keV-energy x-rays from a low-pressure, low-power, low-field, capacitively coupled 27-MHz hydrogen plasma source

We report on the unexpected observation of 0.9--5 keV x-rays coming from a cool (bulk $T_e \sim$ 4 eV), tenuous ($n_e \sim 10^{10}$ cm$^{-3}$) 5-cm-diameter hydrogen plasma column generated in a tandem high-mirror-ratio mirror machine by an external, capacitively-coupled RF (27 MHz) antenna operating at low power, 20--500 W. The x-rays, measured with an Amptek XR-100CR detector, are evidence of energetic electrons that have not been seen previously in experiment or theory in similar plasmas. In the neutral H$_2$ gas pressure range of 0.4 to 1.5 mT, the x-ray emissivity increased with decreasing pressure. No x-rays were observed when operating with argon (or 30/70 argon/hydrogen mixtures) at similar powers and pressures in either capacitively-coupled or helicon modes. X-ray count rate smoothly increased as mirror ratio increased and reached a broad maximum near 80 G, central field. Time-dependent emissivity with pulsed RF power and spatial profiles over a limited axial range have been measured. Possible heating mechanisms, including Fermi acceleration, cyclotron resonance, double layers, and sheaths, are being considered.