Stationary population inversion of hydrogen in an arc-heated magnetically trapped expanding hydrogen-helium plasma jet

A stationary recombining hydrogen-helium mixed plasma is generated in the arc-heated magnetically trapped expanding plasma jet that was developed in the authors' laboratory. The plasma can be generated very stably and a spectroscopic analysis is accomplished with sufficient accuracy. Population densities of atomic hydrogen whose principal quantum number is 3--12 are observed spectroscopically. When the plasma comes downstream, the electron temperature becomes low and population inversion is obtained between the level pairs 3-4, 3-5, and 4-5 in hydrogen atoms. The experimental results are explained very well by a calculation based on the collisional radiative model in which the hydrogen plasma is optically thin. The potential for cw lasing by H I is discussed in terms of atomic processes based on the experimental and numerical results