Patent Publication Number: US-9854660-B2

Title: Ion accelerators

Description:
FIELD OF THE INVENTION 
     The present invention relates to ion accelerators. Its primary application is in plasma thrusters, for example for use in the control of space probes and satellites, but it also has application in chemical vapour deposition (CVD), in lighting systems that require a source of plasma. 
     BACKGROUND TO THE INVENTION 
     Plasma thrusters are known which comprise a plasma chamber with an anode and a cathode which set up an electric field in the chamber, the cathode acting as a source of electrons. Magnets provide regions of high magnetic field in the chamber. A propellant, typically 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 electrons 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. 
     Examples of multi-stage plasma thrusters are described in US2003/0048053, and divergent cusped field (DCF) thrusters are also known. 
     SUMMARY OF THE INVENTION 
     The present invention provides an ion accelerator comprising a first magnet, which may be an inner magnet, and which may have a channel extending through it, for example in an axial direction, and second magnet, which may be an outer magnet, and may extend around the first magnet, the magnets having like polarities so as to produce a magnetic field having two locations of zero magnetic field strength. The locations may be spaced apart, for example in the axial direction. The accelerator may further comprise an anode and a cathode, which may be arranged to generate an electrical potential difference between the locations. 
     The channel may have a central axis. For example it may by cylindrical. The central axis may be an axis of rotational symmetry. One of the locations may be a line that extends around the central axis. One of the locations may be a point. The location that is a point may be forward of the other so that ions will tend to converge when moving between the locations. 
     One of the electrodes, which may be the anode, may be located radially between the inner and outer magnets. This electrode may include a tubular portion which may have an inner diameter greater than the outer diameter of the inner magnet, and an outer diameter less than the inner diameter of the outer magnet. One of the electrodes, which may be the cathode, may be located radially inside the inner magnet, and may be located on, or around, the central axis. 
     The channel may have an inlet end and an outlet end. These ends may be at respective poles of the inner magnet. The outer magnet may extend around at least a part of the inner magnet, and may have an inlet end and an outlet end, which may be at respective poles of the outer magnet. The inlet ends of the two magnets may be of like polarity. The magnets may be of annular cross section. 
     The accelerator may further comprise a housing which may be arranged to support either one or both of the magnets. The accelerator may further comprise a heat sink which may be thermally connected to any one or more of the inner and outer magnets and the housing. 
     The present invention further provides an ion thruster comprising an accelerator according to the invention and a propellant source arranged to feed propellant into the accelerator. The propellant source may be arranged to feed propellant to the cathode. Alternatively or in addition the propellant source may be arranged to feed propellant into a space between the inner and outer magnets. 
     The accelerator may include any one or more features, in any combination, of any one or more of the embodiments of the present invention which will now be described by way of example only with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially cut-away perspective view of an ion accelerator according to an embodiment of the invention; 
         FIG. 2  is a diagram of the magnetic field in the accelerator of  FIG. 1 ; and 
         FIG. 3  is a diagram of the magnetic field in an accelerator of a second embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , an ion accelerator, which in this case forms part of a plasma thruster, comprises an inner magnet  10  and an outer magnet  12 . Each of the magnets  10 ,  12  is in the form of a hollow cylinder or tube, and the magnets are arranged coaxially with the inner one  10  being located inside the outer one  12 . The inner and outer magnets overlap in the axial direction so that the outer magnet  12  surrounds a part, and in the embodiment shown, all, of the inner magnet  10 . A housing  14  supports the magnets  10 ,  12  and comprises an outer annular wall  16  which covers the annular end  18  of the outer magnet  12  at the front end  20  of the thruster, an outer cylindrical wall  22  which is just inside the outer magnet  12  and extends along its length beyond its rear end  24 , a rear annular wall  26  extending inwards from the rear end of the outer cylindrical wall  22 , a middle cylindrical wall  28  extending forwards from the inner edge of the rear annular wall  26  and extending along the outer surface of the inner magnet  10 , an inner annular wall  30  extending inwards from the front end of the middle cylindrical wall  28 , covering the front end of the inner magnet  10 , and an inner cylindrical wall  32  extending rearwards from the inner edge of the inner annular wall along the inner surface of the inner magnet  10 . The inner cylindrical wall  32  surrounds and defines within it a channel  34  which extends through the centre of the inner magnet  12 , and a hollow cathode  36  is located at the rear end of the channel and arranged to generate plasma and introduce it into the channel  34 . A tubular anode  38  is located in the space between the outer and middle cylindrical walls  22 ,  28 , with its front end just forward of the front end of the inner magnet  10 , and well behind the front end of the outer magnet  12 . The anode, or the tubular portion of it, has an inner diameter greater than the outer diameter of the inner magnet  10 , and an outer diameter less than the inner diameter of the outer magnet  12 . The cathode  36  and anode  38  are arranged to set up the electrostatic field required for the accelerator to operate as described below. In other embodiments the cathode for providing the electrostatic field can be separate from the plasma source. 
