DE7228091U - ION SOURCE WITH HIGH FREQUENCY CAVITY RESONATOR - Google Patents

Description

Patent-.vSVca
Dpi-in--. F. a.rr-z aerj

Dr.-lng. R. B £ Γ Z Jr.
eMBn.hen22, Steinedorfetr. 1·

410-19.1 * »J5P (19.1 '» 4H) July 28, 1972

Commissariat A! 'Energie Atomique. Paris (France)

Ion source with high frequency carbon chamber sonator

The invention relates to an ion source with a high-frequency cavity resonator, in particular for Particle accelerator.

The ion sources used in accelerators must have certain properties, in particular;

Adaptability of energy and intensity regulation,

Reproducibility of the results,

long life, and

Possibility of obtaining a large ion density.

o-r (9)

The known ion sources have a certain number of these properties, but none have all of these properties. The ion sources of the "Duoplasmatron ¹¹ " type do not have a very long service life because of the electrodes immersed in the plasma, these electrodes, in particular the cathode, due to the strong bombardment to which they are exposed have a finite lifespan.

Avoid high-frequency discharge sources in the lower high-frequency range, which operate between 20 and 100 MHz this disadvantage, but the electron density of the generated plasma remains low.

There are also known plasma sources that have a high frequency Use cavity resonator. In some of these devices is the interaction between the high frequency Wave and the plasma are considerably increased by a static magnetic field, whatever the conditions for the electron cyclotron resonance are fulfilled.

These plasma sources with a high-frequency cavity resonator and a cyclotron resonance lead to densities Plasmas, however, are not equipped with facilities to obtain the corresponding ions.

It is the object of the present invention to provide an ion source that, on the one hand, has a plasma source with a high-frequency excitation in cyclotron resonance and on the other hand includes withdrawal or extraction devices, which are of the kind used in a duoplasmatron, but which are particularly those of the plasma source used are adapted.

The ion source according to the invention is marked through a source of plaama with a cavity, which corresponds to one of its remaining two uitrehochfroqueiit enegi, is, and with a static magnetic field that acts on the Eiek » troncyclotron resonance is set, and an extraction device for the ions of the plasma with an expansion shell which is provided with an opening which communicates with the cavity, and with electrodes for the extraction of ions, which are at suitable potentials where the plasma density is greatest near the opening and the static magnetic field is substantially zero near the peel electrodes.

The ion source according to the invention accordingly has all of the properties mentioned above and thus represents a considerable "technical advance" compared to known ones Sources.

A further development of the invention consists in that the static magnetic field assumes the value corresponding to the cyclotron resonance only in the vicinity of the opening of the expansion shell.

In one embodiment of the invention it is provided that there is a strong gradient of the static magnetic field in the vicinity of the opening.

Finally, another development of the invention is expressed in that the expansion shell consists of a metal with a weak reluctance and forms a magnetic shield for the static magnetic field.

7228 # 912 & 9.73

The invention will be explained in more detail with reference to the drawing. Show it:

1 shows an ultra-high frequency bank for exciting the ion source ;

Pig. 2 shows an ion source with a rotating cylindrical cavity resonator in which the plasma is confined in a dielectric cylinder;

3 is a graph indicating the relationships between the height and the radius of the cylindrical cavity resonator for various types of excitation waves;

FIG. K shows the electric field lines and the distribution of the amplitudes of this field for the mode types TE _1n and TE ₁₂₂ ; FIG.

5 shows the ^T .7ellen types TE _Q11 and TE _Q22 ;

Fig. 6 shows another embodiment of the invention

a truncated coaxial cavity resonator excited by a TEM mode.

In the following, “high-frequency” is understood to mean, in particular, the highest-frequency range of high frequencies.

In FIG. 1, a high-frequency generator 1 excites an ion source 2 according to the invention via an interposed one high-frequency bank consisting of an adjustable attenuator 3 »a directional coupler 4, an adapter 5»

a stamp 6, a coaxial line 7 and an excitation antenna 9.

A pump device (not shown) makes it possible to create a vacuum in the cavity via a line 21.

