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 referring to fig3 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 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.