EP0781921B1 - Ion source with closed electron drift - Google Patents

Description

Field of the invention

The present invention relates to closed drift ion sources electrons that can be used as propellants, plus particularly for spacecraft, or as sources of ions for industrial treatments such as, in particular, vacuum deposition, ion-assisted deposition (I.A.D., "Ion Assisted Deposition") or dry etching of microcircuits.

Prior art

Industrial ion beam treatments can bring play of ion sources with a grid or a closed electron drift. These two types of ion sources were originally developed for use space (ionic thrusters or plasma thrusters).

Grid sources, known as ion propellants "bombardment thrusters", were invented by Pr. Kaufman in 1961.

These sources produce relatively high energy ions (500 to 1000 eV) with relatively low beam densities (2 to 6 mA / cm ² at the grid). They are well suited for certain applications such as deep fine etching or uniform ion erosion of targets.

For other applications (cleaning of vacuum surfaces, fast ion machining, deposition assisted by ion production (I.A.D.)), it is preferable to decrease the energy of the ions and to increase their density. it is possible with closed electron drift sources (without grid).

• les propulseurs stationnaires à plasma (SPT),
• les propulseurs à couche d'anode (ALT),
• la source d'ions brevetée par le Pr. Kaufman.

There are three types of closed electron drift sources:

• stationary plasma thrusters (SPT),
• anode layer propellants (ALT),
• the ion source patented by Pr. Kaufman.

The latter source is described in the European patent. 0 265 365. It uses a conical anode and an axial counter electrode. This source is mainly used for the I.A.D.

Figure 1 depicts a closed drift plasma thruster of electrons as proposed in an article by L. H. Artsimovitch et al. published in 1974 in "Machinostroenie", pp. 75-84, about the program development of the stationary thruster and its tests on the satellite "METEOR".

Such propellants of the "closed electron drift" type, or stationary plasma thrusters, different from other categories by the fact that ionization and acceleration are not differentiated and that the acceleration zone has an equal number of ions and electrons, which eliminates any space charge phenomenon.

In Figure 1, we see an annular channel 1 defined by a part 2 made of insulating material and placed in an electromagnet comprising parts external 3 and internal 4 polar annulars placed respectively at outside and inside of part 2 made of insulating material, a cylinder head magnetic 12 arranged upstream of the motor and the coils solenoid 11 which extend over the entire length of channel 1 and are mounted in series around magnetic cores 10 connecting the pole piece external 3 to the cylinder head 12. A hollow cathode 7, connected to ground, is coupled with a xenon supply device to form a cloud of plasma in front of the downstream outlet of channel 1. An annular anode 5 connected to the positive pole of an electrical power source, for example 300 volts, is arranged in the closed upstream part of the annular channel 1. A xenon injection tube 6, cooperating with a thermal insulator and electric 8, opens into an annular distribution channel 9 arranged immediately in the vicinity of the annular anode 5.

The ionization and neutralization electrons come from the hollow cathode 7. Ionization electrons are drawn into the channel annular insulator 1 by the electric field prevailing between the anode 5 and the plasma cloud from cathode 7.

Under the effect of the electric field E and the magnetic field B created by the coils 11, the ionization electrons take a trajectory of drift in azimuth necessary to maintain the electric field in the channel.

The ionization electrons then drift along trajectories closed inside the insulating channel, hence the name of the propellant.

The electron drift movement increases considerably the probability of collision of electrons with neutral atoms, phenomenon producing ions (here, xenon).

The specific pulse obtained by ion propellants closed electron drift classics operating with xenon, is around 1000 to 2500 seconds.

Stationary plasma thrusters developed by Pr. Morozov were used intensively for space propulsion.

Figure 2 is an axial section of an example of a thruster developed by Professor Morozov having been the subject of a publication in document FR-A-2 693 770.

This propellant 20 comprises, like the propellant of FIG. 1, an annular channel 21 defined by a piece 22 of insulating material, a magnetic circuit comprising annular pieces external 24 a and internal 24 b , a magnetic yoke 32 disposed upstream of the propellant and a central core 28 connecting the annular parts 24 a , 24 b and the magnetic yoke 32. Coils 31 make it possible to create a magnetic field and an electric field in the annular channel. The hollow cathode 40 is coupled to a xenon supply device to form a plasma cloud in front of the downstream outlet of the channel 21. This motor is characterized by the presence of a still chamber 23 which has a dimension in the radial direction larger than that of the main annular channel 21. An anode 25 is disposed on the insulating pieces 22 delimiting the annular channel 21, in an area located immediately downstream of the stilling chamber 23. An annular ionizable gas distributor 27 is disposed at the bottom of the plenum 23.

In conventional closed electron drift engines such as described with reference to Figure 1, a significant part of the ionization is located in the middle part. Part of the ions hit the walls, which is a cause of rapid wear of the walls and thus reduces the duration of life of the propellant. The energy distribution of electrons in the plasma can be reduced thanks to the magnetic field distribution imposed by your geometry of the pole pieces, which acts on the electrons entering the channel. This results in a lower electrical potential along the lines of magnetic field, which reduces the divergence of the ion beam on the walls and thus avoids ion losses by collision with them, this which has the effect of increasing the yield and reducing the divergence of the harness at the engine outlet. By acting on the ratio of currents in the coils, we can instead create a field distribution (by example a monotonic variation of the radial field in the exit plane between the outer pole piece and the inner pole piece) which does not will not achieve low divergence mode.