This high frequency excitation device works as follows:

The generator 1 contains a high frequency source, for example a magnetron tube, which supplies the electromagnetic field for the excitation. Via the attenuator 3, it is possible to change the power of the wave fed into the device. The position of the plunger 6 and of the adapter member 5 'is changed in order to have the best adaptation for the cavity of the source 2. A mismatch is measured via the directional coupler h , which taps off part of the wave reflected through the cavity to the generator.

The ion source 2 is described in more detail with reference to FIG. 2 in an exemplary embodiment. In Fig. 2 forms a cylinder 11 the side wall of the high frequency cavity resonator, which is excited by a coaxial line 7 which ends with an antenna 9. About a tuning screw 12 is it is possible to easily change the resonance frequency of the cavity. Via a connector 13 it is possible that To introduce gas to be ionized in a dielectric and tight cylinder 14 which is coaxial with the cylinder 11 is. Annular seals 15 and 16 ensure the Leak tightness of the closed interior of the cylinder in

With respect to the remainder of the cylinder 11, a lid 42 closes the cavity upwards. An expansion shell 17, which is screwed onto the jacket of the cylinder 11, completes the resonance cavity 50. It delimits an expansion chamber 40 which communicates with the cylinder 14 via an opening 41 is in communication. Only one of the peel or extraction electrodes is shown. This electrode is with respect to the expansion shell 17 via a voltage source S polarized negatively, being the expansion shell

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an electrical direct current flowing through it, generate a static magnetic field parallel to the axis of rotation of the cavity cylinder 11. The entire device is mounted on a cylinder 21. The tightness with respect to the outside space is achieved by an annular seal guaranteed.

This device works as follows

The high-frequency excitation field propagates in the coaxial line 7 and excites the resonance cavity 50 via antenna 9 · Favor the shape and location of this antenna excitation of the cavity with a transverse electrical wave type, commonly referred to as TE and which is characterized in that the electric field lies in a cross-sectional plane of the cavity, The selection and type of resonance waves are explained in more detail below.

The gas to be ionized is introduced through the opening 13. In this embodiment of the invention it penetrates into the interior of the dielectric cylinder? 14 a

and is partly under the action of the electric field of the high frequency wave and under the action of the static magnetic field ionizes "its amplitude so it is set that the cyclotron frequency is close to the frequency of the high-frequency field. The spools 19 and 20 are not necessarily traversed by the same currents. In an advantageous embodiment they are traversed by very different currents, so that the static magnetic field only in the lower Zone of the cylinder 14 in the vicinity of the opening 4i of the Expansion shell 17 corresponds to the cyclotron resonance. In order to If this opening is in the immediate vicinity of the area in which the density of the plasma is greatest, as a result of which the Diffusion into the expansion chamber is favored.

In another embodiment of the ion source according to the invention, the static magnetic field generated by the coils 19 and 20 is very weak, in particular zero, in the area which is in the vicinity of the extraction electrode 18, in order to avoid that this field does the extraction or extraction of the Ions interfere. This is possible if the shell 17 is made of a metal with a weak reluctance, for example soft iron, in order to form a magnetic screen. The rapid decrease in the amplitude of the static magnetic field when passing through the opening 41 accordingly causes a strong gradient in the magnetic field at this point, which accelerates the plasma and facilitates its expansion into the chamber kO.

This expansion is accompanied by a decrease in the plasma density, i.e. a decrease in the space charge, which usually limits the ion currents that are pulled out

can be. The electrode 18, which is at a strong negative Potential, allows the separation between the electrodes and the ions using a classic method. The shape and the potential of this electrode are such that the ions are guided towards a space which passes through the Tube 21 is limited in its interior, pumps, not shown, maintain a vacuum.

The dimensions of the rotary cylindrical cavity can be determined as described below.

In the embodiment of FIG. 2, the magnetic field is parallel to the axis of the cavity. To those on this field To use based cyclotron resonance, it is necessary that the electric field contained in the cavity high-frequency wave has a component perpendicular to the magnetic field. Transverse electrical ones are therefore preferred Resonance waves which are designed so that the electric field reads in the plane of a cross section. More specifically, a component of the electric field that is parallel to the axis of the cavity makes like them for a transverse magnetic wave is found, the device is not ineffective, but only limits its Efficiency. The aim is therefore to excite the cavity with a TE wave type, which is generally called TE

mnp

is drawn, where the indices m, η, ρ are the field distribution describe according to the usual cylinder coordinates Θ, r, ζ and ζ is the axis of the cavity.

