EP1161855A1 - An ion accelerator - Google Patents

Abstract

In a Hall effect plasma accelerator (1) the propellant gas is introduced into an annular accelerating channel (2) in the region of an anode (9), asymmetrically around the channel. Ionisation of propellant gas and acceleration of the resultant ions out of the channel causes a deflection or displacement in the resultant thrust direction (F) thereby creating a steering or turning effect. <IMAGE>

Description

AN ION ACCELERATOR

This invention relates to an ion accelerator. It arose in connection with the design of a

Hall effect plasma accelerator, also known as a closed electron drift accelerator. Such

accelerators are used as thrusters on satellites or other spacecraft to assist in adjusting

their orientation or position or both when in orbit around the earth, or to move them into

or out of a particular orbit. They could also be used for propelling spacecraft during

long missions.

A conventional Hall effect accelerator comprises an accelerating channel which is

usually annular and which extends circumferentially around a central axis of the

accelerator and which also extends in an axial direction from a closed end to an open

end. An anode is located usually at the closed end of the channel and a cathode is

positioned outside the channel close to and at one side of its open end. An electric field

is generated by the potential difference between the anode and the cathode. A

propellant, for example xenon gas, is introduced into the channel. This is often done

through passages formed in the anode itself or close to it. A magnetic system applies

a magnetic field in a radial direction across the channel which causes electrons emitted

from the cathode to move circumferentially around the channel. Some but not all of the

electrons emitted from the cathode pass into the channel and are attracted towards the

anode. The radial magnetic field deflects the electrons in a circumferential direction so

that they move in a helical trajectory, accumulating energy as they drift towards the

anode. In a region close to the anode the electrons collide with atoms of the propellant,

causing ionisation. The anode serves as a collector of the electrons causing, or caused

CONFIRMATION COPf by, such collisions. The resulting positively charged ions are accelerated by the electric

field towards the open end of the channel, from where they are expelled at great

velocity, thereby producing a thrust. Because the ions have a much greater mass than

the electrons, they are not so readily influenced by the magnetic field and their direction

of acceleration is therefore primarily axial rather than circumferential with respect to the

channel. However, the magnetic field does exert some force on the ions, and thus some

influence on their direction of movement. As the ions leave the open end of the channel

they are neutralized by those electrons from the cathode that do not pass into the

channel.

In referring to Hall effect accelerators the terms "upstream" and "downstream" are used

for convenience to describe directions with reference to the movement of ions in the

channel. In addition the term "axial" is used to describe a direction parallel to the

central axis and "radial" is used to describe a direction perpendicular to the central axis.

Conventional Hall effect accelerators are generally designed to produce a thrust vector

in an axial, fixed, direction. Therefore, to steer a spacecraft either two thrusters are used

so that the relative thrusts produced by them can be changed, or a swivelling mechanism

is used to swivel a single thruster relative to the spacecraft. The use of two thrusters is

expensive and increases weight and the swivelling mechanism is heavy, complex,

expensive and prone to failure.

It has been proposed in EP 0 778 415 to steer a thruster by varying distribution of the

magnetic field to create circumferential non-uniformity of the magnetic field around the open end of the channel. This approach has been shown to work well. However there

are some difficulties. The asymmetry of the magnetic field distribution leads to a

decrease in efficiency, a small but significant increase in operating current and some

erosion of the channel walls. Furthermore, operating parameters of the thruster may

become less stable and oscillations may result. To deal with the problem of erosion, the

walls of the channel can be flared outward at its open, downstream end but this may

further reduce the efficiency of operation of the thruster. It has been shown that this

magnetic technique can deflect the thrust vector by an angle of about three degrees from

its axial direction. However, beyond this amount the thruster characteristics may

deteriorate significantly.

