CA2250917A1 - A hall effect plasma thruster - Google Patents

Abstract

In a Hall effect thruster (3A), especially for use in manoeuvring satellites (1A), a stream or plume of ions, used to produce the thrust, is deflected, by appropriate adjustment of a magnetic field, so as to steer the satellite or other vehicle. The channel (20), along which the ions are accelerated, is preferably flared outwardly at its open end so as to avoid erosion which would otherwise be caused by the deflection. The adjustment of the magnetic field is preferably achieved by dividing an outer magnetic pole, surrounding the channel (20), into separate sectors and winding individual electric coils (18, 19) around the sectors. Control of the current through these individual coils (18, 19) is used to make the appropriate adjustments of the magnetic field.

Description

A HALL E~CT PLASMA THRU~

Field of InvPntion This invention relates to a Hall effect plasma thruster, also known as a closed electron drift thruster. The invention arose when con~;~ienng the design 15 of such thrusters for use on s~qtellites or other spacecraft to assist in adjusting their positions when in orbit around the earth, to move them into the desired orbit or for propelling sracer,r~ft during long missions.

Rqrk~rolln-l of Tnvrntion A conventional Hall effect thruster comrri~es an annular accelerqting 20 channel ext~nrling circumferentially around an axis of the thruster and also extPn-ling in an axial direction from a closed end to an open end. An anode is located, usually at the closed end of the chqnnPl, and a rqthode is positioned outside the chqnnPl close to its open end. Means is provided for introducing a propellant, for example xenon gas, into the Ghqnnel and this is often done 25 through passages formed in the anode itself or close to the anode. A m~gnp~tic system applies a mqgnetir field in the radial direction across the chqnnel and this causes electrons emitted from the cathode to move circumferentially around the chq~mPl. Some but not all of the electrons emitted from the cathode pass into the channel and are qttr.qcted towards the anode. The radial m~gn~tic field deflects30 the electrons in a cil.;ulllÇerential direction so that they move in a spiralllaje~ y, q-~cuml-lqting energy as they gradually drLft towards the anode. In a region close to the anode the electrons collide with atoms of the propellant, causing ionization. The res--lting positively charged ions are ~cce1er~ted by the electric field lo~anls the open end of the ch~nnP1, from which they are expelledat great velocity, thereby producing the desired thrust. Rec~11se the ions have a S much greater mass than the electrons, they are not so readily influenced by the magnetic field and their direction of acceleration is thel~rolc plihll~;1y axialrather than ci~-;ulllfel~;lllial with respect to the ch~nnPl As ralized by thoseelectrons from the c~th- -le that do not pass into the channel.
In this s~ifi~tifm the terms "ulJ~Ll~l" and "down~l~"" will be used 10 for conveniellce to des~ - i1 e directions with l~r~,~el~ce to the movement of ions in the ch~nnP1 The Hall effect thruster has been widely developed and used, and has been the subject of a great number of publications. The op~.dl.l~g principles and rh~"..~ 1;cs of such devices have been well known since the 1970's. In recent 15 years, no fim-l~m~nt~1 challges to the opelalillg princip1Ps of these devices have been made although there have been various proposals for improvement of particular details of design. One difficulty in the design of these thrusters arises from the erosion of ins~ tor m~tPri~1 at the outlet port of the thruseer. If in~r~ s of m~m1f~ctl1re lead to any miC~1ignmPnt between the geometric axis 20 of the t~ Le~ and the thrust axis, this will cause non-symmetrical erosion of the in~ t-)r and lead to a decrease in efflciency and lifetime. One possible way of dealing with this P1IJb1-111 is to set the electri~1 cil~;uill ~ after m~n11f~ctl-re so as to adjust the Gl~C~ ;ull~.nL~ passing through l~pecli~e m~gnPtic coils, thereby collc(;Liilg any mi~1ignmPnt that would otherwise exist between the thrust axis 25 and the geometric axis.
When in use, known L}uusL~l~ of this type have always developed a thrust in a fixed direction. Hitherto this has been accepted without question; steeringof the ~ .,.rL being achieved either by the use of two thrusters and ch~nging their relative amplitude of thrust; or by the use of a mech~nicm which swivels a single thruster relative to the spacecraft. The use of two thrusters is expensive and increases weight and the swivelling merh~nism is heavy, complex and e~ensi~/e. It is believed that the present invention will make it possible to eli...;.~P, the need either for duplication of thrusters or for the use of a complex 5 swivel mP~hqni~m .