Strong beam divergence is beneficial for some industrial applications like IAD (Ion Assisted Deposition) on spherical caps.

More recently, the characteristics of PTS have been described in several publications, including "23rd International Electric Propulsion Conference (Seattle, September 1993) IEPC-93-222 "The Development and Characteristics of High Power SPT Models ", S. Absalyamov, V. Kim and S. Khartov, Moscow Aviation Institute, Moscow, Russia; B. Arkhipov, S. Kudryavisev and N. Masiennikov, Fakel Enterprise Kaliningrad, Russia; T. Colbert and M. Day, Space Systems / Loral, Palo Alto, California; A. Morozov and A. Veselovzorov, Institute of Atomic Energy, Moscow, Russia.

The anode layer propellants, known as ALT, have been described in Russian publications, for example: Fizika Plasmi, Plasmennie uckoriteli i ionnie injectori, Moscow 1984: Plasmennie uckoriteli c anodnim cloem, V.I. Garkusha, L. V. Leckov, E. A. Lyapin and, more recently, in international conferences

23rd IEPC conference

IEPC-93-227 , "Physical Principles of Anode Layer Accelerators", A. Zharinov and E. Lyapin, Central Research Institute of Machine Building, Kaliningrad (Moscow Region), Russia;

IEPC-93-228 , "Anode Layer Thruster: State of the Art Perspectives", E. Lyapin, V. Garkusha and A. Semenkin, Central Research Institute of Machine Building, Kaliningrad (Moscow Region), Russia;

IEPC-93-229 , "Special Feature of Dynamic Processes in a Single-Stage Anode Layer Thruster", E. Lyapin, V. Padogomova and S. Semenkin, Central Research Institute of Machine Building, Kaliningrad (Moscow Region), Russia.

30th IAEA Propulsion Conference

AIAA-94-3011 , "Operating Characteristics of the Russian D-55 Thruster with Anode Layer", John M. Sankovic and Thomas W. Haag, NASA Lewis Research Center, Cleveland, Ohio and David H. Manzella, NYMA, Inc. Brook Park, Ohio.

Figure 3 shows the section through an ALT anode layer propellant. The magnetic circuit is very close to that of a stationary thruster at first generation SPT plasma. It includes a central pole piece 54 around which is wound an internal coil 61 serving as a support to the propellant, and an external annular pole piece 53, these two pieces grounded poles being connected by magnetic cores 60 supporting external coils 62.

Unlike stationary plasma thrusters (SPT) in which the walls of the acceleration channel are insulating, the walls 56 of the acceleration channel 51 of the ALT anode layer thrusters are made of a metallic conductive material. A massive anode 55 and a cathode 59 also serve to distribute the propellant gases. The anode massive 55 occupies most of the acceleration chamber, the channel acceleration 51 being reduced to a very thin area between the anode massive 55 and the conductive walls 56 (hence the name anode layer). In fact, all parts of the propellant in contact with the discharge are metallic.

Examination of SPT stationary plasma thrusters and ALT anode layer thrusters shows that they are not fully suitable for industrial use.

As can be seen in Figure 4A, which relates to a thruster conventional plasma with acceleration channel fully defined by a insulating part 62, the internal surface delimiting the acceleration channel is divides into two zones under the influence of the operation of the propellant. The downstream zone 67, of length L (the length L may have values of the order of 5 to 7 mm for a thruster with a diameter of 100 mm), corresponds to an area permanently eroded by bombardment ionic. The upstream zone 68 corresponds on the contrary to a deposition zone eroded products.

FIG. 4B shows the evolution of the value of the radial component of the magnetic induction B _r as a function of the axial position Z on the imaginary cylindrical surface 65 corresponding to an average radius of the acceleration channel.

Figure 4C represents the value of the potential V as a function of the axial position Z on the same imaginary cylindrical surface 65 corresponding to an average radius of the acceleration channel.

If we simultaneously consider Figures 4A, AB, AC, we see that the eroded area 67 (Figure 4A) corresponds to a high radial magnetic field B _r (Figure 4B).

On the contrary, the deposition zone 68 (Figure 4A) corresponds to an almost zero potential gradient (Figure 4C) and to a relatively weak radial magnetic field B _r (Figure 4B).

The functioning of the propellant is linked to plasma-wall interactions and in particular the secondary emission characteristics of the wall. Secondary emission properties may be different in zones 67 and 68.

The plasma thruster channel comprising boron nitride, erosion of this channel can cause boron atoms on the substrate to treat. This can be particularly troublesome for applications microelectronics because boron is a dopant of silicon.

Furthermore, for industrial treatments, it must be possible to adapt to the treatment gas materials in contact with the landfill. However, the stationary plasma thrusters, such as film thrusters anode, have practically non-removable anodes, which does not allow not for example to easily switch from oxygen to argon.

Finally, plasma thrusters with an acceleration channel defined by fully ceramic parts, as described with reference to Figures 1, 2 and 4A have drawbacks insofar as the ceramic channel must obey contradictory imperatives: resistance to sputtering, mechanical strength, resistance to gradient thermal and thermal shock.

In practice, this results in resistance to spraying by ions leading to a limited engine life.

Furthermore, the need to have a ceramic piece thick enough to ensure its mechanical strength, leads to a relative distance of the pole pieces which can affect the geometry of the field.