In order not to have too large dimensions, the choice of the indices m, η and j is limited to the small values 0.1 or 2. For a given generator frequency, the height is h

7228O912O. ». 73

of the cavity with its radius a via classical relationships linked to the high-frequency cavity resonator theory. In FIG. 3 there are embodiments for four shafts and

IUr öinö i ^: jf oCj / iiöiü ν Oil lü vi-riZ udrgeätexxto ο öüö üiöaox 'äui-

ven has a vertical asymptote, which is a minimal one

Radius a is determined. If for the radius of the cavity min

a value close to a. is chosen, then will germinate

ne overlying waves excited. If for a particular a value something is chosen from a (for a TE ... Wave type, which is at 10 GHz 8.8), then only a TE wave type can in the absence of other modes excited are.

The choice of excitation device of the cavity is linked to the type of wave sought. An antenna (Fig. 2) created by the extension of the middle conductor 9 of the

Tf na Yi oil T * a -ί + ηησ 7 cpäT \ 4 1/1 a + · tt-4 y ^ ri ja ^ -tv * α rr4- / Ί-ϊα VaI 1 av ^ 4- · «* ·» ■ *** - ^

whose electric field has a component perpendicular to the antenna (for example a TE type). If the middle conductor ends with a loop in the plane of the cross-section, then the excited wave types have a magnetic field that has a component that is perpendicular to the plane of the loop (for example, TE _n and TE types). In the case of the FIG. In the embodiment described in Fig. 2, an antenna is provided, but it will be apparent that a device with a loop for coupling is only another embodiment of the present invention. A coupling can also be made via a hole, the cavity then being attached directly to the conductor.

Although the dimensions or dimensions of the cavity

are determined with the aid of the curves of Fig. 3, it may be useful to have a device to achieve a fine match of the resonance frequency of this cavity ϊλλ . In asr Fig. 2, this sln iciitung is formed from metal by the tuning 12th Their penetration into the cavity increases its resonance frequency and disrupts the field lines of a resonance wave. Other tuning devices can be used, such as a moveable lid. It will be apparent that devices using tuning means other than a screw do not go beyond the scope of the present invention.

In Figs. K and 5 some examples of the electric field distribution of the most frequently used resonance waves are shown. It is known that the amplitudes of the radial and tangential components of the electric field of the TE mode are products of

m, n, p

sinusoidal functions of the polar angle, sinusoidal functions of the coordinate ζ and Bessel functions Radius vector.

In FIG. K two shafts with the index m = 1 are shown. The Fig. Ka. shows the directions of the electric field lines of the TE wave type and Fig. ^ b the directions of the electric field lines of the wave type. In Fig. ^ C the changes in amplitude in relation to electric fields of these wave types are shown for a polar angle ψ = 0 and in Fig. 4d for a polar angle ψ = Ι.

TH

- 11 -

In the lig. 5 are two wave types with the index m = shown. Since the first index is set to zero, the changes disappear according to the polar angle, so that the electric field lines are concentric circles form · The changes in the tangential component of this Fields are shown in Figure 5c.

The sinusoidal changes in the electric fields of these four wave types along the coordinate z, parallel to the axis, are shown in dor Fig. 5d.

These possible different distributions lead to different embodiments of the invention:

If a TE ,, _Λ wave type is chosen, then intersects

111

the dielectric cylinder 14 of FIG. 2 is necessary the electric field lines, so this cylinder leads to more or less large losses, which cause a heating of the Material. Nevertheless, it is advantageous to choose this world type, because from the curves of the Pig. 3 it can be seen that for this type of wave the dimensions of the cavity and thus the dimensions of the device very much are small.

The energy dissipated in the dielectric of the cylinder, which limits the plasma, can be reduced considerably, if the radius of this cylinder is chosen so that its walls occupy an area of the cavity in which the amplitude of the electric field is very small, in particular zero. So the amplitude of a component of the electric field is zero at any point other than a point on the walls, it is necessary that the

Bessel function, the dio changes in this amplitude Describes the dependence on the radius vector r, for values other than the trivial values r = 0 or r = a becomes zero, if a denotes the radius of the cavity ·. According to a known property of the Bessel functions, it is necessary that the index η is at least equal to two.

