Feasibility of a low-voltage discharge in pure molecular hydrogen

The feasibility of initiating a low-voltage discharge in pure (free of readily ionizable impurity) molecular hydrogen is studied theoretically. A discharge with cathode fall $\varphi_1$ = 10 V, interelectrode gap $L$ = 2 cm, and total hydrogen concentration $N^{(0)}_{\mathrm{H}_2}\approx$ 2 $\times$ 10$^{15}$ cm$^{-3}$ is considered by way of example. The plasma parameters, including concentration $N_{\mathrm{H}^-}$ of negative hydrogen ions H$^-$, are calculated. The concentration of H$^-$ ions is maximal in the near-anode plasma and may reach $(N_{\mathrm{H}^-})_{\mathrm{max}}\approx$ 0.5 $\times$ 10$^{12}$ cm$^3$. Concentration $N_{\mathrm{H}^-}$ can be increased severalfold by introducing a small amount of cesium into the discharge, $N^{(0)}_{\mathrm{Cs}}\le$ 10$^{13}$ cm$^{-3}$. Cesium ionizes completely and concentrates in narrow near-electrode layers, which are depleted with the plasma in the purely hydrogen discharge. The discharge modifications studied in this work may be of interest as low-voltage volume plasma sources of H$^-$ ions under conditions when a high concentration of cesium in the source plasma is undesirable.