Investigation of the characteristics of the active medium of a helium hydride exciplex laser

A model is developed of plasma-chemical reactions in a helium–hydrogen plasma formed by a hard ionizer. It is shown that, in principle, lasing is possible as a result of a photodissociation transition in the HeH$^*$ exciplex ($\lambda$ = 460 nm, $\Delta\lambda$ = 310 nm). The characteristics of the active medium subjected to pulsed and pulse-periodic pumping are analyzed analytically in the steadystate approximation and also numerically. A considerable gain amounting to $\varkappa=2\cdot10^{-2}$ cm $^{-1}$ and an average efficiency of the active medium $\eta_{\text{av}}\approx$ 3 % can be achieved by pumping a dense medium ($[He]\approx10^{21}$ cm $^{-3}$) with a small amount of hydrogen ($[He]\approx2\times10^{17}$ cm $^{-3}$) by $\sim$ 100 nsec pulses with a peak current density $j_e\approx1$ kA/cm$^2$ in the case of an electron beam and $j_p\approx2$ A/cm$^2$ in the case of a proton beam (the particle energy is assumed to be 300 keV). In the case of pulse-periodic pumping by a beam modulated at the microwave frequency the pulse repetition frequency should be in the range $f\lesssim50$ MHz.