CONTROL OF A SELF-SUSTAINED DISCHARGE BY RUNAWAY ELECTRON BEAMS

Abstract

The method of generating runaway electron beams (EB) in a pulsed discharge in a middle-pressure gas suggested in [1] turns out to be very efficient for pumping of lasers on atomic and ionic transitions in rare gases and metal vapors [2]. Of interest is its practical use for control of the electric discharge parameters. This problem has not yet been considered, and this paper is aimed at filling this gap. In the present paper, we investigated the EB control of a self-sustained discharge to provide the voluminous and homogeneous mode of the discharge. Figure 1a shows the design of a discharge cell and of its power supply circuit. An electron gun contains an 11-stage cathode Kn and a common grid electrode E. A ceramic plate with 11 holes that determine the accelerating distance and the electron beam diameters (~8 mm) is inserted between them. Since the energy of electrons in the beam was several keV and the electron free path in a middle-pressure gas was several centimeters [3], the interelectrode gap of the controlled discharge was chosen equal to 1.8 cm. A grid anode of the gun E represents a steel plate 4 mm thick with 11 holes each 12 mm in diameter. A W-wire 50 μm in diameter was wound on the plate with a pitch of 220 μm. The grid facing the cathode serves as the anode of the gun, whereas the grid on the outside serves as the cathode of the main (controlled) discharge. The aluminum plate A serves as its anode. For this construction of the electrode E, the discharge in the accelerating gap of the gun and the main discharge were separated by a 4 mm space to reduce the negative effect of the main discharge on the EB generation [4]. Eleven capacitors Cn and also the capacitor C1 are charged from a high-voltage rectifier to the voltage U0 = 4– 12 kV through resistors Rn and R1. Capacitors Cn are used to supply power to the electron gun; the energy stored in C1 determines the energy deposited in the gas from the main discharge. The electron gun is actuated through the circuit comprising ground, thyratron T, capacitors Cn, accelerating gap, C2, and ground. Simultaneously, the capacitor C1 is discharged on C2 which then discharges through the cell. A delay between the electron beam current pulse and the current pulse of the main discharge is determined by the characteristic times of the charge exchange circuit and also by the discharge times of Kn–E and E–A gaps. The advantage of this circuit is that it operates with a single commutator T; its disadvantage is the impossibility of independent switching on of the electron gun and main discharge. Figure 1b shows the photograph of the main discharge in the cell. It can be seen that the geometry of the discharge is determined by electron beams, and glow of discharge columns is fairly homogeneous both in the longitudinal direction and in the column cross sections. The column diameter depended on the gas pressure and discharge current. On the electrode E, it was 8–12 mm. We failed to realize the volumetric main discharge mode under any conditions when the electron gun was switched off. Figure 2 shows the peak current of the volumetric main discharge versus the charge voltage U0 in the cell filled with helium. The discharge current pulse duration at half-maximum was 18–22 ns. The EB pulse parameters were measured when the main discharge was switched off. It was found that each element of the electron gun created a peak EB pulse current as high as 10 A for the initial gun voltage 8–9 kV at helium pressures indicated in Fig. 2. In this case, the EB current pulse duration was in the range 15–20 ns. However, when the main discharge was switched on, the voltage on the electron gun, equal to the potential difference between the cathodes Kn and electrode E, did not exceed half the initial applied voltage, and the EB current through each element did not exceed 5 A. The EB was generated simultaneously with the main discharge, and the values of the beam pulse and main discharge duration were almost identical, that is, the main discharge was completely controlled by the electron beam. Quantitatively, the efficiency θ of the