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. The fundamental goal of PSI is to establish a means for the reduction of the spacecraft floating potential to values much smaller than normally associated with spacecraft in low-density plasmas in sunlight.
Spacecraft immersed in plasmas in general attain a potential different from the plasma potential.
The origins of this spacecraft potential are complex, but the fundamental principle is that the net current to the spacecraft surface has to be zero in steady state. A charged particle starting far from the spacecraft and arriving at the spacecraft surface is deflected from its initial direction and arrives at an energy which differs from its initial energy by the spacecraft floating potential. Ions approaching a positively charged spacecraft with insufficient energy are prevented from reaching the instrument, leading to an excluded sphere in velocity space and centered on the spacecraft frame, within which the velocity distribution cannot be measured. Negative floating potentials raise the apparent energy of the ions and may preclude the implementation an energy sampling strategy which is effective in resolving the ambient energy distribution. EUV flux on the spacecraft surfaces.