Further industrial production of ceramic channel parts is delicate due to the complicated shape of these pieces.

An article by H. Kaufman entitled "Theory of ion acceleration with closed electron drift ", published in the Journal of Spacecraft and Rockets, Flight. 21, No. 6, 1984, New York, U.S., pages 558-562, further describes a anode layer propellant in which the cathode is grounded then that the magnetic circuit can be at a floating potential.

Document US-A-3,735,591 describes a magneto-plasma-dynamic propellant comprising an internal tubular pole piece located upstream of a cylindrical channel and a conical outer pole piece located downstream of the cylindrical channel.

Subject and brief description of the invention

The object of the present invention is to remedy the drawbacks known electron drift ion sources and, more particularly, to modify them to allow greater flexibility of use. The improvements of the invention are aimed at particular to reduce the mass of these sources while increasing their longevity, to simplify the manufacture of these sources while facilitating their disassembly and increase their mechanical resistance.

The invention also aims to reduce the emission of particles resulting of the erosion of the walls of the acceleration channel so that these sources may be likely to be used effectively as ion sources in large-scale industrial treatments, while their structure has so far limited their use essentially for the propulsion of satellites or other spacecraft.

All of these benefits are achieved through an ion source at closed electron drift comprising a main annular channel ionization and acceleration open at its downstream end, at least one hollow compensation cathode disposed outside the annular channel main, means of creating a magnetic field in the channel main annular, adapted to produce a field in said channel essentially radial magnetic with a gradient with a maximum induction at the downstream end of the canal, a first means supply of ionizable gas associated with the hollow cathode and a second means for supplying ionizable gas located upstream of the canal main annular, and polarization means cooperating with a anode, characterized in that the main annular channel of this source consists of an electrically conductive material, and in that guard rings brought to a lower potential than that of the anode extend the annular channel downstream of it, these guard rings protecting central and peripheral pole pieces that are part said means for creating a magnetic field and delimiting a air gap in which the radial magnetic field acts with an induction Max.

Insofar as it is mainly the downstream part of the canal which is subject to intensive ion erosion, which could lead to possible pollution of the substrate to be treated by erosion products, depending on the invention, the downstream part extending the channel can be produced using a material different from that of the upstream part of the main annular canal which must be essentially compatible with plasma gas partially ionized.

According to a first embodiment of the invention, a part at less of the main annular channel is electrically polarized by the polarization means so that at least part of the wall internal of the main annular channel directly constitutes said anode.

According to a particular embodiment of the invention, the channel main ring of ionization and acceleration is a set monobloc made of an electrically conductive material.

More particularly, the main annular channel constitutes a main annular canal block closed upstream by a tranquilization supplied with plasma gas by said second means gas supply which includes an annular distributor connected to a supply line.

According to a particular embodiment, the means of creation of a magnetic field include a magnetic circuit consisting of a cylinder head on which is fixed the main annular channel block, said cylinder head comprising an axial core supporting a pole piece lower central and concentric upper central pole piece with the main annular channel block, said cylinder head comprising other share a plurality of tie rods arranged around the annular channel block a once it is mounted on the cylinder head, said tie rods supporting a peripheral upper pole piece, said upper pole pieces central and peripheral constituting said pole pieces defining a air gap in which the radial magnetic field acts with an induction maximum, said guard rings protecting the pole pieces from ion erosion of the plasma. The guard rings arranged at the mouth of the main annular canal are removable.

Advantageously, the main annular channel block is isolated electrically and thermally by vacuum with respect to the elements of the rest of the ion source including electrostatic screens, the space between the annular canal block and the elements of the rest of the source of ions being between 1 and 5 mm.

According to another aspect of the invention, the annular channel block removable is fixed to the magnetic yoke by a plurality of balusters made of thermal insulating material and held in place by removable insulators.

According to another particular embodiment, the pole pieces upper include removable guard rings arranged at the mouth of the main annular canal.

In this case, advantageously, the guard rings are made of one of the following conductive materials: carbon, carbon-carbon composite, nickel alloy, noble metal, composite ceramic consisting of nitrides bonded by silicon, silicon, stainless steel, aluminum.

Alternatively, the guard rings are made in one of the following insulating materials: boron nitride, alumina, quartz.

The main annular channel block is made in one of the following conductive materials: refractory nickel alloy, molybdenum, carbon-carbon composite.

According to another particular embodiment, the internal wall of the annular channel block is plated with a noble metal such as platinum, gold or rhodium in order to eliminate chemical attacks due to the gases present in said channel.

According to yet another particular embodiment of the invention, the wall of the main annular channel is made of a material electrically conductive, and is electrically isolated from the rest of the structural elements of the source, including the anode.

In this case, advantageously, the guard rings are made of dielectric material partially covering the channel main ring finger.

More particularly, the guard rings are produced under form of inserts made of ceramic material which are fixed by means of supports on the pole pieces.

The electrically conductive walls of the annular channel and the plenum are at slightly lower floating potential to that of the anode. This arrangement reduces interactions plasma-wall, therefore the heating of the channel. The latter can by therefore be made of relatively thin sheet metal.

The channel block is held vis-à-vis the magnetic circuit by columns made of weakly conductive material. The anode is held opposite of the channel block by insulators and supplied by a conductor in the axis of one of the columns.

The gas supply is at the potential of the channel block.