During the testing certain Hall effect accelerators it was noted that, even with a

symmetrical magnetic field, there was a deviation of the thrust vector from the axial

direction. This was at first assumed to be an angular displacement and the most likely

explanation for it was thought to be asymmetry of the cathode with respect to the central

axis. However this proved not to be correct and eventually it was found that the

deviation of the thrust vector from the axial direction was caused by a non-uniform

distribution of the propellant within about the channel. Thus the sum of the forces

applied by the electric field to ions on a side of the accelerator having a relatively high

amount of propellant would be greater than the sum of the forces at the opposite side.

This led to an idea of deliberately producing an asymmetry in the strengths of the forces

to produce a steering effect; either on a dynamic basis during operation or between

successive operations; or as a permanent feature of the engine to compensate for other

inaccuracies of manufacture. Thus, according to the invention there is provided an ion accelerator comprising means

for introducing propellant into an ionization region, means for ionizing the propellant

and means for producing an electric field so as to apply forces to the ions and to

accelerate them in a desired direction characterised by adjustment means for varying a

distributioD of the forces, which distribution is lateral with respect to the direction of

acceleration, thereby displacing a resultant reactive force applied to the accelerator.

By deflecting the resultant reactive force on the thruster in this way it is believed it may

be possible to create an even greater steering effect (up to 6 degrees from a central axis)

than can be obtained using the known magnetic method and with less reduction in

efficiency. The problems of non-stability and oscillations can also be expected to be

reduced.

The force applied to accelerate each ion results in a corresponding equal and opposite

reactive force on the accelerator. The sum of these forces is equivalent to an imaginary

single force acting on a particular point in a particular direction. This imaginary force

is referred to as the "resultant" force. The effect of the invention is that by changing the

distribution of the propellant this resultant force is displaced or deflected in some way.

It is believed that this will primarily be a lateral deflection (i.e. its direction remains

constant but its point of application, i.e. the centre of force is changed). However, the

interaction between the ions may be such as to cause an angular deflection. The process

is not yet entirely understood in this respect and the term "displacing" when used in this

specification is to be understood as embracing either a lateral movement of a centre of

force, an angular deflection of the resultant force or a combination of both. The "adjustment means" can work on a number of different possible principles. One

possibility is to control the distribution of propellant in the ionization region so as to

produce a circumferentially non-uniform distribution of ions. This can be achieved by

using separate inlets for propellant, preferably three or more, distributed around a central

axis of the accelerator and feeding propellant at different rates to these inlets under the

control of suitable valves. Other possibilities include the use of a moveable inlet or

inlets or the use of baffles, deflectors or nozzles to deflect the propellant in a

controllable way so as to produce areas of the ionization region where there are different

concentrations of propellant for ionization. It will be understood that, in an area where

there is a greater concentration of propellant, more ions will be generated and therefore

the aggregate force applied to them will be greater than in an area where there is a lesser

concentration. The distribution of propellant may either be controlled by controlling the

way in which it is introduced into different parts of the ionization region or by

redistributing it after introduction. The preferred possibility is to control the rate at

which the propellant is introduced. If the distribution of the propellant is to be

controlled after introduction into the ionization region, the control can take effect either

before or after ionization. When it is to be controlled after ionization an electric field

extending laterally with respect to the direction of acceleration could be used for the

purpose of such control. This could possibly be done by applying different positive

potentials to different circumferentially spaced anodes.

A second possible principle of operation of the adjustment means is to feed different

propellants, or propellants mixed in different proportions to different parts of the

ionization region. Typical suitable propellants are xenon, krypton and argon. A third possibility is to control the relative electric field strengths at positions spaced

laterally with respect to the direction of acceleration. This latter principle can easily be

employed in combination with the first mentioned principle by using an anode structure

serving also as a propellant inlet. Such a structure can be formed by a number of

separate parts arranged around an axis of the accelerator with propellant being

controllably fed to each individual part and the potential of different anode parts being

individually controlled. Alternatively more than one cathode (preferably at least three)

may be spaced around an axis of the accelerator.

