Reference: Moore, T.E., C. R. Chappell, M. O. Chandler, S. A. Fields, C. J. Pollock, D. L. Reasoner, D. T. Young, J. L. Burch, N. Eaker, J. H. Waite, Jr., D. J. McComas, J. E. Nordholt, M. F. Thomsen, J. J. Berthelier, and R. Robson, The Thermal Ion Dynamics Experiment and Plasma Source Instrument, Space Sci. Rev., 1995.
PSI Description:
The PSI consists of the plasma source, a gas feed system with appropriate tanks, plumbing, and valving and an electronics system capable of powering and monitoring the operation of the PSI. The relationship among these components is illustrated in Figure 8.
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Figure 8. PSI functional block diagram illustrating the relationships among the principal components.
The plasma source is a compact arrangement of a hollow-cathode, keeper and anode electrodes, a magnetic structure, and a grounded shield. Xenon gas flowing through the plasma source is ionized by bombardment with electrons released from a low-work-function surface within the hollow cathode. The ionized gas flows out of the plasma source, providing a medium-density (~1010 cm-3), inert-gas plasma to neutralize differential charge buildup between various surfaces of the satellite and also to form an electrically conducting "bridge" between the satellite and the natural space plasma.
It is anticipated the plasma source will bias the satellite such that the plasma source anode is within a few volts of space potential. To hold the satellite within +1 V of space potential it may be necessary to bias the plasma-source anode relative to spacecraft ground. The PSI design provides a vernier bias control to permit this. The plasma source has the characteristics listed in Table 2.
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Table 2. PSI Specifications
The plasma source has a porous-tungsten insert. The cathode, keeper, and anode are all electrically isolated from the outer can so that the return current from the satellite can be measured. The outer can is fabricated from cold rolled steel and thus serves as a magnetic shield for the permanent magnets inside the source. A solenoid valve in the gas feed plumbing is an additional source of stray magnetic fields. Both have been tested thoroughly, and their stray fields have been found acceptable.
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