Brief description of the drawings

• la Figure 1 est une vue en coupe axiale montrant un exemple de propulseur à plasma à dérive fermée d'électrons selon l'art antérieur ;
• la Figure 2 est une vue en coupe axiale montrant un autre exemple de propulseur à plasma à dérive fermée d'électrons selon l'art antérieur ;
• la Figure 3 est une vue en coupe axiale montrant un exemple de propulseur à couche d'anode selon l'art antérieur ;
• la Figure 4A est une vue partielle en coupe axiale d'un propulseur à plasma de l'art antérieur montrant l'érosion de la partie aval du canal ;
• la Figure 4B est un diagramme donnant la valeur de la composante radiale B_r de l'induction magnétique en fonction de la position Z selon une direction axiale correspondant au rayon moyen du canal de la Figure 4A ;
• la Figure 4C est un diagramme donnant la valeur du potentiel électrique V du plasma en fonction de la position Z selon une direction axiale correspondant au rayon du canal de la Figure 4A ;
• la Figure 5 est une vue en coupe axiale d'une source d'ions selon un premier mode de réalisation de l'invention ;
• la Figure 6A est une coupe schématique afin d'expliciter le fonctionnement de la source d'ions selon l'invention ;
• la Figure 6B est un diagramme donnant la valeur du potentiel électrique V du plasma en fonction de la position Z selon une direction axiale correspondant au rayon moyen du canal de la Figure 6A ;
• la Figure 7 est une vue en coupe axiale d'une source d'ions montrant une disposition alternative du premier mode de réalisation de l'invention ;
• la Figure 8 est une vue en perspective montrant le montage des différents éléments constituant la source d'ions selon le premier mode de réalisation de l'invention ;
• la Figure 9 est une vue en demi-coupe axiale en perspective d'une source d'ions selon le premier mode de réalisation de l'invention, montrant l'alimentation du canal en solide sublimable ;
• la Figure 10 est une vue en demi-coupe axiale du canal annulaire d'une source d'ions selon le premier mode de réalisation de l'invention, montrant le dépôt partiel d'une couche isolante sur les parois internes du canal annulaire ;
• la Figure 11 est une vue en coupe axiale d'une source d'ions à dérive fermée d'électrons selon un deuxième mode de réalisation de l'invention ; et
• la Figure 12 est une vue de détail montrant un exemple de liaison brasée pouvant être réalisée entre un insert en matériau diélectrique et un support électriquement conducteur assurant le centrage du canal d'accélération d'une source d'ions selon le deuxième mode de réalisation de l'invention.

Other characteristics and advantages of the invention will emerge from the following description of particular embodiments given by way of nonlimiting examples with reference to the appended drawings in which:

• Figure 1 is an axial sectional view showing an example of a closed electron drift plasma thruster according to the prior art;
• Figure 2 is an axial sectional view showing another example of a closed electron drift plasma thruster according to the prior art;
• Figure 3 is an axial sectional view showing an example of an anode layer propellant according to the prior art;
• FIG. 4A is a partial view in axial section of a plasma thruster of the prior art showing the erosion of the downstream part of the channel;
• FIG. 4B is a diagram giving the value of the radial component B _r of the magnetic induction as a function of the position Z in an axial direction corresponding to the mean radius of the channel of FIG. 4A;
• FIG. 4C is a diagram giving the value of the electric potential V of the plasma as a function of the position Z in an axial direction corresponding to the radius of the channel of FIG. 4A;
• Figure 5 is an axial sectional view of an ion source according to a first embodiment of the invention;
• Figure 6A is a schematic section to explain the operation of the ion source according to the invention;
• Figure 6B is a diagram giving the value of the electrical potential V of the plasma as a function of the position Z in an axial direction corresponding to the mean radius of the channel of Figure 6A;
• Figure 7 is an axial sectional view of an ion source showing an alternative arrangement of the first embodiment of the invention;
• Figure 8 is a perspective view showing the mounting of the various elements constituting the ion source according to the first embodiment of the invention;
• FIG. 9 is a view in axial half-section in perspective of an ion source according to the first embodiment of the invention, showing the supply of the channel with sublimable solid;
• Figure 10 is an axial half-sectional view of the annular channel of an ion source according to the first embodiment of the invention, showing the partial deposition of an insulating layer on the internal walls of the annular channel;
• Figure 11 is an axial sectional view of a closed electron drift ion source according to a second embodiment of the invention; and
• FIG. 12 is a detail view showing an example of a brazed connection which can be produced between an insert of dielectric material and an electrically conductive support ensuring the centering of the acceleration channel of an ion source according to the second embodiment of the invention.

Detailed description of particular embodiments

We will first refer to Figure 5 which shows a view overall view, in axial section, of a first example of an ion source at closed electron drift according to the invention.

The design and construction of an annular channel are significantly simplified compared to the case of a source for space use like the one in Figure 2.

A 123 plenum, which is of dimensions reduced, and the upstream part of a main annular acceleration channel form a one-piece metallic assembly 122 which will be referred to below "channel block" and which plays in particular the role of an anode 125.

A magnetic circuit, consisting of a yoke 136, a core axial 138, a pole piece 132, an internal pole piece 135, tie rods 137 and an external pole piece 134, determines a magnetic field maximum in the air gap defined by the pole pieces 134, 135.

