Plasma density in discharge sustained in inhomogeneous gas flow by high-power radiation in the terahertz frequency range

We have measured the density of plasma (electron concentration) in discharge maintained in inhomogeneous argon flow under the action of high-power pulsed radiation of gyrotron (frequency, 0.67 THz; power 40 kW; pulse duration, 20–30 $\mu$s) in a range of background gas pressures in the discharge chamber from 10$^{-3}$ to 300 Torr. The electron concentration at low pressures (10–3 to 7 Torr) was determined using Starkeffect induced broadening of the H$_\alpha$ atomic emission line (656.3 nm) of hydrogen present in discharge as a small impurity in residual gases. The maximum observed Stark broadening of the H$_\alpha$ line corresponded to a plasma density on the order of 2 $\times$ 10$^{16}$ cm$^{-3}$, which exceeded the critical value for the given frequency of radiation sustaining the discharge. At background pressures above 7 Torr, the plasma density was estimated from analysis of the spatiotemporal patterns and waveforms of discharge glow in the visible spectral range. These estimations gave electron concentrations on the level of (1–2) $\times$ 10$^{15}$ cm$^{-3}$.