     The rear ends of the two magnets  10 ,  12  are aligned with each other in the axial direction, and the outer magnet  12  is longer than the inner magnet  10  and extends forward of the front end of the inner magnet. The region inside the front end of the outer magnet  12  and forward of the inner magnet  10  forms a chamber  40  in which plasma generation and ion acceleration takes place as will be described in more detail below. The housing  14  shields the magnets  10 ,  12  from the channel  34  and plasma chamber  40 . At the rear end of the accelerator a heat sink  42 , in this case in the form of a copper block, is located against, and in thermal contact with, the rear end of the housing  14  and the rear ends of the inner and outer magnets  10 , 12 . The heat sink  42  has an aperture through which the hollow cathode  36  can be inserted and through which gas can be supplied to the hollow cathode  36 . Four propellant channels  44  are provided extending radially through the heat sink  42  and connect to apertures  46  in the housing, in the rear end of the outer cylindrical wall  22 . As the anode  38  is spaced from the outer and middle cylindrical walls  22 ,  28 , propellant introduced into these propellant channels  44  can flow into the space between the outer and middle cylindrical walls  22 ,  28 , and therefore between the inner and outer magnets  10 ,  12 , past the anode  38 , and into the main plasma chamber  40 . 
     In operation, the general principle of the accelerator is similar to known accelerators. The anode  38  and cathode  36  set up an electric field which accelerates electrons and ions in the plasma chamber  40 . The accelerated electrons ionize the propellant introduced into the chamber  40  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 therefore tend to travel in a direction dictated by the electric field. 
     Referring to  FIG. 2 , the polarities of the inner and outer magnets  10 ,  12  are in the same direction. For example if the front end of the outer magnet  12  is its north pole and the rear end is its south pole, then the front end of the inner magnet  10  is also its north pole, and the rear end is its south pole. The polarities are therefore opposed to each other, and not complementary as they would be if the polarities were opposite to each other. This sets up a complex magnetic field having a point  50  of zero magnetic field located on the central axis of the accelerator and forward of the front end of the outer magnet  12 , and a line  52  of zero magnetic field that is circular and extends around the central axis just forward of the front end of the inner magnet  10 . A similar zero point  54  and zero line  56  are set up to the rear of the magnets  10 ,  12  but these are not relevant to the operation of the accelerator. 
     As is well understood by those skilled in the art, in a plasma, magnetic fields act as an electrical resistance to electrons trying to move perpendicular to them, as the electrons are deflected by the magnetic field, but lines which do not have significant magnetic field perpendicular to them have low electrical ‘resistance’ and therefore can be considered to act as ‘conductors’ as electrons can move relatively freely along them. Therefore it will be appreciated that the zero point  50  at the forward end of the accelerator is held at an electrical potential close to that of the cathode, because of the ‘channel’ of low transverse magnetic field between it and the cathode. Similarly the line  52  of zero magnetic field is held at a similar electrical potential to the anode, as there is little magnetic field transverse to the direction between them and a similar ‘channel’ of low transverse field can be seen between the front end of the anode  38  and the zero line  52 , so electrons can move relatively freely between them. 
