Generation of uniform plasma emitters in discharges with a magnetic field

© 1998 Beam Organizing Committee. Development of sources for generation of beams with largely different dimensions encounters special and even opposite requirements with respect to the emitting plasma parameters. For example, in narrow beam sources extraction of charged particles is enhanced, as a rule, by producing a sharply nonuniform plasma having the maximum concentration near the emission aperture. For this purpose, special measures are taken in order to increase ionization in the said region and impede escape of particles therefrom. In particular, strong magnetic fields are often applied to improve the extraction efficiency. In the case of broad beam sources one of the main requirements consist of the generation of a uniform plasma in the discharge system of the source. This problem seems to be solvable in quite the opposite way: stop applying magnetic fields and provide a uniform ionization in the discharge chamber, In high-pressure discharges, when particles are lost through volume recombination, a uniform ionization indeed causes appearance of a uniform plasma. However, the situation is largely different in low-pressure discharges, when particles are lost at the walls of the discharge chamber. A simultaneous analysis of ionization processes and motion of particles needs be performed to determine conditions necessary for formation of a uniform plasma in these discharges. The results of such analysis are presented. They show that under certain conditions a uniform plasma may be produced just in the presence rather than in the absence of a magnetic field. Therefore these systems may serve as generators of broad beams.