This field includes a minimum in the vicinity of the pole piece 132. The control is created by an internal coil 133 and one or more external coils 131, which makes it possible to adjust its distribution and adjust thus the divergence of the ion beam.

The channel block 122 comprises, at its upstream part, a tranquilization 123 which is equipped with a gas injection manifold 127 supplied by a pipe 126. This channel block 122 serving as anode 125 is held by at least three balusters 121, one of which can be constituted by the line 126 itself. These balusters 121, 126 are fixed on insulators 145 by nuts 146. The balusters 121, 126 can thus be separated from the insulators 145 to allow the disassembly of the annular channel block 122. Electrostatic covers 147, 148, 154 make it possible to prevent discharges. The gas supply is performed using ground tubing 150, isolator 151 and a fitting comprising a gasket 152 and a nut 153. This assembly is housed in a base 130 which serves as a support for the source.

The electric discharge producing the ion beam is established between a hollow cathode 140 supplied with rare gas and the channel block 122 forming anode 125, supplied with pure gas or a mixture of gases, at least one of these gases can be reactive.

The nature of the material of the channel 122 can be adapted to the gas to be ionize while the nature of the guard rings 164, 165 which are placed in the extension of the channel block 122, downstream of it and which are subject to ion erosion, can be adapted to both the nature of the gas and the requirements of the substrate to be treated (for example semiconductor or layer thin optic). As such, these removable guard rings 164, 165, which are arranged respectively in the outer pole pieces 134 and internal 135, can be made of carbon (with a low rate of erosion), ceramic composite materials (such as a composite consisting of silicon, silicon nitride and titanium nitride) in aluminum, stainless steel, noble metal (such as platinum or gold).

Screens 139, 159, 160, arranged outside the channel block 122, play both a thermal and electrostatic role vis-à-vis the channel block 122. They prevent excessive heating of the pole pieces and the coils and determine, around the channel block 122, a field preventing landfills. The main annular channel 122 is thus electrically isolated and thermally from the rest of the source 139, 159, 160 by vacuum, space between the annular channel 122 and the rest of the source being included typically between 1 and 5mm.

Tests show that with a channel block 122 produced entirely of conductive material cooperating downstream with end pieces 134, 135, 164, 165 brought to a lower different potential, in this case to mass, we get a profile of the plasma potential along the median axis of channel 122 (Figure 6B) practically identical to that of the thrusters first generation stationary plasma (SPT) (Figure 4C). It is therefore possible to generate a progressive acceleration of the ions in a channel formed by two zones brought to different potentials. The profile of magnetic field in plasma is determined by the thickness of guard rings 164, 165 protecting the pole pieces from erosion ionic plasma.

Determining the nature of the wall of the canal according to the nature of industrial treatment using ions produced by the source, is basically a chemical problem due to the reaction of the wall with the partially ionized plasma gas. With an ion source conforming to the invention it is now possible, thanks to the material walls driver, to use this source for a whole range of treatments for which conventional channel sources made of ceramic material were disreputable.

The electrical insulation of the channel block 122 with respect to the cylinder head is carried out by means of the three balusters 121 provided with insulators 145. The electrical insulation of the front, side and rear faces of the channel block 122 from the grounded parts (i.e. pole pieces 134 and 135 and heat shields 139 and 159) is provided by vacuum. Indeed, the small distance between these walls (of the order of a millimeter) and the low pressure (2.10 ^-4 to 5.10 ^-4 mbar) lead to a discharge voltage much higher than the operating voltage (according to Paschen's law ).

Channel block 122 receives the heat flow, radiated and dissipated (resulting from inelastic collisions of ions and electrons) by the plasma. This corresponds to a power of a few hundred watts for a 1.5 kW source. To avoid overheating of the parts polar (whose temperature must always remain below the point of Curie) coils and dismountable connecting members 145, 153, 152, the heat losses from the channel block 122 forming the anode 125, to the rest from the source, are limited by specific constructive provisions.

Thus, the only conductive link with the source is constituted by the hollow support columns 121 and the gas inlet duct 126.

These columns can be made of weak material conductor (stainless steel, Inconel), so that the heat flow leads can be greatly reduced.

In addition, it should be noted that these balusters (and / or the inlet duct allow a differential expansion of the channel block 122 forming the anode 125 with respect to the magnetic yoke 136.

(a) en donnant une faible émissivité aux faces externes du bloc canal 122 formant l'anode 125 (par exemple par polissage de ces faces externes),

(b) en disposant un écran antirayonnement 159 entre le bloc canal 122 formant l'anode 125 et la bobine 133, cet écran jouant aussi le rôle d'écran électrostatique ;

(c) en disposant un écran externe 139 qui prévient le rayonnement sur les bobines 131 et la pièce polaire 134.

Furthermore, the radiated heat flux is limited:

(a) by giving a low emissivity to the external faces of the channel block 122 forming the anode 125 (for example by polishing these external faces),

(b) by placing an anti-radiation screen 159 between the channel block 122 forming the anode 125 and the coil 133, this screen also playing the role of electrostatic screen;

(c) by having an external screen 139 which prevents radiation on the coils 131 and the pole piece 134.

This screen can be for example either a solid block 139, such as can see it in Figure 5, discharging the heat flow over a large surface, i.e. a screen fitted with screened windows 179, represented on the Figure 7, allowing direct radiation from the channel block 122 forming the anode 125 in a certain solid angle.

The disassembly of the channel block is facilitated by the provisions sources, as shown in Figure 8.

