AN INVESTIGATION OF LOW-VOLTAGE BEAM DISCHARGE IN HELIUM .2. THEORY

The collisional regime of the low-voltage beam discharge in helium is investigated theoretically. The conditions under consideration corresp ond to the case in which the distribution function of fast beam electr ons is weakly asymmetric in velocity space. It is shown that the beam energy relaxation can take place either due to binary collisions or by excitation of Langmuir waves, depending on the beam current magnitude . The theoretical value of the beam threshold current, above which the change of relaxation mechanism occurs, is determined. In the case of collisional relaxation the distribution function of fast electrons is found, and the concentration of metastable excited atoms is determined . Good agreement between theoretical and experimental results is obtai ned.