15 cm mercury multipole thruster

A 15-cm multipole ion thruster was adapted for use with mercury propellant. During the optimization process, the magnetic field was found to: 1) define the region where the bulk of ionization takes place; 2) influence the magnitudes and gradients in plasma properties in this region; and 3) control impedance between the cathode and main discharge plasmas in hollow cathode thrusters. The mechanisms for these functions are discussed. Data from SERT II and cusped magnetic field thrusters are compared with those measured in the multipole thruster. The performance of this thruster is shown to be similar to that of the other two thrusters. Means of achieving further improvement in the performance of the multipole thruster are suggested. that the magnetic field configuration within an ion thruster discharge chamber plays a prominent role in determining thruster performance. It has been found1 that in addition to containing primary electrons long enough to provide significant ionization probability, a magnetic field which diverges toward the screen electrode provides more uniform ion density there. It was suggested by King, et al., 5 and substantiated by Beattie3 that a critical field line passing from the downstream end of the cathode pole piece to the anode pole piece, when revolved about the thruster axis to form a surface of revolution, represents the upstream boundary of the region within which the bulk of the propellant ionizations occur. The screen grid forms the downstream boundary of this region which is commonly called the primary electron region. The distance between the surface of revolution of the critical field line and the screen grid at a given radius deter- mines the relative ionization probability for a neutral propellant atom at that radius. Beattie2 also found that there was a strong interrelationship between the proximity of the anode to that critical field line and the plasma properties obtained in the discharge chamber, particularly Maxwellian electron temperature. Isaacson and Kaufman6 reported a multipole magnetic field thruster design which yielded a primary electron region that was essentially cylindrical in shape. It was shown to operate efficiently using argon and xenon as propellants and to have a very flat beam current density profile. Because it appeared to operate as efficiently as could be expected with these propellants, and because this design can be scaled easily to various sizes, an investigation of its use with mercury propellant was undertaken. This multipole design had not been operated on mercury nor had it been operated using a hollow cathode, so some optimization of the chamber was required before meaningful comparisons could be made against other thruster designs. Parameters varied during this optimization process included the discharge chamber length, the field strength, and the axial location of electron injection from the cathode discharge chamber into the main discharge chamber.