Center for Laser Applications

Lockheed Martin Hall Thruster Test

A new class of Russian-developed plasma thrusters has been receiving considerable attention for possible application in satellite station keeping. These thrusters operate on xenon propellants and have better efficiency and specific impulse than competing propulsion systems. Diagnostic tests of the TAL D-55 thruster were carried out in the CLA large vacuum tank to determine the exhaust plume flowfield using the CLA developed multiplexed LIF technique by the use of cw laser radiation and Langmuir probes.

The LIF technique enables simultaneous measurement of two components of velocity with a precision of approximately 10 m/s by measuring the Doppler shift in an excited exhaust atom or ion. The Langmuir probe provided measurements of plume plasma potential, electron temperature and electron number density. A continuous 200 hour erosion test was also performed in addition to the diagnostic measurements. Scientists from Lockheed Martin, Primex Corp., Russia and the University of Michigan observed and/or participated in the test program.

The CLA large vacuum tank provides a unique capability for testing of this class of plasma thrusters. The 20 K helium cryopump enables continuous operation of thrusters generating more than one kilowatt of power in a vacuum environment that is close to actual space conditions. The multiplexed LIF system was originally developed for diagnostic measurements on arcjet thrusters operating with argon propellant. It has since been employed for diagnostic measurements on hydrogen, nitrogen and xenon ion propellants. The advantage of the multiplexed LIF system is that it permits simultaneous measurement of two components of the velocity. This allows data to be acquired in less time and insures that operating conditions do not change between separate measurements of two velocity components. The narrow linewidth of the dye laser that is used to excite the fluorescence transition in the propellant enables the precision and accuracy of the velocity measurement to approximately 10 m/s. The accuracy of the measurement is assured by simultaneous measurement of a stationary discharge, along with the measurement of the two velocity components. New techniques were developed to better analyze the Langmuir probe data. These involved the use of optimal digital differentiating filters to determine the plasma potential and nonlinear SIMPLEX methods for extraction of electron temperature, ion current and electron current densities.