  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.
