Abstract:
A plasma thruster includes a plasma chamber having first and second axial ends, the first of which is open, an anode located at the second axial end, and a cathode. The cathode and anode are arranged to produce an electric field having at least a component in the axial direction of the thruster. A magnet system including a plurality of magnets is spaced around the thruster axis, each magnet having its north and south poles spaced around the axis.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to plasma thrusters which can be used, for example, in the control of space probes and satellites. 
       BACKGROUND TO THE INVENTION 
       [0002]    Plasma thrusters are known which comprise a plasma chamber with an anode and a cathode which set up an electic field in the chamber, the cathode acting as a source of electrons. Magnets provide regions of high magnetic field in the chamber. A propellant, typicaly a noble gas, is introduced into the chamber. Electrons from the cathode are accelerated through the chamber, ionizing the propellant to form a plasma. Positive ions in the plasma are accelerated towards the cathode, which is at an open end of the chamber, while electons are deflected and captured by the magnetic field, because of their higher charge/mass ratio. As more propellant is fed into the chamber the primary electrons from the cathode and the secondary electrons from the ionization process continue to ionize the propellant, projecting a continuous stream of ions from the open end of the thruster to produce thrust. 
         [0003]    Examples of multi-stage plasma thrusters are described in US2003/0048053, and divergent cusped field (DCF) thrusters are also known. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a plasma thruster comprising a plasma chamber having first and second ends. The first end may be open. There may be an anode located at the second end. There may be a cathode. The cathode and/or the anode may be arranged to produce an electric field having at least a component in the axial direction of the thruster. The system further comprises a magnet system comprising a plurality of magnets. The magnets may be spaced around the thruster axis. Each magnet may have its north and south poles spaced from each other around the axis. The plurality magnets may comprise an even number of magnets with alternating polarity so that each pole of each magnet is adjacent to a like pole of the adjacent magnet. Each of the magnets may be orientated so that its poles are spaced apart in a direction perpendicular to the axial direction. 
         [0005]    The plasma thruster may further comprise a supply of propellant, which may be arranged to supply propellant into the chamber, for example at the second end of the chamber. 
         [0006]    At least one of the magnets may be an electromagnet arranged to produce a variable magnetic field. 
         [0007]    Indeed the present invention further provides a plasma thruster comprising a plasma chamber having first and second axial ends, the first of which may be open, an anode, which may be located at the second axial end, and a cathode, wherein the cathode and anode are arranged to produce an electric field which may have at least a component in the axial direction of the thruster, and a magnet system comprising a plurality of magnets located around the chamber so as to generate magnetic fields in the chamber, and wherein at least one of the magnets is an electromagnet arranged to produce a magnetic field which is variable. This may be arranged to vary the net direction or the net position of thrust of the thruster. 
         [0008]    Each of the magnets may be an electromagnet arranged to produce a variable magnetic field. 
         [0009]    The present invention further provides a plasma thruster system comprising a thruster according to the invention and a controller arranged to receive a demand for thrust, and to control the at least one electromagnet so that the thruster generates the demanded thrust. 
         [0010]    The controller may be arranged to generate a non-axial thrust by controlling the magnetic field generated by each of two adjacent magnets so that it is less than the magnetic field generated by each of at least two other magnets. 
         [0011]    Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a longitudinal section through a thruster according to an embodiment of the invention; 
           [0013]      FIG. 2  is a transverse section through the thruster of  FIG. 1 ; 
           [0014]      FIG. 3  is a diagram of the magnetic field in the thruster of  FIG. 1 ; 
           [0015]      FIGS. 4   a  and  4   b  show the effect on the magnetic field of reducing the current in one of the electromagnets of the thruster of  FIG. 1 ; 
           [0016]      FIGS. 5   a  and  5   b  show the effect on the magnetic field of reducing the current in two of the electromagnets of the thruster of  FIG. 1 ; 
           [0017]      FIGS. 6   a  and  6   b  show the distribution of electron density in the thruster of  FIG. 1  with equal current in all four electromagnets; 
           [0018]      FIGS. 7   a,    7   b  and  7   c  show the distribution of electron density, and the variation in thrust centre offset with axial distance from the channel exit, in the thruster of  FIG. 1  with reduced current in two of the electromagnets; 
           [0019]      FIGS. 8   a  and  8   b  illustrate alternative magnet arrangements to that of the thruster of  FIG. 1 ; and 
           [0020]      FIG. 9  shows the magnetic field in a thruster having a similar topology to that of  FIG. 8   b.    