The part which extends the channel block 122 downstream thereof is divided into two removable and interchangeable rings. The outer ring 164 is screw mounted on the outer pole piece 134, while the ring internal 165 is locked in position by the internal pole piece 135. For change the rings 164 and 165, so you just have to disassemble the parts polar.

The gas distributor 127 is an integral part of the tranquillization 123.

The channel block 122 also constitutes a metal part easily interchangeable. To dismantle the channel block 122, you must first remove the assembly constituted by the external pole piece 134, the ring of guard 164 and the screen 139 and the assembly constituted by the internal pole piece 135 and the guard ring 165. This first level of disassembly can be done without adjustment by keeping the source in place.

Then simply remove the covers 148 and 154 to access the nuts 146 used to separate the balusters 121 and the tubing 126 for axially extracting the channel block 122.

The connection between the gas supply and the tube 126 is hermetic. A flat seal 152 seals between the two parts. He is crushed by the nut 153. In order to allow easy access to the nuts 146 and 153, the base 130 is removable (Figure 5). It is provided with a degassing 176 screened, in order to prohibit the entry of the plasma prevailing in the vacuum chamber inside the space formed by the base 130 and the magnetic yoke 136. The cable 143 for polarizing the anode 125 and the gas supply 150 pass advantageously through the interface between cylinder head 136 and base 130 so as not to hinder disassembly of the latter.

Figure 9 shows a device for supplying the channel block 122 in particles 195 of a sublimable solid under vacuum (metals with strong vapor pressure, volatile oxides). This ionizes these vapors (partially) to carry out vacuum deposition, reactive or not.

In order to ensure a fine thermal control of the channel block 122, we may provide the external screen 139 with a heating element 191. It will be noted that the shape of the plenum affects that of a crucible which helps to standardize the flow of steam. If necessary, we can introduce this room a conical cornice 192.

Figure 10 shows a variant of the channel block 122 provided with a internal insulating deposit 193 delimiting the conductive area 198 constituting the anode 125 opposite the minimum field.

Figure 11 shows an overall view, in axial section, of a second example of a closed electron drift ion source conforming to the invention.

This ion source includes the following components: a hollow compensation cathode 240 disposed outside the source itself, downstream of it; a magnetic circuit comprising a cylinder head 236 disposed upstream from the source and from the bars 237, 238 connecting the cylinder head 236 to external pole pieces 234 and inner 235 in the form of rings, arranged downstream of the ion source; means 231, 233 for creating a magnetomotive force constituted by coils can be arranged for example around some of the connecting bars 237, 238 and auxiliary pole pieces 232, 239 determining a minimum of field in the vicinity of the anode; a channel block ring 222 of ionization and acceleration, delimited downstream by walls cylindrical external 281 and internal 282 metallic, and extended in the acceleration zone by two annular pieces 264, 265 of material dielectric (ceramic) held against internal pole pieces 235 and external 234, either by mechanical mounting (positioning between the pole piece and a metal holding piece), either by brazing each ceramic ring 264, 265 on a metal support itself fixed by screw on the corresponding pole piece 234, 235.

The bottom of the plenum receives a cylindrical anode 225 and a gas distributor 227, the anode 225 being held in place by insulators 283, compressed by the distributor 227 against the bottom of the chamber using tie rods 221 and spacers 221 a .

These tie rod-spacer assemblies 221, 221a are mounted on 245 insulators ensuring positioning with respect to the magnetic circuit (and more particularly the cylinder head 236).

The distributor 227 is supplied with gas by a line 226 and a connector 252 mounted on an insulator 245.

The polarization of the anode is ensured by a tie rod 221b and a wire polarization 243.

The anode 225 and the distributor 227 remain easily removable.

The ion source further includes electrostatic screens conductors 259, 339 which surround the annular channel 222.

Screens 259, 339 can slide at their downstream end respectively on the outer ceramic ring 264 and the ceramic ring internal 265.

It is the same for channel 222, the ends of which can be fitted with a metal wire eliminating peak effects, therefore the risks of dump.

Free space created between electrically conductive screens 259, 339 and the metal walls 281, 282 have a width approximately constant (typically between 1 and 5mm) so as to avoid risk of electric shock between the walls 281, 282 and the screens 259, 339. The screens 259, 339 can be provided with a grid so as to allow the degassing of the space between these screens and the walls 281, 282.

The end pieces 264, 265 have a length along the acceleration channel 222 which extends at least over an area corresponding to the length L of Figure 4, i.e. over the erosion zone due to ions.

As can be seen in Figure 11, the walls electrically conductive 281, 282 define a width of the acceleration channel 222, in the radial direction, which can be greater than the width of the channel acceleration 222 defined in the radial direction by the end pieces 264, 265 of dielectric material.

Indeed, this provision avoids the appearance of a discontinuity due to the deposition zone / erosion zone transition, the deposition producing progressively on surfaces 281 and 282.

However, it should be noted that one can also realize a source where the surfaces 281 and 282 would be at the diameter of the end pieces 264, 265 or even a smaller (281) and larger (282) diameter with a conical connection, this makes it possible to reduce the air gap of the parts auxiliary poles 232, 239.

As can be seen in Figure 11, the walls electrically conductive 281, 282 are electrically connected to each other by a bottom conductor 270 constituting, with the conductive walls 281, 282, a monobloc assembly which can itself be integral with the assembly 227 gas distributor.

