Experimental Study of Breakdown Time in a Pulsed 2.45-GHz ECR Hydrogen Plasma Reactor

An electron cyclotron resonance (ECR) plasma reactor developed at the European Spallation Source Bilbao has been operated in pulsed mode at 50 Hz to study the breakdown-process dynamics by time-resolved diagnostics. Injected power, reflected power, electrical-biased probe saturation current, and light emission were measured simultaneously for three different magnetic fields: under ECR, ECR, and asymmetric over ECR profiles. Gas pressure, power, and duty cycle have been used in a parametric study obtaining information about microwave (MW) coupling and plasma formation stages during the breakdown process. The study is relevant for designers that need to extract short beam pulses from a 2.45 ECR ion source for any application because the total breakdown time measured is defined as corresponding to reach the steady-state plasma parameters. A simple model of residual electron density evolution between pulses is proposed to describe the MW coupling as a function of incoming power and duty cycles.