Although the invention is considered to be of special value when applied to Hall effect

accelerators it may also be applicable to other ion accelerators where there is a need to

displace the plume of ions. Also, although the mvention was devised when considering

the design of a thruster where the deflection of the force will produce a turning effect,

the invention could possibly also find application in accelerators used for ion cleaning,

ion milling, deposition of coatings or in vacuum processing to change surface

characteristics of metals or other materials and where, for some reason, there is a need

to control the lateral position or direction of the centre of a plume of ions. Reference

to "propellant" and "reactive force" when used in this specification should not therefore

be interpreted as implying that the accelerator is used to "propel" a space thruster.

The propellant, which is typically xenon gas, is preferably introduced through or in the

region of an anode. An axial electric field may be applied, as is conventional, between

the anode normally located inside an accelerating channel and one or more cathodes

located outside the channel close to its open end. The anode may be formed from two or more separate units or compartments into which propellant is supplied, each

compartment having an associated control which regulates the rate of supply of

propellant and/or the type of propellant mixture which it receives. Propellant may be

supplied into the ionizing region through a single outlet or a plurality of outlets,

provided in each compartment.

When the invention is applied in the construction of a Hall effect thruster it is proposed

that an effect similar to that described in patent specification 0 778 415 could be used

by which a magnetic field at the downstream end of the channel is tilted at an angle α

(preferably not more than 5° or 10°) to a plane perpendicular to the central axis thereby

causing the ions to initially converge as they exit from the channel. In this way any

abrasion against the edge of the channel wall at its downstream end can be reduced

without the need for that edge to be made with a flared shape which would reduce

efficiency. The deflection (at the outside of the converging cone of ions) should

preferably by at least 1 ° or 2° and preferably more than 3 ° . In one embodiment the

deflection is 5° to 10°. In a preferred arrangement the magnetic field is normally

circularly symmetrical about the central axis but can be varied so as to become non-

symmetrical (when desired) so as to add to the steering effect of the invention.

In a Hall effect thruster employing the invention, a number of individually controllable

sources of magnetic field may be used to vary the magnetic field so as to control the

direction of the resultant reactive force.

In a Hall effect thruster, the ionization region will normally be bounded by a ceramic material because of the high temperatures which are generated. It preferably has a

circular cross-section in the perpendicular plane, although other, non-circular

configurations are possible. For example, where there are a number of coils or

permanent magnets arranged around the outside of the ionization region, there can be

an advantage in making it wider in regions adjacent those coils or permanent magnets.

Although the invention is expected to find its principal value creating a steering effect

during operation, or between successive operations of a thruster, it can also be used

simply to correct for inaccuracies of manufacture of assymetries which might occur after

manufacture and which would create an undesired displacement of the resultant thrust

vector from the axial direction. Thus, according to a second aspect of the invention

there is provided an ion accelerator comprising means for introducing propellant into

an ionization region 4, means for ionizing the propellant and means for producing an

electric field so as to apply forces to the ions and to accelerate them in a desired

direction characterised by means 43, 44, 45 for adjusting and/or creating an

asymmetrical distribution of propellant in the ionization region so as to reduce any

deviation of a resultant force F from an axis 3 of the accelerator.

An embodiment of the invention will now be described, by way of example, with

reference to the accompanying drawings in which:

Figure 1 shows a plan view of a thruster according to the invention (a cathode not being

shown); Figure 2 shows a cross-sectional view of the thruster through plane A- A of Figure 1 ; and

Figure 3 shows a schematic view of a control system for the thruster of Figures 1 and

2.

A thruster 1 constructed according to the invention comprises an annular accelerating channel 2 having a central axis 3, extending in an axial direction from a closed, upstream, end 4 (defining an ionization region) to an open, downstream, end 5. The channel 2 is made of a suitable refractory material such as boron nitride. It has an inner wall 6, an outer wall 7 and a floor 8 which closes off the channel 2 to form the closed end 4. Located adjacent the closed end 4 is an anode 9. The anode 9 is made of a suitable refractory metal such as molybdenum. In addition to being a source of positive potential it is also used as a means to introduce propellant into the channel 2. Located adjacent the open end 5 outside the channel 2 is a cathode 10. The cathode 10 is typically of a hollow configuration containing a suitable thermo-emitter.