The inv be suitably designed to cope with a mi~vlignPd axis of thrust, such "mi~l;~ " can be created delihP~tqtely in a dynqmicqlly chal~geable basis so as to steer the satellite or other space vehicle in response to a signal in~ ting a desired change in the direction of thrust.

10 S--mm~ty of InvPntinn According to the invention there is provided a Hall effect thruster co~ ising a çhqnnPl eYtpn~ing around an axis of the thruster and ~ytpn~ltng in an axial direction from a closed end to an open end, means for introducing a propellent into the ch~nnPl and means for applying an axial electric field and a15 radial ~n~1ic field so as to cause ioni7qtion of the propellant and ~cceletq~tion of the resulting ions from the open end of the ~hqnnel, chq-tq~ ~1 by steering means for varying the m-q.~nPtic field in response to a steering signal thereby ch~nging the direction in which the ions are accelerated and the,~rol., the direction of thrust.
It is believed that by employing this technique it will be possible to steer the direction of the thrust through up to 5 degrees from the ~ ic axis ;.~.1r,~,P....~ y in two orth~gonql directions. This 5 degree dPflP~tion is believed to be sllffi~ nt for most pulposes. Erosion of the in~llqtor dPfining the channel along which ions are accelerated can be reduced to an acceptable level by 25 de~igning this in~llqtor so as to be flared oulwdldly at its open, downstream, end. In arrqn~ P~ where the down~ll.,alll end of the t~hqnnP~l is flared, it is pl~;Çe~d that the flare be created by an oul~anl slope of the outer wall of the chqnn~,l A similar inward slope of the inner wall would be possible.
Alternatively, the design may be such that the inner wall extends dowllsl,~n less far than the outer wall for the puIposes of erosion limit,qtion.
The propellant, which is typically xenon gas, is preferably introduced S through or in the region of an anode and this anode plcf~ably extends ~ub~ ially all of the way aroupplied, as is conventional, between the anode at the closed end of the çh,qnn~,l and one or more cathodes located outside the channel close to its open end. The or each c-q-th-)de may either be located at aposition which is radially outwardly of the channel or radially inwardly of the 10 chqnn~,l.
The m,qgnetic field is preferably ge~ d by a central source of otive force (e.g. a coil or a ,~ ent magnet) around which a number Of q~liti~n,ql sources of electromotive force (which may also be coils or electrom-qgnet~) are pochion~d, Where m~gnPti~ coils are used, the required 15 variation of the m-q~nPtic field can be achieved by indepçndently varying the current through dirÇ~ coils.
The chqnnPl will normally be defined by a ce~amic colllpollent because of the high tk~ G.,~ s which are gellGlated. It preferably has a circular CI'OSS3~;liOn in a plane perpe,nf~ Ul-q-r to the axis of the thruster, although other, 20 non-circular co.~ are possil,le. For .oYqmple, where there are a numberof coils or pe....~ -~,-1l mqgn~,t~ arranged around the outside of the channel, there can be an advantage in m,qking the c,h-q-nn~l wider in regions qdj,qcent those coils or ~ -l ma~nP.t~.
In a ~lcrGllcd form of the invention the means for creating the m,q.~n~,tic 25 field c~)...l..;~es inner and outer ~ bodies ~efining opposite m~qgnetic poles and located lc~ ely to the inside and outside of the ch,qnn~,l These mq~nf~,ti~,bodies may be pe.lll;lnP,I~I magn~,t~ or clecl~ gnetc. Where the outer m~gnetic body is an ~ E~ it IJlGÇ~.ably has an associated ell~,tric~l coil e, rten~ing ch1ulllrelGI~lially around the axis of the thruster. This coil is preferably located radially to the inside of the mAgnP,tic body between the m~gnetiC body and an ~or:~'~ s;~ g colllpollc lt which serves to reduce the m~gnptic field in the area of the anode. The outer m~n~tic body may be formed with gaps located circumferentially around it so that the m~gnP.tiç pole which it defines iof the S m~gnPtic body between the gaps. Each of these sub-poles has an associated electrical coil çxten-ling around it and passing through the gaps. The steçring means varies the respective electric cull~nls through the dirr~,~n~ coils so as to deflect the direction of thrust during operation of the thruster.

