Evaluation of Back-Pressure Effect on Thrust Measurement

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Electric propulsion device must be tested in a vacuum chamber for simulating the space environment. In order to evaluate the back-pressure effect on thrust measurement in a simulation chamber, we bring our Hall thruster, which is one of electrostatic accelerators, to the university of Michigan for testing in a good vacuum condition.The department of Aerospace engineering of the university of Michigan has a huge vacuum facility (6 m - diameter, 9 m -long, 180,000 litter/s pumping speed, ) which can produce one order higher vacuum environment than our facility at Nagoya university (1 m diameter, 2 m - long, 8000 litter/s pumping speed.)The discharge characteristics recorded in the big chamber was almost same as that recorded in a small chamber in Japan. Therefore, it can be concluded that the back-pressure hardly affect the discharge inside the thruster.The measured total beam-current is exponentially decreased according to the distance from the thruster exit. This tendency is well explained by theoretical prediction depending on the mean free path for charge exchange reactions between beam ions and ambient neutral particles. This result shows that the total beam-current can be estimated even in a relatively low vacuum if the effect of charge exchange reactions is taken into account.Two dimensional plasma potential distribution shows the valley of potential around the cathode. The diameter of the valley is approximately one meter. It indicates that the chamber-wall effect can arise in a vacuum chamber whose diameter is less than one meter.Thrust was measured using the thrust stand made by electric propulsion laboratory of the university of Michigan. However, the thrust level of our thruster was to low to be measured with high accuracy. Therefore, we failed to have a thruster performance directly from the thrust measurement. The performance is calculated from measured total beam-current, mean ion velocity, and divergence angle.