Optimization of a ribbon diode with magnetic insulation for increasing the current density in a high-current relativistic electron beam

Abstract

The geometry of the ribbon diode of the U-2 accelerator is optimized to increase both the current density and the total current of the relativistic electron beam for its subsequent injection into the plasma of a multimirror GOL-3 trap. Beam simulation in the diode was performed using the POISSON-2 applied software modified on the basis of the results obtained using the theory of a planar diode in an inclined magnetic field. As a result of the optimization, the diode geometry and the magnetic field configuration were found that should provide a factor of 1.5-2 increase in the current density in experiments with a small angular divergence of electron velocities.Introduction. Studies of dense plasma heating by a high-current relativistic electron beam [2] and plasma confinement in a multimirror magnetic trap have been performed on the GOL-3 facility of the Institute of Nuclear Physics of the Siberian Division of the Russian Academy of Sciences [1]. During injection of a beam with a current density j ≈ 1.5 kA/cm 2 into a plasma of density n p ≈ 10 21 m −3 , the plasma electron temperature in the experiments reached values T e ≈ 2-3 keV and the ion temperature values T i ≈ 1-2 keV. The plasma energy lifetime in the trap was 0.5-1.0 msec. The obtained plasma parameters are among the best results achieved for the class of open traps and are even comparable to the parameters of medium size tokamaks. This implies that the multimirror confinement of a plasma heated by an electron beam is a promising concept for use in thermonuclear reactors.To further increase the plasma parameters, it is necessary to improve the beam characteristics, namely, to increase its density with retention of a small angular divergence of electron velocities and to increase the beam pulse duration. For this purpose, the diode geometry and magnetic field configuration were optimized by numerical calculations using the POISSON-2 applied software package [3]. To calculate the beam characteristics with the required accuracy, we modified the algorithms of the package taking into account the results of a previous theoretical analysis of the operation of a planar diode in an inclined magnetic field [4,5].1. Parameters of GOL-3 Experiments. The layout of the GOL-3 facility and the magnetic field distribution on the trap axes are shown in Figs. 1 and 2. The magnetic system of the facility consists of N = 55 magnetic mirrors (the length of each mirror l = 22 cm, mirror ratio B max /B min = 4.8 T/3.2 T) connected in series between the strong end magnetic mirrors with a field B entr ≈ 6 T at the beam entrance point and B exit ≈ 9 T at the exit point. A deuterium plasma of density n p ≈ 10 20 -10 22 m −3 is produced in a cylindrical conducting chamber located in the solenoid of the GOL-3 facility.