Rrief nescriptioll of nrawi~c One way of ~.rOllllillg the invention will now be desclibed with reference to the acco,llpallying drawings in which:
Figure 1 illllstrAtec, in s.~lh ...~I;c form, a known tecl~ique for steering ~t~.llit~c;
Figure 2 is an illllstr~tinn similar to Figure 1, but showillg sçhpm~ti~ ~lly 15 a s~tellite ArrAnged to be steered using a techni~ue in accordance with the invention;
Figure 3 is a pP~spective view of a stationary Hall effect plasma thruster constructed in accor~lce with the invention and illllstrAtçd as if cut through its rliAmeter to reveal fca~ules of intPrn~l construction;
Figure 4 is a cross-section through the axis X-X of Figure 3 showing the magnP,ti~ components only and lines of magnp~tic force;
Figure S is a plan view of the colll~ollelll~ shown in Figure 4 but showing a design variation in which the ...~elic poles are divided into four S~ale paIts; and Figure 6 is a sÇ~ view of an AltPrn~tP embodiment of the invention in which the gap between the walls of the thruster is larger near the mAgnP,tic poles than elsewhere.

W O 97~7126 rcTnusg7/os207 RPct Mode for ('~ i~ Out Inv~nti~n R~r~,ling firstly to Figure 1, this shows, in very S~ h~ ..AI;C form, a known ~"Ai~ge..-ent in which a Q~ttq.llitlq 1, co..~ ~;nin~ a power supply 2, is propelled by means of a plasma thruster 3. The Ih~uslel 3 contains an inner magn~.tic coil 4 S and four outer m~nPtiC coils 5, 6, 7 and 8 col n~octecl in series with the power supply 2 so as to receive equal CO~I.C1A-~I ~;un~ ls. The thruster 3 can be adjusted mech~nir~lly by a swivel m~xl--n;.c.~ 9 under the control of a direction controlcircuit 10. The swivel merh~-ic... comprises a platform 11 hinged at 12 to the cAtellite and at 13 to a driven shaft 14 of an actuator 15.
An ~IAu~r~"rnt constructed in accol~lce with the invention is shown in Figure 2, also very ~ . AI;~11Y, and collll,lises a s~tP-llite lA having a thlusta 3A c~ ~d rigidly to it; i.e. wilhoul the interposition of a swivel ~ xll~nicm The thruster 3A has an inner coil 16, an outer coil 17 and four ancillary sl~
coils 18, 18/, 19, 19/. A ~tPllite power supply 2A is connected to a control circuit lOA which receives a direction signal SIG (e.g. from an earth station via a radio link) defining a desired direction of thrust. The circuit lOA has twelveoutput lines arranged in pairs al, a2; bl, b2; cl, c2; dl, d2; el, e2; and fl, f2 and can apply a selected voltage of either polarity at each pair of outputs. Thevoltage a applied across al and a2 is conctAnt as is the voltage b applied across bl and b2. The voltage c across cl and c2 is of a~lv~-ul,ately the same value as the voltage d across dl and d2 though there may be a small offset between these values to correct for any mi~lignm~nt that might otherwise exist bel~
the axis of the thrust vector and the physical axis of the thruster; or to ~elib~", 1~Jy create such miQ~lignm~nt The voltages c and d may be of the same or ol,l,osi~ signs dep~n~ling on the ~ ~1ion~ to the coll~onding coils 18, 18/
and are selectfYl by the circuit lOA so as to ensure that current flows in opposite d;~;L;~I15 tl-l~ugl- coils 18, 18/, thereby controlling the direction of thrust in one plane. Voltages e and f are varied in the same way as (but inflepen~lPntly of) voltages c and d so as to control the cu~nls through coils 19, 19/ and thus the direction of thrust in an o~ gol-ql plane. In this way the direction of thrust can be adjusted through a total angle of up to 10 degrees in each plane as in-lir3t~in broken lines.
R~Çel,ing now to Figures 3 and 4, the lhl.l~t~. is genP,r~lly symmetr~
about an axis X-X. It comprises an annular accelerating chqnnPl 20 defined bt;lweell inner and outer walls 21, 22 respectively of a a closed, dowl.sll~ll end (the bottom as shown on Figure 3) to an open, u~ll end where the outer wall 22 extends in the dowl.stl~am direction slightly further than the inner wall 21.Both inner and outer walls 21 and 22 are of incre~d thic~nP-ss at their respective downstream ends and the outer wall 22 is ch~lrel~d at 22A so as to provide a slightly flared open end of the channel.
At the u~ ~n end of the channel there is located a circular anode 24 in the form of a hollow square section tube having a groove eYtPn~1ing continuouslyaround it. A pipe 25 delivers a propellant (which is xenon gas in this particular ~nl~le but could alternatively be klypton or argon) into this hollow anode from which it is delivered to the ch~nnel 20 through the circular groove. Baffles (not shown) may be supplied inside the anode in order to improve distribution of the propellant gas around the çh~nnPl An elP~trir?l connectinn 26 supplies positive enlial to the anode.
A cathode 27 is mnlmtP~ on a .~ flic north pole, to be described later, close to the do~ slr~ll end of the ch~nnPl 20. This c~thn~lP is supplied with xenon gas through a connection 28 and with a source of negative po~e..lial via clr~t~ ;n~l co~nP~r 29. The .~ system incl~ldes two m~gnPtir~lly sep~r~te bodies or yokes, namely an inner yoke 30A and an outer yoke 30B, both made 25 of m~ntetic~lly permeable m~teri~l The inner yoke 30 is in the shape of a spool and has a central cylindrit~l core part 31 having a central bore for the purposes of weight reduction. An inner coil 16 is wound around this cylin(lrit~l part so that current passes in acl~wi~e dil~tion as viewed from the duwll~ end. At the dowl~sl~ end of the ~;yl;~ l part 31 is a radially oulwardly ext~n~iing end-piece in the formof a flange 32 which defines at its free edge a first, circular pole 33 (m~gnetic south) of the inner m~gnPtic yoke. Another end-piece in the form of a radially outw~dly ~ g fll ~u~?o~ls a cylin(1ri-~1 wall 35 which paltly encloses the 5 inner coil 32 and defines a second (m~n~tir north) pole at its free edge 36, as seen best on Figure 4.