The cylindrical surfaces 281 and 282 are connected to the bottom of chamber 270 by radii of curvature ensuring a smooth surface evolving gradually. So the electric field between the surfaces conductive 281, 282 and the conductive screens 259, 339, which are at the mass, does not undergo a significant increase which can cause a breakdown.

The upstream part of the acceleration channel 222 is separated from the parts polar 232, 239 as well as electrostatic screens 259, 339 by a empty space. Thus, as in the case of the embodiment of the figure 5, the main annular channel 222 is electrically and thermally insulated from the rest of the source 259, 339, 232, 239, 236 by the vacuum, the space between the main annular channel 222 and the rest of the source included typically between 1 and 5mm. In the embodiment of Figure 11, the walls 281, 282 of the annular channel 222 are electrically isolated from the rest of structural elements of the source, including anode 225.

It is also possible to bring the auxiliary pole pieces 232, 239 in contact with the electrostatic screens 259, 339, always for the purpose reduce the air gap and improve the control of the field profile magnetic.

- The external surface of the walls 281, 282, 270 as well as the surfaces external and internal screens 259, 339 can be polished so that decrease the radial radiative losses. This allows in particular to decrease the heat flux on the central coil 233 (Figure 11).

According to an alternative embodiment, the outer surface of the wall external 281 of the chamber, and this one only, can be on the contrary covered with a high-emissivity coating, as well as the faces of the screen 339, the part of the screen 259, which faces the internal wall 282, remaining polished. This arrangement improves radiation cooling of the conductive channel while preventing the heating of the central coil 233.

The life and efficiency of the ion source depends on the functional phenomena that occur within the layer ionization.

The main phenomenon that determines lifespan is erosion end pieces 264, 265 of the discharge chamber-channel assembly acceleration 222, due to the projection onto the walls of the ions which have been accelerated.

Integrity characteristics of the closed drift ion source of electrons are largely determined by the geometry and intensity of the magnetic field in the acceleration channel and remain stable even when the downstream outlet part of the discharge chamber has widened by further ion projection (see Figure 4A). A significant degradation of the operating efficiency of the thruster is observed only when a complete projection of the ions was carried out on the walls of the chamber of discharge in the interpole space of the magnetic system and when the poles 234, 235 themselves have undergone significant projections. In this cases, changes in topology and intensity of the magnetic field are the main causes of performance degradation.

In the case of the present invention, parts are used terminals 264, 265 of the walls of the discharge chamber-channel assembly acceleration, inserts of sufficiently thick dielectric material which have increased resistance to spraying by accelerated ions, which increases the lifespan of the entire ion source.

In traditional closed electron drift ion sources, to choose the walls of the discharge chamber (Figure 4A), materials having a high resistance to thermal shock and accelerated ion projections. We know that oxide ceramics aluminum (alumina) have a very high resistance to projections of accelerated ions, but have thermal resistance insufficient, which quickly leads to cracking of the walls of the chamber following several source start-up cycles. These effects are attributed to the high temperature gradient that occurs during starting, along the relatively thin walls of the chamber. However, in the case where, as according to the present invention, no use is made that inserts 264, 265 of relatively small dimensions, produced under the form of rings arranged in the vicinity of the exit from the chamber, it is possible to obtain resistance alumina inserts satisfactory thermal.

Furthermore, taking into account the shape of the curve (Figure 4C) of the plasma potential V, which remains substantially constant as long as the radial component B _r of the magnetic induction remains less than 0.6 B _rmax or 0.8 B _rmax , according to the operating regime, where B _rmax denotes the maximum value of this radial component B _r (FIG. 4B), the substitution, in accordance with the invention, of a conductive wall 281, respectively 282, for a wall in dielectric material for the area of the discharge chamber corresponding to the substantially constant part of the curve V does not appreciably affect the functional process within the source. This has been verified by various functional tests.

The fact that the internal conductive walls 282 and external 281 are electrically isolated from the rest of the ion source structure, allows to give great stability to the operating process of the ion source and equalize the plasma parameters in the near area from anode 225.

In some cases, the walls 281, 282 can however be also connected to the anode 225 by an electrical resistance.

The use of electrically conductive walls 281, 282, made of metal or composite material, leads to a reduction in the mass of the entire ion source.

It should however be taken into account that the walls electrically conductive 281, 282 have a potential close to that of the anode while in operation the structural elements of the magnetic system (elements 236, 237, 238) are going to be at a potential close to that of the cathode. This is to avoid the appearance of electrical discharges between the magnetic system and the chamber that this is surrounded by conductive screens 339, 259 which are placed at an approximately constant small distance from the walls 281, 282 and 270.

The creation of a very strong bond between the pieces of ceramic 264, 265 and support parts 274, 275 can be obtained by brazing.

Figure 12 gives an example of a brazed connection allowing to allow differential expansion between a part 264, respectively 265, and a metallic support 274, respectively 275, while respecting the electric field requirements between the screen 339 respectively 259 and the wall 281, respectively 282.

To this end, the support 274 has an inverted end 272 which is wetted by the solder 271 and the support 275 can be made of identical way.

