Resonant increase of x-ray emission in a microwave discharge at half-gyrofrequency

Described in the present paper is the observation of a resonant increase of x-ray emission in a microwave discharge at half-gyrofrequency, where the equality ω= 1 2 ω Be is satisfied (here, ω is the microwave frequency and ω Be is the gyrofrequency of electrons). These experiments show, for the first time, that the x-ray flux produced at subharmonic resonance is five times higher than that produced at the cyclotron resonance, the photon energy being higher.