     Another effect that is well known to those skilled in the art and relevant to the operation of the accelerator is that a high degree of ionization, and therefore a high density of ions, tends to occur at points of zero magnetic field. This is because the magnetic field around such points tends to enclose the electrons and prevent them from moving away. 
     In the accelerator shown, when it is in operation, plasma is introduced into the channel  34  from the hollow cathode and the electrons and ions are accelerated due to the electric fields in the channel and plasma chamber  40 . The electrons tend to cause further ionisation of any propellant that is added into the plasma chamber  40  thereby replacing any ions and electrons that leave the chamber. The positively charged ions accelerate towards regions of low electrical potential. As there is a lot of ionisation taking place in the region of the zero field line  52 , a large number of positive ions are accelerated from the region around that line, which is in the shape of a torus, towards the zero field point  50 . This forms a converging stream of ions moving towards the front end of the accelerator. As the electric field strength in front of the zero point  50  is relatively weak, the positive ions are not significantly decelerated after passing the zero point  50  and form a continuous stream of ions ejected forwards from the front end of the accelerator. Meanwhile electrons gradually move towards the anode  38  and are collected there. 
     While this arrangement can be used to generate ion beams for many applications, in this embodiment as the accelerator forms part of an ion thruster, propellant can be introduced into the plasma chamber  40  via the inlet channels  44  during operation of the accelerator to keep up a continuous beam of ions which produce thrust. Other configurations of propellant supply could of course also be used. In other applications of the ion accelerator, the hollow cathode may be able to provide sufficient plasma and a separate supply of gas for ionisation may not be necessary. 
     In still further embodiments, the hollow cathode is replaced by a simple cathode and the only supply of gas is via the inlet channels  44 . 
     It will be noticed that the magnetic field forward of the zero point  50  is in approximately parallel to the direction of travel of the ion beam. This helps to contain the ion beam as the positive ions tend to follow the magnetic field direction, though to a much lesser extent than the electrons due to the difference in charge to mass ratio. 
     It will be appreciated that the geometry of the accelerator can be modified in many ways. For example the zero point  50  and zero line  52  at the front end of the accelerator are spaced apart in the axial (forward/backward) direction much more than those  54 ,  56  to the rear of the accelerator. This is because the front ends of the inner and outer magnets  10 ,  12  are not level, in the axial direction, with the front end of the outer magnet  12  being forward of the front end of the inner magnet  10 , whereas their rear ends are level in the axial direction. It will be understood that the relative lengths and axial positioning of the two magnets, and their relative size, can be selected so as to achieve the axial spacing of the two regions of zero magnetic field and their relative size, suitable for a particular application. For example the inner and outer magnets can in some cases be of equal length. In some cases their front ends can be approximately level in the axial direction. However this means that the axial offset between the two zero field regions will be less than in the embodiment of  FIG. 1 . 
     Referring to  FIG. 3 , in a further embodiment the positions of the inner and outer magnets  110 ,  112  is the same as that of the first embodiment, but the relative strengths is different, in this case the inner magnet being stronger than the outer magnet. This results in a magnetic field pattern that still includes a zero point  150  on the central axis of the accelerator and a zero line  152  in the form of a ring around that axis, but in this case the ring is forward of the point  152 . Therefore, for the accelerator to accelerate positive ions in the forward direction, the electrode  138  that is radially between the inner and outer magnets  110 ,  112 , is the cathode, and an anode is placed on or around the central axis and radially inside the inner magnet  110 . The resultant ion beam is divergent which may be desirable in some circumstances.