Utaro Furukane

Simulation of gas-contact cooling for VUV laser oscillation in recombining plasmas

Possible rapid cooling of a higher temperature plasma is investigated in a gas-contact cooling method in which the effect of finite time for mixing the plasma and the contact gas is considered. The calculation has shown that cooling under these conditions is much more rapid than that achieved by radiative loss cooling and that the gain per unit length of the He II 164 nm line is approximately 2 cm-1 for the laser oscillation under optimum conditions. The possibility of realising a laser at a shorter wavelength in a plasma with higher charge number Z is also discussed.

Population Inversion in Mixing Hydrogen Gas with a High Temperature Plasma

Inverted population distributions in the hydrogen atom which appear in the contact phenomenon of hydrogen gas with high-temperature helium plasma are analyzed by a collisional-radiative model associated with energy balance equations. By including the ion temperature effect in the energy balance equations, the collisional-radiative model indicates an inverted population of the hydrogen in the same-order range as the experimentally observed ones. The possibility of lasing at the 656 nm line of hydrogen is discussed in connection with initial conditions of helium plasma and injected hydrogen gas.

Lasing conditions in recombining helium plasmas

Abstract Lasing conditions for He have been evaluated numerically. We have used a collisional radiative model to calculate overpopulation densities Δ n ij , which are defined as differences between population densities per unit statistical weight of the upper and lower excited levels i and j , respectively. Laser oscillations for the level pairs 2 1 P -3 1 S , 2 1 P -4 1 S , 2 1 P -3 1 D , 2 1 P -4 1 D , 3 1 D -4 1 F , 3 1 P -4 1 S , 3 1 P -4 1 D , and 3 3 P -4 3 S are possible when the electron densities are within well defined limits at low electron temperature ( T e = 0.1 eV). For level pairs of the singlet state, the inversion mechanism for He is the same as for H. Only collisional processes produce population inversion in the triplet level pair 3 3 P -4 3 S .

Population inversion in optically thick, recombining hydrogen plasmas

Abstract The plasma condition is investigated theoretically for population inversion between the first two excited states of hydrogen atoms in a recombining plasma. The rate equation, including atom-atom collision terms, is solved consistently with the optical escape factors. The upper bound of the ground level population density (n1)max necessary for inversion in the optically thick plasma at specified electron density and temperature is nearly inversely proportional to the mean radius of the plasma rO. With a decrease in the atom temperature, the upper bounds increase in the optically thin plasma but decrease in the optically thick plasma.

Population inversion in recombining weakly ionized hydrogen plasma.

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 (kTe\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.

Numerical study of selective double-electron capture by α particles in collisions with hydrogen molecules in a cooled plasma

Abstract A numerical study based on a collisional radiative model is performed to understand the enhanced He I (n = 3 → n = 2) line intensities recently observed in an He plasma in contact with hydrogen gas: accidental resonant double-electron capture into the excited states by α particles in the process He2+ + H2 → He∗ (1s3l) + H+ + H+ whose cross-sections are very large at low energies. The calculated results agree fairly well with the observed temporal evolutions of the enhanced He I line intensities.

Fast Numerical Calculation for a Set of Nonlinear Rate Equations: Application to Rapid Ionizaton Phase in Plasma

To make a useful contribution to the development of the compact X-ray laser, a modified quasi-steady-state approximation is applied to the fast numerical calculation of rate equations for the rapid ionization phase of a laser-produced plasma, to which the usual quasi-steady-state model by Bates et al. is not applicable. The set of nonlinear rate equations for the excited level populations coupled with the electron density and temperature for the plasma are numerically calculated with sufficient accuracy for their temporal evolution. The calculation also shows that a suitable combination is possible between the power and pulse width of a heating laser so that a high-density plasma can be obtained as the X-ray laser medium. The mathematical meaning of our approximation is briefly mentioned and its applicability is discussed.

Fast Calculation Method for the Rate Equations of Plasma

The rate equations for the plasmas in which the electron or ion density and the temperature changes substantially in a finite time are solved by using a fast numerical calculation method. The scheme of the method is presented. A recombining plasma mixed with cooling gases is investigated, and some other cases in which the effect of the electron temperature change on \dotn e is significant are briefly discussed.

Spectroscopic study of rapid mixing and cooling of a high-density He plasma flow penetrating into hydrogen gas

Rapid mixing and cooling of a high-density helium plasma flow injected into hydrogen gas was experimentally confirmed for the first time, which is essentially of importance to generate a recombining plasma as a short-wavelength laser medium. The plasma flow was produced by a small Z-pinch gun. The electron temperature and density was spectroscopically measured to be 12 eV and 6×1016 cm-3, respectively, before injected into the hydrogen gas. After injection of the gas, the plasma was rapidly cooled down to 4 eV while the density increased only by about 30%. It was also found that the cooled plasma tended to a recombining phase. A simplified calculation was also performed on the collisional radiative model to show that the rapid cooling was due to the atomic collisions between the electrons and hydrogen atoms mixed with the plasma.

Contribution of the Charge-Exchange Process for Inverted Population in a Recombining Plasma

We have numerically determined that high gain for the laser oscillation of the CVI 13.5 nm line is promoted by the contribution of the charge-exchange process, C6++H(1S) →C5++H+. In particular, the contribution shows a notable effect in some lower-temperature (~10 eV) recombining plasmas.