The flexible magnetic field thruster

Abstract

The flexible magnetic field thruster is a unique research tool for studying the behavior of direct current electron-bombardment ion thrusters. It utilizes a long wire anode shielded by a circumferential magnetic field. The magnetic field around the anode, which is produced by passing a direct current of up to 150 A through the wire, restricts primary electron flow from the discharge plasma to the anode. Different magnetic field configurations (divergent, cusped, multipole, etc.) can be created by routing the anode wire in various ways through the discharge chamber. The thruster is also designed so ion currents to various internal surfaces can be measured directly. This allows the determination of the distribution of ion currents within the discharge chamber. Experiments indicate that the distribution of ion currents is strongly dependent on the shape and strength of the magnetic field but independent of the discharge current, discharge voltage, and neutral flow rate. Measurements of the energy cost per plasma ion indicate that this cost decreases with increasing magnetic field strength due to increased anode shielding from the primary electrons. Energy costs per argon plasma ion as low as 50 eV were measured. The energy cost per beam ion was found to be a function of the energy cost per plasma ion, extracted ion fraction, and discharge voltage. Part of the energy cost per beam ion goes into creating many ions in the plasma and then extracting only a fraction of them into the beam. The rest of the energy goes into accelerating the remaining plasma ions into the walls of the discharge chamber. Measurement of ion fluxes across a virtual anode surface appears to indicate that ions cross this surface with velocities approaching their random thermal velocity rather than the Bohm velocity. NomenclatureA =area, m 2 B = magnetic flux density, T E = electron energy, eV e = electronic charge, C / = extracted ion fraction J = current, A k = Boltzmann's constant = 1.38 x 10~2 3 J/K m =mass, kg n -plasma density, m~3 P = specific power, eV/ion r = radial position, m T = temperature, K v = velocity, m/s V = voltage, V ju 0 = permeability constant = 1.26 x 10 ~6 H/m Subscripts B =beam b = Bohm back = back surface of discharge chamber D = discharge E= emission e = electron / = magnetic field / = ion P = production or plasma S = virtual anode surface side = side surface of discharge chamber screen = screen grid w = all thruster cathode potential surfaces