The processes leading to the population inversion are investigated in a recombining hydrogen plasma which is interacting with a cool and dense neutral hydrogen gas by using the rate equations on the basis of a collisional-radiative (CR) model and the energy equations for electrons, ions and neutral particles. The quasi-steady state (QSS) approximation is used only for the levels i lying above a certain level /* which is not the first excited level. The calculations have shown that the quasi-steady state cannot be realized while intense energy-flows due to the collisional processes exist between different kinds of the particles such as the electrons and the ions in the plasma, and that the population inversion is realized only in the quasi-steady state following the transient phase. The effects of the initial conditions of the hydrogen plasma and of the introduced neutral hydrogen gas on the overpopulation density are also discussed.

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A high potentiality of plasma cooling and its mechanism are theoretically revealed for a helium plasma which comes into contact with hydrogen molecular gas in the TPD-I machine. The stationary population inversion between the levels n=2 and 3 of the He+ ion, which is due to mixing of the stationary high-temperature helium plasma and the hydrogen molecular gas, is simulated and discussed in comparison with cooling due to the mixing of the plasma and the atomic hydrogen gas. The calculation also shows that the gain per unit length of the He II 164 nm line is ∼ 2 cm-1 for the laser oscillation under optimum initial helium plasma (T e ∼ 30 eV and n e ∼ 3.5 × 1016 cm-3) contacted with hydrogen molecular gas at a pressure of 0.1 Torr.

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Abslrad Anomalously intense photoon emission from He I (n = 3 -* n = 2) has been observed when helium plasmas contact with cold hydrogen molecules. This enhancement of photon emission can be undentwd to be due to accidental resonant double electrun capture by alpha particles, Hezi, when molecular hydmgen gases ar? injected into helium plasmas: He"tH2 + He'(ls31)tH:+. The present observation hasalsoconfirmed theoretical prediction of strongly selective double electron captwe by alpha panicles in He?+ + H2 collisions.A number of interesting phenomena occur when a plasma interacts with cold neutral gases and play an important role not only in basic sciences but atso in several applications: the formation of double layers between hot and cold plasmas (Hora 1983). the population inversion leading to soft x-ray lasers by charge-exchange (Dixon and Elton 1977, Dixon et at 1978) or by rapid cooling of plasmas through mixing with gases (Furukane et ai 1989) and plasma diagnostics (Fonck er al 1984) by charge exchange of hot ions with neutral atoms.In the present letter, we report the observation of the unexpected intense He I (n = 3 + n = 2) photon emission when helium plasmas are in contact with cold hydrogen molecules. The present experiment has been performed at the TPD-I plasma machine at the National Institute for Fusion Science, as schematically shown in figure 1, which produces quiescent DC helium plasma flows of diameter -10 mm with an electron density of and an electron temperature of several eV (Otsuka et al 1975). Helium plasmas, diffused out through an 8 nun diameter anode hole, extend for 2 m at a speed of about lo4 cm s-' along the axial magnetic field of 5 kG before reaching a grounded target plate. The pressure is measured to be 1.2 x Torr (He) in the observing region, which is 80 cm downstream from the anode hole, when no additional cooling gas is introduced. The pulsed gases of the order of IO-' Torr are introduced into the helium plasma through a piezo-valve which is opened for a duration of 12.5 ms. A spatially scanned pressure measurement with a fast vacuum ion gauge shows that the injected hydrogen gas is fairly well localized (also see later discussion), at least for the early period of the gas puffing.

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A new calculation method is presented, which is suitable for investigating a population inversion between the energy levels of weakly ionized hydrogen plasma in which atom-atom collision processes have to be considered. As an example, the plasma conditions required for inversion between energy levels of the principal quantum number 4 and 5 of hydrogen atom are calculated by this method. In contrast to the recombining hydrogen plasma with moderate temperature (k T e\gtrsim1 eV), it is shown that population inversion is possible in a larger range above the lower bound of the effective recombination rate of the plasma.