An instrument for the investigation of interplanetary plasma

The instrument is designed to determine the density, direction, and magnitude of the bulk velocity of the protons of the interplanetary plasma. It is essentially a Faraday cup containing four grids which serve (a) to keep the electrons of the plasma from reaching the collector and (b) to suppress the photoelectric current by modulating the incoming protons without modulating the photoelectrons produced when the cup faces the sun. A transistorized electronic system amplifies, compresses, and demodulates the a-c signal from the collector before transmitting it to the telemetry system of the vehicle. Current densities from 10−12 to 10−8 amp/cm2, and proton kinetic energies from 10 to 3000 ev, can be measured.