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 Operations:
The plasma source component of TIDE/PSI has required less development than TIDE itself, being a direct copy of the plasma source device developed for the USAF by Hughes. Moreover, its operation is essentially passive in that it reacts very weakly to its environment and is self-regulating in terms of the equilibrium floating potential of the spacecraft. Nevertheless, there are significant choices to be made in operating the PSI, in terms of the potentials and internal currents which are set so as to provide optimal performance. In general, there is a tradeoff between stability of the PSI discharge and its emission capability. Since the current requirements for the POLAR spacecraft (tens of mA) are far below the capabilities of the plasma source (>1 mA), its operating point is chosen so as to strongly favor stable operations (i.e., without fluctuations of the plasma output) at the expense of maximum emission capability.
To illustrate this point, we provide a plot of the operating parameter space for PSI in Figure 15, based on laboratory tests. The design operating point is indicated, as well as a wide range of parameters which would produce considerably larger plasma output in some regions at the expense of smooth and stable operation.
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Figure 15. PSI operating parameter space illustrating the wide range of operating points which are possible. Selection of an operating point in flight is based upon minimum fluctuation in the discharge and emitted plasma current. The triangle indicates approximately the conditions at which PSI will operate on POLAR, while the circle indicates a high-current capability alternative.
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