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Referring to  FIGS. 1 and 2 , a plasma thruster comprises a plasma chamber  10  having four ceramic side walls  12  arranged symmetrically around the central axis Z of the thruster. One end  14  of the plasma chamber is open. At the other end  16  an anode  18  covers the end of the plasma chamber so that that end is closed. A cathode  20  is located at the open end  14  of the chamber  10  offset from the axis Z. The anode  18  and cathode  20  are therefore arranged to generate an electric field which extends generally in the axial direction of the thruster. A propellant inlet  21  is arranged to allow propellant to enter the chamber  10 . The propellant inlet  21  is located at the closed end of the chamber  10 , approximately on the Z axis. The inlet is connected to a supply of propellant which in this case is krypton, though other propellants such as argon and xenon can be used. 
         [0022]    Four electromagnets  22  are spaced around the plasma chamber  10 , each having its poles spaced apart from each other around the axis Z so that they are located at adjacent corners of the chamber  10 . The magnets are arranged perpendicular to the Z axis. They are aligned with each other in the Z direction, i.e. in a common X-Y plane. The polarities of the magnets  22  alternate, so that each has its north pole adjacent to the north pole of one of the adjacent magnets and its south pole adjacent the south pole of the other adjacent magnet. While straight magnets, parallel to the walls  12  of the chamber  10  could be used, in this embodiment the core of each magnet  22  has two straight arms  22   a,    22   b  joined together to form a right angle, and the magnet  22  is arranged such that each of the arms is at 45° to the chamber wall  12 . Each arm  22   a,    22   b  of each magnet is in the form of a plate which extends along substantially the whole of the length of the chamber  10  in the axial Z direction. Each of the electromagnets has a coil  24  wound around the arms  22   a,    22   b  of its core, and the coil is connected to a power supply which is controlled by a controller  26  so that the current through the coils  24  can be varied. The controller  26  is arranged to control the current in each of the coils  24  so as to control the strength of the magnetic field generated by each of the electromagnets  22 . The controller  26  is also arranged to control the other parameters of the thruster, such as the voltage of the cathode and anode and the supply of propellant. When the thruster is used to control the orientation of a probe or satellite, the controller  26  is arranged to receive a demand for thrust from a main controller and to control the current in each of the coils  24  so as to produce the demanded thrust. 
         [0023]    Referring to  FIG. 3 , in which the magnets  22  are shown but not the chamber walls  12 , if all of the electromagnets are generating an equal magnetic field, that field has four cusps  30 , each of which is located at a pair of adjacent and opposite poles of two of the adjacent electromagnets  22 , and a further central cusp  32  at the centre of the chamber  10  on the Z axis. Simulations show that this magnetic field pattern is reasonably constant along the length of the chamber  10 , and diverges gradually at the ends of the of the chamber. 
         [0024]    In operation, the anode  18  and cathode  20  set up an electric field approximately axially along the length of the chamber  10  in the Z direction, and electrons from the cathode  20  are therefore accelerated through the chamber  10  towards the anode  18 . As krypton propellant is introduced into the chamber  10 , the accelerated electrons ionize the krypton producing positive ions and further secondary electrons. The electrons, because of their relatively high charge to mass ratio, are deflected by the magnetic field in the chamber and tend to follow the magnetic field, while the positive ions are relatively unaffected by the magnetic field and are therefore ejected from the open end of the chamber  10  producing thrust. The chamber  10  therefore forms a thruster channel along which the ions are accelerated. It will be appreciated that varying the magnetic field within the chamber or channel  10  can be used to vary the electron density at different points across the channel  10 . It is anticipated that varying the magnetic field strength in different areas around the Z axis of the thruster can be used to provide thrust vectoring. 
