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 Specifications:
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. The Plasma Source Instrument (PSI) is an essential adjunct to TIDE, because control of the potential of the POLAR spacecraft will allow measurements of low-energy ions in regions of very low plasma density. 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. The currents flowing between the spacecraft and the plasma are all functions of this potential, and the potential adjusts self-consistently, on very short time scales, to changes in plasma environment or solar irradiance so as to satisfy a zero-current condition. Equilibrium spacecraft potentials can range from several volts positive (to retain emitted photoelectrons) when the spacecraft is in a low-density plasma, to thousands of volts negative when the spacecraft is simultaneously in the shadow of Earth and in a very hot plasma such as is found in the plasma sheet during substorm activity.
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.
Experience with plasma source and accelerator devices on the ATS-6 and SCATHA spacecraft has shown that it is feasible to maintain the spacecraft floating potential at small values in the presence of changes in external plasma or sunlight conditions. This is done by establishing a plasma in the vicinity of the spacecraft with the following properties:
(a) The generated plasma must have an ion thermal flux which is larger than the electron thermal flux of the photoelectron cloud generated by the incident EUV flux on the spacecraft surfaces. In addition, the local plasma electron thermal flux must be larger than that of any naturally occurring plasma expected to be encountered by the spacecraft.
(b) The generated plasma electron temperature is small, a few tenths of eV at most.
(c) The generated plasma ions are chemically inert, with a mass per charge which is distinct from that of naturally occurring ions.
Condition (a) insures that the thermal flux of local plasma ions is capable of neutralizing the flux of photoelectrons away from the spacecraft which results if the spacecraft floating potential is maintained at values much less than the mean energy of the photoelectrons. Condition (a) further insures that the thermal flux of local plasma electrons is sufficient to neutralize the flux to the spacecraft of very hot electrons frequently encountered near synchronous orbit. Condition (b) insures that the equilibrium floating potential is much smaller than either the several volts corresponding to the photoelectron mean energy, or the thousands of volts corresponding to plasma sheet electron temperatures. Condition (c) insures that the operation of the plasma source does not create a plasma which can be confused with the ambient plasma.
Table 2. PSI Specifications
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The PSI design of a source which produces a suitable plasma is based on earlier experiences with ATS-6 and SCATHA. This design has been developed by the Hughes Aircraft Corp. under a contract with the U.S. Air Force. Flight hardware has been previously produced, tested, and flown by Hughes in connection with these programs, and the design is mature. Table 2 provides a summary of the technical specifications for the PSI which details the capabilities of the system.
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