A magnetically permeable soft metal yoke 11 applies a magnetic field 12 in a radial direction across the channel, its maximum strength being close to the open end 5. The magnetic yoke 11 comprises an inner tube 13 located radially inwardly of the inner wall 6, three outer rods 14, 15 and 16 located radially outwardly of the outer wall 7 and a base plate 17. The rods 14, 15, 16 may in an alternative construction be replaced by curved upstanding walls running parallel to arcuate sections of the channel 2.

The inner tube 13 terminates with a radially extending flange, or pole-piece, 18 forming a magnetic South pole and the rods 14, 15, 16 terminate with flanges, or pole-pieces, 19 foπning magnetic North poles. A coil 20 is wound on the tube 13 so that current passes in a clockwise direction as viewed from downstream and coils 21, 22, 23 are wound on the rods 14, 15, 16 so that current passes in an anticlockwise direction as viewed from downstream.

The outer rods 14, 15, 16 and coils 21, 22, 23 are identical in the sense that they produce magnetic fields having the same magnitude and direction when the coils 21, 22, 23 are energised with the same current. In the illustrated embodiment of the invention, gaps 24 are provided between adjacent pole-pieces 19 so as to allow independent magnetisation to be applied to each pole-piece 19. In this way different magnetic fields can be applied to different 120 degree sectors of the channel 2.

A tubular inner magnetic shield 25 (Fig. 2) is located between the inner wall 6 of the channel 2 and the inner coil 20 and a tubular outer magnetic shield 26 is located between the outer wall 7 of the channel 2 and the outer coils 21, 22, 23. The shields 25 and 26 are fixed to the base plate 17. They serve to reduce the magnetic field in the channel in the region of the anode 9.

The magnetic yoke 11 comprising the tube 13, the rods 14, 15, 16, the base plate 17, the pole piece 18, 19, and the shields 25 and 26 is made of a magnetically soft material. In the illustrated construction it is shown as made of a single piece of material, but in practice it would be formed by several parts connected together. The anode 9 has a circular configuration and lies along the bottom of the channel 2. It

has a dual function of providing a source of positive potential and as a plenum or

distributor to supply propellant into the channel 2. The anode 9 is in the form of a

hollow rectangular section tube which is divided into three adjacent 120° degree

compartments 27, 28 and 29 by end walls 30, each compartment being aligned

circumferentially with a respective pole-piece 19. Although the anode 9 may be a single

unitary piece comprising the three compartments, it is preferred that the compartments

are separate arcuate pieces which are assembled together to provide the complete anode

9.

Each compartment has a single inlet pipe 31 and a single outlet 27 A, 28A and 29A in

the form of a slot extending along its arcuate length. Propellant is supplied from the

pipes 31 into the compartments and around baffles 9B located inside the compartments

to distribute the propellant uniformly to all parts of the outlet slots 27 A, 28 A or 29 A.

An electrical connection 32 supplies a positive potential to the anode compartments 27,

28 and 29.

The cathode 10 is mounted close to the downstream end of the channel 2 and is supplied

with xenon gas through a pipe 33 and with a source of negative potential via electrical

connection 34.

Figure 2 shows lines of magnetic field 12 generated when current passes through the inner coil 20 and the outer coils 21, 22, 23. If the outer coils 21, 22, 23 are carrying

equal current, the magnetic field is symmetrical about the central axis 3. It can be seen from Figure 2 that there is an offset in the axial direction between the inner pole-piece

18 and the outer pole-pieces 19. This offset results in the magnetic field 12 being tilted

at an angle α to a plane perpendicular to the central axis 3 in an annular zone 35 close

to the downstream end of the channel where, in operation, the ions are accelerated.

