Measurement of the electron concentration in a plasma generated by a high-velocity electron beam

Abstract

The probing of gas and plasma flows by beams of accelerated electrons is now coming into wide use [1,2]. In colliding with particles of the gas being investigated the beam electrons ionize these particles and either generate an independent plasma or increase the ion concentration in the plasma under investigation. This can introduce consideable error into the measurements of plasma parameters. Consequently, the problem can arise of the need to evaluate such error. This can be done by our investigating parameters of the plasma generated by ionization of the gas by a beam of accelerated electrons. The ionization of a gas by an electron beam produces a plasma whose parameters, such as electron concentration N, the effective frequency of electron collisions, etc . , can be easily varied by a suitable selection of gas, or by our varying the gas pressure, the electronbeam current and energy, etc. Modification or displacement of the zone occupied by the plasma can be important in certain applications. Such modification can evidently be most readily achieved with a plasma generated by an electron beam. Available experimental data [8, 4] pertain to ionization of a rarefied gas by means of pulsed beams (pulses on the order of a few psec at pressures ranging from 10 -s to 10 "~ mm Hg. At low pressures and short pulses the ionization processes play a fundamental role in plasma generation, since in this case there is little elee~on recombination and diffusion. ConsequenrIy, data obtained under these conditions are not applicable to the case of gas ionization by a steady electron beam. Preliminary results from measurement of electron concentration in plasma generated in air, helium, and argon by a high-velocity electron beam at current intensities varying between 0.1 and 0.6 mA are presented in this paper. The mean concentration of electrons in the plasma was measured along the beam cross section, with the plasma-column diameter assumed equal to the bright region of the gas ionized by the beam (Fig. 1). This model is somewhat crude approximation of phenomena actually taking place in fire beam, since it does not take account of irregularity in electron distribution across the beam, and of electron diffusion outside the bright zone. However, consideration of these phenomena is extremely complex. The electron concentration in the plasma was measured by means of a superhigh frequency resonator in the three-centimeter range of wave-lengths [6]o A cylindrical resonator with TMox 0 oscillations and an electron beam aIong its axis was selected for these experiments. The resonator was 26.4 mm in diameter and 10 mm in height. The electron concentration in the pIasma was determined in the usual manner from change in the transmission ratio and from the shift in the resonance frequency of the resonator during passage of an electron beam through it.. The experimental setup is shown diagrammatically on Fig. 2. The energy from superhigh frequency oscillations at 8680 mHz was fed from a klystron generator to the resonator located in a vacuum chamber together with the waveguides. The controlled admission of gas to the chamber made possible variation of the ionized gas pressure within the required limits,