Claims (21)

1. A closed electron drift ion source comprising a main annular channel (122) for ionisation and acceleration having an open downstream end, at least one hollow compensation cathode (140) disposed outside the main annular channel (122), means (131, 132, 133, 134, 135, 136, 137) for creating a magnetic field in the main annular channel, adapted to produce an essentially radial magnetic field in said channel (122) having a gradient with maximum induction at the downstream end of the channel (122), first ionisable gas feed means associated with the hollow cathode, and second ionisable gas feed means (126, 150, 151) situated upstream from the main annular channel (122), and bias means (143, 121) co-operating with the anode (125);
      the ion source being characterised in that the main annular channel (122) is made of an electrically conductive material, and in that guard rings (164, 165) raised to a potential that is lower than the potential of the anode (125) extend the annular channel (122) downstream therefrom, with said guard rings protecting central and peripheral pole pieces (135 and 134) belonging to said means for creating a magnetic field and defining an air-gap in which the radial magnetic field acts with maximum induction.
2. An ion source according to claim 1, characterised in that at least a portion of the main annular channel (122) is electrically biased by bias means (143, 121) so that at least a portion of the inner wall of the main annular channel (122) directly constitutes said anode (125).
3. An ion source according to claim 1 or claim 2, characterised in that the main annular channel (122) is electrically and thermally insulated by vacuum from the elements constituting the remainder of the ion source and comprising electrostatic screens (159, 160, 139; 259, 339), the gap between the main annular channel (122; 222) and the elements constituting the remainder of the ion source lying in the range 1 mm to 5 mm.
4. An ion source according to claim 2 or claim 3, characterised in that the main annular channel (122) for ionisation and acceleration is a one-piece unit constituted by an electrically conductive material.
5. An ion source according to claim 4, characterised in that the main annular channel (122) constitutes a main annular channel block that is closed upstream by a plenum chamber (123) fed with plasma-generating gas by said second gas feed means comprising an annular manifold (127) connected to a feed pipe (126, 150).
6. An ion source according to claim 5, characterised in that the means for creating a magnetic field comprise a magnetic circuit constituted by a yoke (136) on which the main annular channel block (122) is fixed, said yoke (136) comprising an axial core (138) supporting a central lower pole piece (132) and a central upper pole piece (135) that are concentric relative to the main annular channel block (122), said yoke (136) also comprising a plurality of tie rods (137) disposed around the annular channel block once it has been mounted on the yoke, said tie rods supporting a peripheral upper pole piece (134), said central and peripheral upper pole pieces (135 and 134) constituting said pole pieces that define an air-gap in which the radial magnetic field acts with maximum induction, said guard rings (164, 165) protecting the pole pieces from erosion by ions in the plasma.
7. An ion source according to claim 6, characterised in that the removable annular channel block (122) is fixed to the magnetic yoke (136) by a plurality of posts (121, 126) made of thermally insulating material and held in place by removable insulators (145).
8. An ion source according to any one of claims 1 to 7, characterised in that the guard rings (164, 165) disposed at the outlet of the main annular channel are removable.
9. An ion source according to claim 8, characterised in that the guard rings (164, 165) are made of one of the following conductive materials: carbon; carbon-carbon composite; nickel alloy; precious metal; ceramic composite constituted by nitrides bonded by silicon; silicon; stainless steel; and aluminium.
10. An ion source according to claim 8, characterised in that the guard rings (164, 165) are made of one of the following insulating materials: boron nitride; alumina; and quartz.
11. An ion source according to claim 5, characterised in that the main annular channel block (122) is made of one of the following conductive materials: refractory nickel alloy; molybdenum; and carbon-carbon composite.
12. An ion source according to claim 5, characterised in that the inner walls of the annular channel block are plated in a precious metal such as platinum, gold, or rhodium, in order to eliminate chemical attacks due to the gases present in said channel.
13. An ion source according to claim 6, characterised in that the means for creating a magnetic field further comprise induction coils (131, 133) or permanent magnets interposed in the magnetic circuit.
14. An ion source according to claim 13, characterised in that the induction coils (131, 133) are mounted on tie bars (137).
15. An ion source according to claim 13, characterised in that a toroidal coil (133) provided with an annular magnetic screen is mounted around the axial core (138).
16. An ion source according to claim 1, characterised in that the walls (281, 282) of the main annular channel (222) are made of an electrically conductive material, and are electrically insulated from the remainder of the structural elements of the source, including the anode (225).
17. An ion source according to claim 16, characterised in that the guard rings (264, 265) are made of a dielectric material covering a portion of the main annular channel (222).
18. An ion source according to claim 17, characterised in that the guard rings (264, 265) are made in the form of inserts of ceramic material that are fixed by means of supports (274, 275) on the pole pieces (234, 235).
19. An ion source according to claim 18, characterised in that the electrically conductive walls (281, 282) define a width of the annular channel (222) in the radial direction that is greater than the width of the annular channel (222) defined in the radial direction at said guard rings (264, 265).
20. An ion source according to any one of claims 16 to 19, characterised in that the portions of greater diameter and of smaller diameter of the electrically conductive walls (281, 282) of the annular channel (222) are electrically interconnected by a conductive end wall co-operating with said portions of the electrically conductive walls (281, 282) to constitute a one-piece unit (281, 282, 270), which unit is at a floating potential that is slightly smaller than that of the anode (225).
21. An ion source according to claim 20, characterised in that the portions of greater diameter and of smaller diameter of the electrically conductive walls (281, 282) of the annular channel (222) are connected to the conductive end wall (270) by radii of curvature that ensure a smooth surface.

Images