 and this
latter communicates with a tube 26 which is brazed to the cylinder
23 ; the central conductor of the coaxial line 7 terminates in a
coupling loop 27. An expansion cup 17 defines an expansion chamber
40 which communicates with the cavity 50 through an aperture 41 ;
an extraction electrode 18 is negatively polarized by the source S.
A sole plate 28 of soft iron, two columns 29 and 30 and an
end-plate 31 constitute together with the magnetic metallic
cylinder 23 a magnetic circuit for the static magnetic field
produced by the two coils 32 and 33 which are mounted in parallel ;
O-ring seals 34 and 35 ensure leak-tightness of the cavity 50 and
of the guide 21.
The operation of said device is as follows : the
microwave-frequency excitation field penetrates into the cavity
through the coaxial line 7. A current passes through the loop 27
and the resultant magnetic field excites the modes of resonance of
the cavity which have a magnetic component at right angles to the
plane of the loop. This coupling mode thus excites a
transverse-electric-magnetic (TEM) mode. This mode of excitation is
strictly the TEM mode only in that portion of the cavity in which
this latter may be assimilated with a coaxial cavity ; the gas to
be ionized is injected via the tube 26 through the duct 25. This
gas is subjected to the microwave-frequency field only as it
emerges from the duct 25 ; it is also in this zone that the static
magnetic field produced by the coils 32 and 33 enhances the
microwave-plasma interaction as a result of the cyclotron resonance
phenomenon, with the result that the aperture 41 is located very
near the zone in which the plasma has the highest density. The
shape of the magnetic circuit, and especially the presence of the
soft iron soleplate 28 produces a high magnetic-field gradient
across the aperture 41 which promotes the extraction of the plasma
as in the previous alternative embodiments. The plasma then
diffuses within the expansion chamber 40 and the electrode 19
performs its ion-extracting function substantially without
disturbance by the magnetic field.
In this alternative mode of application, the lines of electric
force in the vicinity of the plasma-formation zone are less well
defined than in the alternative embodiment which employs the
cylindrical cavity. Although the microwave-frequency electric field
is not perpendicular to the static magnetic field, the electron
cyclotron resonance phenomenon which has been described still
remains possible since the microwave-frequency electric field has a
non-zero transverse component except possibly along the axis of the
cavity. The electrons accordingly follow a helical path and no
longer describe a plane curve.
The ion source in accordance with the invention can operate with a
wide range of gases. Hydrogen in particular has led to the
achievement of good results.
* * * * *