Figure 3 shows in schematic form a control system. A digital signal on line 36, derived

from an attitude sensor (not shown), is compared in an error detector 37 with a similar

signal on line 38 indicating a desired attitude of the central axis 3. The output of the

error detector 37 defines the angular adjustment required, in magnitude and direction,

and is applied to processors 39, 40, 41 which respectively control: the supply of

propellant to anode compartments 27, 28, 29; the voltages applied to the anode compartments; and the currents through coils 21, 22, 23.

Propellant is supplied by a propellant supply 42 to a set of digitally operated valves 43,

44, 45 which independently control the amount of propellant entering the pipes 31 as

determined by the output of the processor 39. This processor calculates the amount of

opening of each valve required to achieve a thrust deflection in the direction indicated

by the output of the error detector 37. The propellant supply 42 also supplies propellant

on line 33 to the cathode 10.

A power supply 46 is connected by line 34 to the cathode 10 and via line 34 to three

voltage controllers 47, 48, 49 to which it supplies a high voltage relative to the cathode

10. The voltage controllers independently control the voltages applied on lines 32 as

determined by the output of the processor 40 which operates in a manner analogous to that of processor 39.

Power supplied on Une 50 is distributed to coil 20 and coils 21, 22, 23 the current

supplied to coils 21, 22, 23 being controlled by the processor 41 in a manner analogous

to the operation of the processors 39 and 40.

Operation of the thruster 1 is as follows. Electrons are emitted from the cathode 10 and

are divided into two streams. One stream of electrons is effective to neutralise ions as

they are expelled from the thruster so as to avoid leaving a resultant negative charge on

the thruster. The other stream is attracted into the channel 2 towards the anode 9. The

radial component of the magnetic field within the channel 2 causes these electrons to

travel circumferentially as they drift towards the anode 9. In the closed, upstream, end

4 of the channel 2 there is only minimal magnetic field because of the magnetic

screening effect of the shields 25 and 26, and the electrons, having acquired energy

during their helical movement along the channel, cause ionization of the propellant

supplied through the anode 9.

The resulting ions, which are positively charged, are accelerated in a downstream

direction by an electric field produced by a potential difference of about 300 volts,

between the anode 9 and the cathode 10.

The magnetic field lines in the accelerating zone 35 are inclined at an angle α to the

plane perpendicular to the central axis 3. This causes the ions to leave the channel

initially in directions which define a converging cone 47, thus limiting erosion of the edges of the channel 2 at its downstream end. The angle α is about 5 to 10 degrees in

the illustrated embodiment (shown exaggerated in the drawing) but a useful effect can

be obtained for α values of between as little as 2.5 to 3 degrees. Erosion of the edge of

the inner wall 6 is reduced by the fact that it does not extend as far in the upstream

direction as the corresponding, opposing, edge of the outer wall 7.

When the steering signal on line 36 indicates that central axis 3 is aligned with the

desired centre of thrust, the processors 39, 40, 41 operate so as to cause the pipes 31 to

carry substantially equal rates of flow of propellant, the anode sectors 9 to carry

substantially equal voltages and the coils 21 to carry substantially equal currents. This

will result in the plume of ions having an axis which is coincident, in direction and position, with the central axis 3. The processors may be set, during an initial trimming

operation to a datum state where there are slight deviations in these flow rates, voltages

and currents to compensate for inaccuracies of manufacture.

When the output of the error detector 37 indicates that a steering manoeuvre is required,

the supply of propellant through the pipes 31, and thus into each of the anode

compartments 27, 28 and 29, is varied by the processor 39 so as to provide a

substantially non-uniform distribution of propellant circumferentially around the

channel. This deflects the thrust vector laterally from the central axis 3 to provide a

turning effect in the desired direction. At the same time the processor 40 causes

different potentials to be applied by controllers 47, 48, 49 to the anode compartments

27, 28, 29 with a similar effect; and the processor 41 causes the magnetic fields

generated by the coils 21, 22, 23 to be varied so as to tilt the resultant thrust vector relative to the central axis 3, but without lateral displacement.