The outer yoke 30B is formed by a cylin-lrir~l wall 37 coaxial with the axis X-X and having a circular rim 38 of increased thi~ n~.cs. This rim is divided by four slots or gaps 39 ~Figure 3) serving to divide the rim 38 into four equal sectors 38A, 38B, 38C and 38D. Each of these sectors has a subsidiary, steering, coil wound around it. These steering coils are the same coils as are shown at 18, 18/, 19, 19/ on Figure 2 and they are arranged so that current passes clockwise around one and ~nti~ wise around the opposite coil. A first, radially inwardly e~t~n-ling end-piece, in the form of a flange 40, is ~tt~hP~1 to lS the four sectors of the rim 38. This flange 40 is circular and bridges the gaps b~w~n the rim sectors 38. It is shown partly broken away on Figure 3 so as to reveal the underlying parts. The circular, radially inner, edge of the flange 40forms a first (...~n- lic north) pole of the body 30B and is positioned, as is best seen on Figure 4, slightly dow~ l of the m~nPtiC south pole 33 of the body 20 30A. The uySIl~ll end of the cylin-~ri~l wall 37 extends into another inwardly extçn~ling circular flange 41 which in turn extends into a cylil~l;c~l wall 42 coaxial with the axis X-X. The walls 37, 41 and 42 define an enclosure which cont~in.c a main outer coil 17 (also shown on Figure 2) which is wound around the wall 42 and is connected so that current flows in the direction shown in 25 Figure 4 such as to create a m~gnPtic south pole at the downstream edge 43 ofthe wall 42 and a m~ nP.tic north pole at the inner edge 44 of the flange 40.
Figure 4 shows the lines of m~P.tic field when current is passing through the inner coil 16 and the outer coil 17 but not through the steering coils 18, 18/, 19 and 19/. It will be seen from Figure 4 that the offset between the WO 97/37126 PCTtUS97tO5207 poles 33 and 44 results in the m~gnP~tic field being tilted in an annular accele,~tillg zone 45 where, in operation, the ions are acceler~ted. This tilt of the magn~tir field causes the ions to be accelerated in a direction shown by thearrows V tc,~ards the axis X-X. The purpose of this is to limit the div~ .~ence 5 of the resulting plume of ions fr~m the ~ sler. A point worth noting is that the cylin~1ri-~l walls 35 and 42 serve to screen the area 45/ where the anode is located from the effects of the mqgn~-tic field.
Operation of the ilh~st~ted thruster is as follows. Electrons are emitted from the c~thode 27 and are divided into two streams. One stream of such electrons is ~tt~- tecl tow~ds the anode 24 into the annular ch~nn.o1 20. The radial ~lllpo~ l of the m~gnPtic field within the ch~nn~l causes the electrons to travel in a cilcwnfe,~n~ial direction, gradually drifting in an axial direction the anode. In the region 45/ of the anode, where there is only minim~l m~gnetic field, the electrons, having acquired energy during their spi~al 15 movement down the çh~nn~1, cause iOn~ ll of the propellant gas supplied along the pipe 25.
The res~lting ions, which are positively charged, are accelerated in a downstrearn direction by an electric field produced by a potential dilre~nce of about 300 volts, between anode and c~thode. Rec~use of their relatively high 20 mass, as co~ d with the mass of electrons, the propellant ions are not greatly r~ by the ma~ti~ field. There is however some such inflnence and the in~lin~l nature of the ~..~.~el;~ field in the açcel~ ~Ih~g region 45 between poles 33 and 44 causes the stream of ions, issuing from the dow"sl~ end of the thruster, to tend to converge in thethrough the coils 18, 18/, the effect of the25 magnetic field on the ions issuing from one side of the thruster is increasedbecause the m~gnttic field ~llengl~l there is increased, whilst the effect is decreased on the opposite side of the thruster. Thus a deflection in the direction of the vector is achieved as shown at V/ and Vll on Figure 4. By controlling the WO 97t37126 PCTtUS97/0~207 ~;u,~ through coils 18, 18/ on the one hand and 19, 19/ on the other hand, thedirection of thrust can be ~ ced in any direction.
It was mentioned earlier that the electrons emhted from the cathode 27 were divided into two streams and that one of these streams entered the 5 acc~ g rh ~ m~l The other stream of electrons is effective to neutralize the ions as they are ejected from the thruster so as to avoid leaving a res~-lt~nt negative charge on the IhlllSItl. Erosion of the downstream edge of the outer ceramic wall 22, caused by ~ g of the thrust vector, is reduced by the presence of the cl~,.fer 22a, whilst reduction in erosion of the corresponding 10 edge of the inner wall 21 is reduced by its ~ Jl~ nt in the u~sll~ll direction relative to the ~.lGsyonding challlre,Gd edge of the outer wall 22.
An i~ feature of the ill--st~t~ embodim~nt of the invention is that the ".~g~lf~i~ bodies 30a and 30b are ...~ liC~lly se~ P, each co~ ;.-g an individual ele~ r~ having its own north and south poles. Rec~llse of this, 15 it is possible to obtain the required m~g~Pti~ char~ctçri~tics within the accelerating channel with a wide variety of dirre,f.ll overall ~limton~ions of the ..~nf~;r system, dirr~ t from those ~linn~o.n~i~m~ shown in Figures 3 and 4. ForeY~mple, it is predicted that it will now be possible to rn~m-f~ctl-re Hall effect thmsters which, for a given power, are shorter in the axial direction and wider 20 in di~meter (or vice versa). A thruster can tller~fol~ now be designed which makes better use of available space on a satellite or in the launching vehicle.
Iody 30A ~l~.r..,:.-g the inner ...~ ;c south pole is divided by radial slots into four se.E;,-.~ ; Sl, S2, S3 and S4; and the circular flange 40 (lr~ ng the outer m~gn~ti~ north pole is ~imil~rly divided into four segments Nl, N2, N3 25 and N4. The slots or gaps between the thus formed individual north poles is notably smaller than the much larger slots or gaps 39 which accommodate the steering coils 18, 18/, 19, 19/. The individual north poles thus overlap end portions of the coils where they pass through the gaps. This design variation shown in Figure 5 provides improved steering capability.

