Development of a 45-Degree Parallel-Plate Electrostatic Energy Analyzer for Hall Thruster Plume Studies: Preliminary Data (1999)

Several successful Hall thruster plume studies have been conducted at the University of Michigan using a 45degree parallel-plate electrostatic energy analyzer mounted externally to the vacuum chamber. This requires the thruster to be in close proximity to the chamber walls potentially resulting in some set-up induced inaccuracies of the ion voltage distributions. A new design has been developed that aims to mitigate these effects by substantially reducing the analyzer size and mounting it internally to the chamber. The device has an inter-slit distance of 66 mm, a plate separation of 18 mm, and a resolution of 1.4%. Existing data from the P5 laboratory Hall thruster on centerline was used to calibrate the spectrometer constant of the analyzer. The analyzer was tested with the P5 1 m downstream of the exit plane over angular positions from 0 to 80 degrees. The analyzer traces qualitatively match the existing data, but closer examination reveals that the spectrometer constant is variable over angular position. The variation is most likely attributed to high pressures being maintained inside the analyzer. Future work will concentrate on alleviating this problem so that the spectrometer constant does not vary over the density ranges encountered in a Hall thruster. a) Graduate Student, Aerospace Engineering. b) Associate Professor, Aerospace Engineering and Applied Physics, Laboratory Director. Copyright  1999 Richard R. Hofer. Published by the Japan Society for Aeronautical and Space Sciences with permission. INTRODUCTION In a Hall thruster ions are accelerated by an axial electric field established between a downstream cathode and an upstream anode. An applied radial magnetic field in an annular discharge chamber impedes the motion of migrating electrons due to the crossed electric and magnetic fields creating an azimuthal closed electron drift, the Hall current. Propellant is injected at the anode and collisions in the closed drift region create ions that are then accelerated producing thrust. Commercially developed Hall thrusters typically operate between 50–60% efficiency, achieving specific impulses between 1500–2500 seconds. In recent years, several plume studies of Hall thrusters have been conducted at the University of Michigan. These have included the use of a retarding potential analyzer (RPA) and a parallel-plate electrostatic energy analyzer (ESA). Although both devices yield data proportional to the ion voltage distribution function, RPAs are subject to numerical noise because the data must be differentiated. Parallel-plate energy analyzers do not require differentiation and the resulting traces are therefore more accurate representations of the distribution function. A parallel-plate ESA consists of two plates, a grounded baseplate and a backplate held at some positive potential with respect to ground. An ion beam is passed through the entrance slit of the baseplate. By setting the voltage on the backplate, the ESA acts as an energy per charge filter, allowing only ions of a particular voltage to pass through the exit slit of the baseplate where they are then recorded as current on a detector. The existing ESA at PEPL is a stationary device mounted externally to the main vacuum chamber. This leads to some important considerations, which will be discussed below, with the experimental set-up because the thruster must be mounted in close proximity to the chamber walls. The primary motivation for this new design was therefore to build a compact and mobile energy analyzer that could be mounted anywhere within a vacuum chamber and thereby minimize some of the set-up induced effects that the larger energy analyzer introduces. In the following, the motivation for such an analyzer design is discussed. The theory of operation of electrostatic energy analyzers is reviewed and the design and set-up is also presented. Finally, the method of analyzer calibration and a preliminary set of data are also given.