High Power Vacuum Ultraviolet Source Based on Gasdynamic ECR Plasma

Abstract

We report experimental results of vacuum ultraviolet (VUV) emission from the plasma of an electron cyclotron resonance (ECR) discharge in hydrogen, sustained by powerful millimeter-wavelength radiation of a gyrotron. Distinctive features of the considered discharge are the high plasma density ($10^{13}$ cm$^{-3}$ order of magnitude) and, at the same time, the high electron average energy ($10 - 300$ eV), which makes it possible to significantly increase the efficiency of VUV re-emission of the energy deposited into the plasma by the microwave radiation. Experiments were performed with the plasma confined in a simple mirror trap and heated by pulsed gyrotron radiation $37.5$ GHz / $100$ kW under the ECR condition. The measured volumetric VUV emission power of Lyman-alpha line ($122 \pm 10$ nm) overlapping with the Werner band, Lyman band ($160 \pm 10$ nm), and molecular continuum ($180 \pm 20$ nm) reached $45$, $25$, and $55$ W/cm$^3$, respectively. The total absolute radiation power in these three ranges integrated over the plasma volume is estimated to be $22$ kW i.e. $22$% of the incident microwave power, which matches theoretical predictions. Further optimization of the conditions of the ECR hydrogen discharge sustained by powerful gyrotron radiation provides an opportunity for the development of effective technological VUV sources of a kilowatt power level.