An investigation of internal ion number density and electron temperature profiles in a laboratory-model Hall thruster

Abstract

Experiments have been conducted at the University of Michigan Plasmadynamics and Electric Propulsion Laboratory (PEPL) to investigate the acceleration and ionization mechanisms in the P5 laboratory-model Hall thruster. A cylindrical, double Langmuir probe was used to measure electron temperature and ion number density in the discharge chamber of the PS. Probe residence times inside the thruster, and hence thruster perturbation, were minimized by use of PEPL's High-Speed Axial Reciprocating Probe (HARP) system. Discharge voltage for this experiment was fixed at 300 V and two discharge current settings were considered: 5.4 A (1.6 kW) and 10 A (3 kW). Axial profiles of temperature and number density at multiple radial positions spanning the width of the discharge channel are presented for the two cases along with previously measured profiles of the radial magnetic field and plasma potential. At 1.6 kW, the number density exhibited a dual-peak axial profile indicating two regions of ionization. The maximum temperature and number density was approximately 38 eV and 2.1xlO m, respectively. This structure essentially disappeared at 3 kW, with a single number density peak of 2.8el0. Electron temperature reached a maximum of 32 eV at roughly the same axial location. INTRODUCTION The role of the Hall thruster as a primary propulsion device continues to expand and evolve as new missions are developed which can benefit directly from the Hall thruster's unique combination of thrust and specific impulse. Along with this changing role comes the need to scale existing thrusters to both higher and lower power levels without sacrificing the thruster's inherent performance characteristics. Central to this idea of efficient scaling is the need to fully understand the Hall thruster ionization and acceleration mechanisms. This will be accomplished by a thorough mapping of plasma parameters inside the discharge chamber. The resulting data will enable the development of accurate computer models, which will be invaluable in generating the next generation of high-efficiency Hall thrusters. Toward this end, the University of Michigan Plasmadynamics and Electric Propulsion Laboratory (PEPL) has developed a comprehensive experimental program aimed at fully exploring the underlying physics of the Hall thruster. The centerpiece of this effort is the P5, a 5 kW laboratory-model Hall thruster. The P5 has been shown to have performance characteristics very similar to commercially available, state-of-the-art thrusters. It was designed specifically to facilitate internal plasma parameter measurements. As part of the same program, a High-Speed Axial Reciprocating Probe (HARP) system was assembled to allow rapid positioning of electrostatic probes inside the thruster. Using the HARP, probe residence times under 100 ms are routinely and consistently achieved. Correspondingly, perturbations to the discharge current during data collection are generally less than 10%. Graduate Student, Student Member, AIAA. Associate Professor, Director of Lab, Associate Fellow, AIAA. Copyright © 2000 by James M. Haas Published by the American Institute of Aeronautics and Astronautics with permission. (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. OBJECTIVE The objective of this research is to measure the electron temperature and ion number density inside the discharge chamber of a Hal! thruster while avoiding significant perturbation to thruster operation. The resulting data are combined with previous measurements of the axial electric field and radial magnetic field to further extend our understanding of the magnitude and spatial structure of the ionization and acceleration regions. EXPERIMENTAL SET-UP Thruster The thruster used is the University of Michigan/Air Force Research Laboratory P5 5 kW laboratory-model Hall thruster. This throster was developed specifically to provide extensive diagnostic access to the discharge chamber. Compared to smaller thrusters, the P5 provides a larger discharge chamber for better spatial resolution for electrostatic probes as well as a lower power density to reduce heat flux to the probe. Thrust, specific impulse, and efficiency have been measured and correspond very closely to commercially available thrusters. Performance characteristics and plasma parameter profiles in the plume have been reported in a previous work. The P5 incorporates a lanthanum hexahoride (LaB6) cathode. Thruster discharge voltage was fixed at 300 V for all experiments. Two discharge current levels were considered: 5.4 A and 10 A. These corresponded to anode mass flow rates of 63 seem and 112 seem, respectively. Cathode mass flow rate remained constant at 6 seem. Vacuum Chamber All experiments were conducted in the University of Michigan's 6 m diameter by 9 m long Large Vacuum Test Facility (LVTF). The pumping system consists of four CVI model TM-1200 ReEntrant Cryopumps providing a measured xenon pumping speed of 140,000 1/s. The ultimate base pressure of the facility is 2xlO7 Torr. The operating pressures for this experiment were S.SxlO Torr and 9.6X106 Torr when corrected for xenon and corresponded to discharge currents of 5.4 A and 10.0 A, respectively. Details of the facility have been presented in a previous work. Positioning System The double probe is positioned inside the Hall thruster discharge chamber using the PEPL HARP system. The HARP system allows the probe to be inserted into, and removed from, the thruster on a time scale under 100 ms. This allows measurements to be made with very little perturbation to thruster operation. The extent of thruster perturbation is determined by monitoring the discharge current during probe movement. Use of the double probe caused a slight perturbation in the discharge current but this remained less than 10% of the nominal discharge current value during all measurements. Double probe data were collected during both insertion and removal of the probe and agreed reasonably well for most sets of data. However, data from the outward sweep generally exhibited more noise due to the presence of the probe insulator body in the channel. It was concluded that the inward sweep was more representative of the true data. Therefore all data presented, unless otherwise noted, are from the inward sweep. Figure 1 shows the area inside the discharge chamber where electron temperature and ion number density were measured. The exit plane is defined as the end of the discharge channel. Radial movement is accomplished by mounting the thruster on a linear table. Between axial sweeps with the HARP system, the thruster is moved radially such that a 2-D cross section of the discharge chamber and near-field region is covered. Note that the axial position throughout this paper corresponds to the tip of the double probe electrodes.