Referring to Figure 2, it will be understood that, for example, an increase in propellant

flow or voltage applied to the anode compartment 27, relative to each of the

compartments 28 and 29, will result in a displacement of a resultant reactive force F

from alignment with the central axis 3 to a position as shown at F thereby causing an

anticlockwise turning moment to be applied about a point, such as point P, on the axis

of the thruster; and vice versa as indicated by F₂. Similarly an increase in the current

through the coil 21 relative to the coils 22 and 23 will cause an angular deflection of the

ions to the right, as viewed on Figure 2, thereby deflecting the reactive force to the left

as shown at F₃.

The processors 39, 40, 41 can be programmed to calculate their output signals according

to a predetermined algorithm, or alternatively may use a look-up table or equivalent

containing a record of control signal values found empirically, to give the required effect

in response to different error signals.

It will be appreciated 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, rather than the invention

being applied to so-called stationary plasma thrusters which have channel walls

comprising dielectric material, the invention is also applicable to the so-called anode

layer thrusters which have metal channel walls. Although the foregoing describes a

thruster having three independently controllable supplies of propellant into the accelerating channel there may be more or less than three. The thruster may only be

provided with steering by varying the distribution of propellant in the channel.

Magnetic steering may be omitted. In yet another embodiment of the invention one or

more outlets may be provided which are circumferentially movable about the central

axis, that is relative to the floor of the accelerating channel. In such an embodiment

only one outlet would be needed to provide thrust in any radial direction although more

than one could be used.

Claims

CLAIMS 1. An ion accelerator comprising means for introducing propellant into an

ionization region (4), means for ionizing the propellant and means for producing

an electric field so as to apply forces to the ions and to accelerate them in a

desired direction characterised by adjustment means (39, 43, 44, 45; 40, 47, 48,

49) for varying a distribution of the forces, which distribution is lateral with

respect to the direction of acceleration, thereby displacing a resultant reactive

force (F) applied to the accelerator.

2. An accelerator according to Claim 1 in which the adjustment means comprises

means (39 43, 44, 45) for controlling the distribution of propellant in the

ionization region (4).

3. An accelerator according to Claim 1 or Claim 2 characterised in that the

adjustment means includes means (43, 44, 45) for controlling the rate at which

the propellant is introduced into different parts of the ionization region (4).

4. An accelerator according to Claim 2 or Claim 3 comprising two or more

propellant inlets (27 A, 28A, 29A) associated with different parts of the

ionization region (4).

5. An accelerator according to any preceding claim characterised in that the

adjustment means includes means (47, 48, 49) for controlling the relative

electric field strengths at positions spaced laterally with respect to the direction (3) of acceleration.

6. An accelerator according to any preceding claim characterised in that the

adjustment means includes means for controlling the relative composition of

propellant at positions spaced laterally with respect to the direction of

acceleration.

7. An accelerator according to any preceding claim characterised by an accelerating

channel (2), an anode (9) in the channel (2), a cathode (10) downstream of the

anode (9) and means (20, 21) for applying a magnetic field in a region (35) close

to the open end.

8. An accelerator according to Claim 7 characterised in that the means (20, 21) for

applying the magnetic field has associated with it, means (41) for variably

controlling the field so as to create asymmetry of the field about an axis (3) of

the channel (2) so as to deflect the resultant reactive force (F).

9. An accelerator according to any preceding claim comprising means (37) for

generating a steering signal and arranged to apply that signal to the adjustment

means so as to displace the reactive force (F) during operation of the accelerator

or between successive operations.

10. An ion accelerator comprising means for introducing propellant into an

ionization region (4), means for ionizing the propellant and means for producing an electric field so as to apply forces to the ions and to accelerate them in a

desired direction characterised by means ( 43, 44, 45) for adjusting and/or

creating an asymmetrical distribution of propellant in the ionization region so

as to reduce any deviation of a resultant force (F) from an axis (3) of the accelerator.