         [0025]    Referring to  FIGS. 4   a  and  4   b,  simulations show that, if one of the four electromagnets  22  is turned off, the central cusp  32  of the magnetic field does not shift significantly from the centre of the channel  10 . However, referring to  FIGS. 5   a  and  5   b,  if two adjacent electromagnets are turned off, or redcued to 10% of the current of the other two, then the central cusp  32  of the magnetic field shifts significantly, towards one corner of the channel  10 . 
         [0026]    Referring to  FIGS. 6   a  and  6   b,  simulations show that, with all four electromagnets receiving equal currents, and the magnetic field therefore being symmentrical, the electron density shows a sharp peak at the cusp  32  in the magnetic field at the centre of the channel  10 . This peak radiates out in a cross configuration following the magnetic field lines towards the magnetic poles. The occurrence of this strong confinement of the electrons by the magnetic field, which is a result of the configuration of the magnets  22 , leads to a high ionization efficiency in the thruster and hence a high thrust efficiency. If electron temperature is simulated, the temperature follows the same pattern as the electron density, being highest at the central cusp  32 . 
         [0027]    Referring to  FIGS. 7   a  and  7   b,  if two adjacent magnets  22  are reduced to 10% of the strength of the other two, then the electron density peak shifts with the cusp  32  in the magnetic field, so that the peak is offset to one side of the Z axis of the thruster. Again, the electron temperature distribution shifts in the same way. 
         [0028]    From the results of the simulation discussed above and shown in  FIGS. 6   b  and  7   b  we can see that the plasma properties vary considerably across the channel for the case of a ‘steered’ magnetic field. This non-uniform distribution in electron density and temperature is expected to give rise to a non-uniform distribution of plasma potential, leading to an inclined electric field that will enhance thrust vectoring. However, in the worst case scenario the electric field will remain exactly parallel to the thruster Z axis, and the intensity of the ion beam will be relocated in a 2-dimensional x-y plane. 
         [0029]    Assuming the electric field is uniform across the channel, there will be a small amount of thrust vectoring from the action of ambipolar diffusion of the ion beam. As the ions are accelerated from the thruster chamber they will diverge at a theoretically predictable rate. In the case of a non-uniform beam, such as that of  FIG. 7   b,  this will result in a shift of the center of thrust varying with the axial distance from the chamber exit. If the center of thrust as a function of axial location from the channel exit is analysed, the results are as shown in  FIG. 7   c.  It can be seen from these results that in the worst case scenario there should be a beam vectoring capability of 30.5°, with a 8.4 mm offset of the center of thrust compared to the axis of the thruster, in a chamber with a 35 mm square cross section. It will therefore be appreciated that both the net position of the thrust and the net direction of the thrust can be varied under the control of the controller  24 . 
         [0030]    Referring to  FIG. 8   a,  in a further embodiment of the invention the chamber walls  82  are aligned with the arms of the magnets  84  so that the magnetic poles are located in the centre of each side of the ceramic chamber rather than in the corners of the ceramic chamber. 
         [0031]    Referring to  FIG. 8   b,  in a further embodiment of the invention each of the electromagnets  92  is in the form of a horseshoe magnet having two parallel arms  92   a,    92   b  joined by a backpiece  92   c.  This arrangement allows for more coil windings per magnet and therefore allows higher field strength to be generated for a given maximum electrical current. However the design is obiously bulkier and heavier than the design of  FIG. 2  or that of  FIG. 8   a.  The magnetic field in the design of  FIG. 8   a  is shown in  FIG. 8   b.  As would be expected, as shown in  FIG. 9 , the magnetic field within the chamber for the magnet topology of  FIG. 8   b  is similar to the design of  FIG. 2 , because the magnetic poles are located in the same place relative to the chamber  10 . 
         [0032]    While each of the embodiments described above has four magnets, it will be appreciated that other numbers of magnets can be used. For example six or eight magnets arranged in a simiar configuration, with alternating polarities around the Z axis, would produce similar peaks in electron density, and would be steerable in a similar manner. It will also be appreciated that the use of electromagnets to steer the thrust can be carried over to other thruster topologies in which the magnets are aligned differently.