The TE ₁₂₂ and TE_ ₂₂ wave types, their distributions in the Pig. 4c, kd and 5c have this property. For the TE ₁₂₂ wave type, the field is zero for Y = 0 and - = 0.35. For f = ^ the field is zero for f = 0.72. A dielectric cylinder with a radius between 0.35a and 0.72a is therefore only subject to a very weak high-frequency field. As for the TE _{QO2 wave} type, the electric field is zero for - = 0.55 for each polar win-

SL

keif «A thin dielectric cylinder with a radius r = 0.55a lying coaxially in a resonance cavity for the TE _Q22 wave type means no high frequency loss at all. With these devices it is therefore possible to use the maximum electric field in order to generate the plasma, because this maximum lies within the cylinder.

The use of resonance waves with regions in which the field in the cavity is weak or zero, in order to reduce the losses and the heating of the cylinder to a minimum, is of course not limited to the TE ₁₂ 2 "" ^and TE _{Q22 described} -Wave types. ^{The use} of other shafts with an index η ^ 2 therefore only forms another embodiment of the invention without leaving its scope cu.

The description given of the operation of the device with reference to FIGS. 1 and 2 does not take a position on the disturbances generated by the presence of a plasma in the interior of the cavity. The two main disturbances concern, on the one hand, the rejection of the resonance frequency of the cavity and, on the other hand, the drop in its overvoltage. Let f be the vacuum frequency of the cavity. The creation of the plasma creates a change in the resonance frequency, which becomes f. If the initial overvoltage of the cavity is Q, then the losses in the plasma cause this overvoltage to change to a value Q. If a value equal to f was chosen for the fixed frequency of the high-frequency generator, then the distortion of the resonance frequency of the cavity must be smaller than the bandwidth of the resonance with a plasma, so that the interaction between the wave generated by the generator and the plasma of the cavity is still noticeable is "This condition ^v therefore .ANN be described as follows:

- ^f pl < ^f / ² Qp

For example, if the power absorption by the plasma is such that Q = 1000, then the resonance frequency of the cavity from that of the magnetron do not differ by a size higher than 1.23 MHz for f = 2459 MHz or higher than 5 MHz for f = 10 GHz.

The values for the rejection of the frequency and for the drop in the overvoltage depend on the resonance wave under consideration, on the diameter R of the dielectric cylinder '\ k, on the static magnetic field used and on the electron density η of the plasma:

At low plasma densities (η <10 m ~ ³ at 2459 MHz and at a pressure of 1 θ "Torr) and for a plasma with small dimensions or dimensions (- ^ 0, i) the rejection of the frequency is not very significant: it can Assume that the cavity is resonating at the same frequency as when there is a vacuum in it, that is, at the generator frequency, and the warping is so

R R

smaller, the smaller - is. For the same value of - it is

a a

smaller for TE wave types than for TE ₁ wave types. onp lnp

This distortion disappears when the value of the cyclotron resonance (875 Gauss for 2459 MHz), in which case the power absorption by the plasma is greatest.

At higher densities (nG £ iO ⁱ⁷ m " ³ at 2459 MHz) and without a magnetic field, if one wants to obtain an acceptable warpage, it is also necessary for TE -We11ypen

R.
if small values of - are to be used, while larger values can be used for a TE wave type. With a magnetic field that is close to the resonance value, one can increase the value of - · by an acceptable distortion

for the TE and TE_ wave types.

With increased density values (ηκ 1O ¹⁹ In "" ³ at 2459 MHz) only the TE _Q -We11 type leads to weak distortions in the vicinity of the cyclotron resonance.

The rotating cylindrical cavities are not the only shapes that can be used with the invention. In another embodiment of the invention, which is shown in the As shown in Fig. 6, the high frequency cavity has a composite shape. He's on most of it

Length coaxial and cylindrical on the remaining part.