CA 022509l7 l998-lO-Ol W O 97/37126 PCTrUS97/05207 Figure 6 depicts sçh~ qlly an q~ e design to that of Figure S
with the alteration being in the shape of the channel 20. The ~ t~qnce between the walls of the chqnn~l 20 is greater at Dl ~q.-ljqcellt the ancillary ~ g coils 18, 18', 19, 19', and is lesser at D2 at positions removed from the ancillary S steering coils 18, 18', 19, 19'. It will be noted that erosion is reduced in this design during st~ring. A combination of cl al,lre~ g, as seen at 22A in Figure 3, and varying the gap between the walls of the channel 20, as shown in Figure 6, can be l~ti1i7~A.
It will be a~cialed that the particular embodiment of the invention shown in the drawings has been described only by way of example and that the invention is in no way limited to particular features of this example. For example, the invention is also applicable to the so-called anode layer thruster.Where a construction similar to that illustrated is used, various variations of design would be possible. For example, the steering effect could be improved by dividing the flange 40 into four S~lnte sectors collesponding with the sectors 38a, 38b, 38c and 38d, thereby fo,~ g four s~ P. main mqgnetic north poles. Another variation would be for the coil 17 to be located on the outside, rather than the inside, of the cylin~lricql mqgnPtic wall 37. Another possible variation would be to omit the coil 17 and Also, either or both of the coils 16 and 17 could of course be replaced by p~ enl magn~tS. ~lternqtively the mqgn~tic bodies 30A and 30B could be formed as permqnPnt mqgn~t~ Yet another possible variation would be to have just three steering coils, or any number greater than four.
While the invention has been described in connection with specific embo~liment~ thereof, it will be understood that it is capable of further modification, and this application is intenrled to cover any variations, uses, or ~rtqtionc of the invention following, in general, the principles of the invention and inrln~ling such de~a,lul~ s from the present disclosure as come within knownor cuslu-~ practice in the art to which the invention pe;llilins and as may be W O 97~7126 PCTAUS9710~207 applied to the ç.cse.nti~l r~Lules h~ b~rol~ set forth, and as fall within the scope of the invention and the limits of the ayy~nded claims.

