Production of large area low-energy electron beams by extraction from a multidipolar plasma (abstract)

Abstract

The production of large area low-energy electron beams by extraction from a low-pressure plasma is investigated. In order to extract electron beams of given energy from a plasma, the plasma potential, as referred to wall potential, must be stationary and independent of the extraction potential. The macroscopic neutrality of the plasma therefore requires that reactor walls are metallic and that extraction to reactor wall area ratio is less than the square root of the electron to ion mass ratio. These specifications are fulfilled in the source presented in this study where the plasma is produced in a cylindrical chamber by seven elementary dipolar sources distributed over the end flange of a cylindrical chamber opposite to the extraction grid. The plasma is sustained at electron cyclotron resonance by microwaves at 2.45GHz in the chamber whose wall potential (extraction potential) can be varied from 0to−200V with respect to the ground potential. The electron beam extracted from the plasma through the grid then expands in a second chamber whose walls are set at the ground potential. The details of the experimental arrangement built on the above specifications are presented and the characteristics of argon plasmas measured by using cylindrical Langmuir probes as a function of the extraction voltage. In the same way, electron beam characteristics in the ground potential chamber are also measured using both planar and cylindrical Langmuir probes. In particular, variations in plasma and beam potentials are simultaneously determined as a function of extraction voltage. Current-voltage extraction characteristics are also reported, which show that electron beams in the ampere range can be extracted between 0 and −200V. The experimental results are in good agreement with the expected electron source behavior.