An electron beam-probe for potential measurements in plasmas

A method is described for measuring elevations of potential in a region, such as a plasma column, by detecting variations in the time of flight of an electron beam directed through the region. The use of a very small current of relatively high energy electrons as a probing beam results in an extremely small disturbance of the space charge distribution within the beam, and enables measurements to be made in the interior of a plasma without the insertion of metallic electrodes. The electron beam is intensity modulated at high r-f frequency by a conventional crt gun, and is made to impinge on a collector specially designed to minimize the effects of deflection due to the electric fields encountered in flight. Variations in time of flight are detected by comparing the phase of the collector signal with that of the modulating waveform in a coincidence-type phase detector whose output is linearly proportional to phase difference. The output signal is proportional to the integral of the potential elevation over the path length. Sensitivity of the measurement is of the order of 10 volt-centimeters, with a frequency response extending from zero frequency to 1 megacycle. Means for calibrating the instrument are described. Circuit diagrams and construction details of the collector are presented. This technique has been developed as a diagnostic tool, and is currently being used in studies of pulsed plasmas for electrostatic propulsion.