In this FIG. 6, a rotary cylindrical cavity 50 is traversed on its axis by a magnetic metal cylinder 23 which extends from a cover 2k . The cylinder 23 is pierced by a channel 25 which is connected to a tube 26 which is soldered onto the cylinder 23 * The central conductor of the co .xial line 7 ends with a coupling loop 27. An expansion shell 1? defines an expansion chamber kO, which is in communication with the cavity 50 via an opening 41. A pull-out electrode 18 is negatively polarized by a power source-S. A plate 28 made of soft iron, two supports or posts 29 and 30 and a flange 31 form with the magnetic metal cylinder 23 a magnetic circuit for the static magnetic field generated by the two coils 32 and 33, which are connected in parallel. Annular seals 34 and 35 ensure the tightness of the cavity 50 and the guide member 21.

This device works as follows:

The high-frequency excitation field penetrates the cavity through the coaxial line 7. The loop 27 is traversed by a current and the resulting magnetic field excites the resonance waves of the cavity, which have a magnetic component perpendicular to the plane of the Loop is. This coupling wave thus excites a transverse electromagnetic type of TEM-Ve11. This excitation wave is exactly of the TEM type only in that part of the cavity, in which this can be equated to a coaxial cavity. The gas to be ionized is through the pipe 26 introduced the channel 25. This gas is only available at the exit of the

7228Q9120.I.73

Channel 25 subjected to an ultra-high frequency field. In
this area is also reinforced by the coils 32 and 32
33, the static magnetic field created the interaction between the high frequency wave and the plasma through the phenomenon of cyclotron resonance, so that the opening
41 is very much in the area where the plasma is most dense. The shape of the magnetic circuit, and in particular the use of the soft iron plate 28, impose a significant gradient in the magnetic field on the implementation of the
Opening kl , whereby, as in the previous exemplary embodiments, the withdrawal or extraction of the plasma is favored.This a then diffuses into the expansion chamber 40, and the electrode 18 draws off the ions,
especially in the protection of the magnetic field.

In this embodiment, the electrical
Field lines in the vicinity of the area in which the plasma is formed are less precisely defined than in the exemplary embodiment in which a cylindrical cavity is used. Although the high-frequency electric field is not perpendicular to the static magnetic field, a cyclotron resonance of the electrons, as described above, is possible because the high-frequency electric field possesses
a non-zero transverse component except perhaps on the axis of the cavity. The electrons then describe a helix and no longer one
flat curve.

The ion source according to the invention can be used with many oases work. In particular, good results have been obtained with hydrogen.

Claims (7)

Patent Attorneys Dlpl.-Ing. R. B £ T Z sen. Dlpl-In.?· K. LAfjiP..ICHT Dr.-Ing. R. CS E E T Z Jr. ^ S Munich 22, Steinadorfitr. 10 · <44 (5) 6.7.1973 New claims for protection 1. Ion source with a static magnetic field adjusted to electron cyclotron resonance, c t characterized by, that in the area of influence of the magnetic field a plasma source with a resonantly excited cavity (50) and an ion extraction device is arranged with an expansion shell (17) which, in operation, is the largest Plasma density is connected to the cavity (50) via an opening (4l) and in a magnetic field area with a Field strength close to zero contains extraction electrodes (l8) for the extraction of ions. 2. Ion source according to claim 1, characterized in that only the vicinity of the expansion shell (17) with the Opening (41) connecting the cavity (50) in a magnetic field area with a field strength corresponding to the cyclotron resonance lies,, 3. Ion source according to claim 1, characterized in that the cavity (50) with the expansion shell (17) connecting opening (4l) in a magnetic field area with a strong field strength gradient. 4. ion source according to claim 3 »characterized in that the expansion shell (17) consists of a metal with weak reluctance. 5. Ion source according to claim 4, characterized in that the expansion shell (17) consists of soft iron. 6. Ion source according to claim 1, characterized in that that the cavity (50) is designed as a rotary cylinder and is axially aligned with the static magnetic field and a dielectric cylinder (14) arranged coaxially and at a radial distance therein and, on the other hand, tightly sealed for receiving the plasma, relative to which the expansion shell (17) lies in an axial extension. 7. Ion source according to claim 1, characterized in that the cavity (50) as a hollow cylinder with an axially inserted Metal cylinder (23) is designed with weak reluctance, which has a cover (24) for the cavity (50) touches and opens near the opening (41) leading to the expansion shell (17) in the cavity (50).