Claims (43)

13That which is claimed is:

1. In a Hall effect thruster comprising a channel extending around an axis (X-X) of the thruster and extending in an axial direction from a closed endto an open end, means for introducing a propellent into the channel and means for applying an axial electric field and a radial magnetic field so as to cause ionization of the propellant and acceleration of the resulting ions from the open end of the channel, the improvement comprising:
steering means for varying the magnetic field in response to a steering signal (SIG) thereby changing the direction in which the ions are accelerated and therefore the direction of thrust.

2. A thruster according to claim 1, characterized in that the channel is flared outwardly at a downstream end of the channel to reduce erosion of its outer surface.

3. A thruster according to claim 2, characterized in that the steering means allows a modification of the magnetic field independently along two axes located in a plane perpendicular to a geometric axis of the thruster.

4. A thruster according to claim 3, characterized in that the steering means comprises two pairs of magnetic coils, one on each of two opposite sides of the thruster, connected so that the electric current passes through the coils in dependence on the steering signal SIG.

5. A thruster according to claim 4, further including:
two pairs of magnetic coils arranged to control the direction of thrust in respective orthogonal directions.

6. A thruster according to claim 5, characterized in that the means for steering the magnetic field comprises inner and outer magnetic bodies defining opposite magnetic poles and located respectively to the inside and outside of the channel, the outer magnetic body extending around the channel andbeing formed with gaps so that the magnetic pole which it defines is divided into individual sub-poles each extending around an arcuate portion of the magnetic body and having an associated electrical coil, the steering means being effective to vary the respective currents through the different coils.

7. A thruster according to claim 6, in which the outer magnetic body includes a circular pole piece which bridges the said gaps.

8. A thruster according to claim 6, in which the outer magnetic body includes separate pole pieces arranged in a circle and serving partially to bridge the gaps but having spaces between them smaller in the circumferential directionthan the gaps.

9. A thruster according to claim 8, including a main coil extending circumferentially with respect to the outer magnetic body.

10. A thruster according to claim 9, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

11. A thruster according to claim 1, characterized in that the steering means allows a modification of the magnetic field independently along two axes located in a plane perpendicular to a geometric axis of the thruster.

12. A thruster according to claim 11, characterized in that the steering means comprises two pairs of magnetic, coils, one on each of two opposite sides of the thruster, connected so that the electric current passes through the coils in dependence on the steering signal SIG.

13. A thruster according to claim 1, characterized in that the steering means comprises two pairs of magnetic coils, one on each of two opposite sides of the thruster, connected so that the electric current passes through the coils in dependence on the steering signal SIG.

14. A thruster according to claim 4, characterized in that the means for steering the magnetic field comprises inner and outer magnetic bodies defining opposite magnetic poles and located respectively to the inside and outside of the channel, the outer magnetic body extending around the channel andbeing formed with gaps so that the magnetic pole which it defines is divided into individual sub-poles each extending around an arcuate portion of the magnetic body and having an associated electrical coil, the steering means being effective to vary the respective currents through the different coils.

15. A thruster according to claim 14, in which the outer magnetic body includes a circular pole piece which bridges the said gaps.

16. A thruster according to claim 15, in which the outer magnetic body includes separate pole pieces arranged in a circle and serving partially tobridge the gaps but having spaces between them smaller in the circumferential direction than the gaps.

17. A thruster according to claim 16, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

18. A thruster according to claim 17, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

19. A thruster according to claim 15, including a main coil extending circumferentially with respect to the outer magnetic body.

20. A thruster according to claim 16, including a main coil extending circumferentially with respect to the outer magnetic body.

21. A thruster according to claim 14, including a main coil extending circumferentially with respect to the outer magnetic body.

22. A thruster according to claim 21, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

23. A thruster according to claim 6, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

24. A thruster according to claim 23, including a main coil extending circumferentially with respect to the outer magnetic body.

25. A thruster according to claim 6, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

26. A thruster according to claim 14, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

27. A thruster according to claim 7, including a main coil extending circumferentially with respect to the outer magnetic body.

28. A thruster according to claim 8, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

29. A thruster according to claim 1, characterized in that the channel forms a larger gap adjacent the steering means than elsewhere to reduce erosion of its outer surface.

30. A thruster according to claim 29, characterized in that the steering means allows a modification of the magnetic field independently along two axes located in a plane perpendicular to a geometric axis of the thruster.

31. A thruster according to claim 30, characterized in that the steering means comprises two pairs of magnetic coils, one on each of two opposite sides of the thruster, connected so that the electric current passes through the coils in dependence on the steering signal SIG.

32. A thruster according to claim 31, further including:
two pairs of magnetic coils arranged to control the direction of thrust in respective orthogonal directions.

33. A thruster according to claim 32, characterized in that the means for steering the magnetic field comprises inner and outer magnetic bodies defining opposite magnetic poles and located respectively to the inside and outside of the channel, the outer magnetic body extending around the channel andbeing formed with gaps so that the magnetic pole which it defines is divided into individual sub-poles each extending around an arcuate portion of the magnetic body and having an associated electrical coil, the steering means being effective to vary the respective currents through the different coils.

34. A thruster according to claim 33, in which the outer magnetic body includes a circular pole piece which bridges the said gaps.

35. A thruster according to claim 34, in which the outer magnetic body includes separate pole pieces arranged in a circle and serving partially tobridge the gaps but having spaces between them smaller in the circumferential direction than the gaps.

36. A thruster according to claim 35, including a main coil extending circumferentially with respect to the outer magnetic body.

37. A thruster according to claim 36, in which the gaps are slots extending in an axial direction along part but not all of the axial length of the outer magnetic body.

38. A satellite or other spacecraft or space platform comprising a thruster according to claim 1, means for producing a steering signal SIG
indicating a thrust vector required to be applied to the satellite, spacecraft or platform, and means for applying the steering signal to the steering means.

39. A satellite, spacecraft or space platform according to claim 38, comprising a main body and characterized in that the thruster is connected rigidly to the main body.

40. A satellite or other spacecraft or space platform comprising a thruster according to claim 2, means for producing a steering signal SIG
indicating a thrust vector required to be applied to the satellite, spacecraft or platform, and means for applying the steering signal to the steering means.

41. A satellite, spacecraft or space platform according to claim 40, comprising a main body and characterized in that the thruster is connected rigidly to the main body.

42. A satellite or other spacecraft or space platform comprising a thruster according to claim 29, means for producing a steering signal SIG
indicating a thrust vector required to be applied to the satellite, spacecraft or platform, and means for applying the steering signal to the steering means.

43. A satellite, spacecraft or space platform according to claim 42, comprising a main body and characterized in that the thruster is connected rigidly to the main body.