Transverse heating of a cold dense helium plasma by a pulsed CO2 laser beam

Abstract

We present experimental evidence for the transverse heating of a dense (3.2 × 1017 cm−3) cold (≃4 eV) helium θ-pinch plasma by the focused beam of a TEA CO2 laser having a pulse duration of 180 nsec and a peak power of approximately 5 MW. The laser light was incident on the plasma during the first pinch compression, when streak photographs taken with a high-speed camera indicate a stable quiescent plasma lasting about 1.5 μsec. Time-resolved spectroscopic measurements were made to determine the plasma density and temperature before and after irradiation by the CO2 laser. Maximum temperature increases of 0.8 eV and density increases of 0.5 × 1017 cm−3 were observed for a laser energy of 0.7 J incident on the plasma. The theoretical description of the interaction considers the quasistatic temporal response of the plasma to the absorption of laser energy by inverse bremsstrahlung, and includes deviations from local thermodynamic equilibrium (LTE) before, during, and after irradiation. The absorbed energy is consumed in expansion, heating, and further ionization of the plasma. Highly excited atomic or ionic states are assumed to be in Saha equilibrium with each other and with the ionization continuum, whereas the ground states of successive ionization stages are assumed to be in coronal equilibrium. Thermal conduction losses are negligible. The theory correctly predicts the observed temperature increases but not the observed density changes, since inertial effects are neglected in order to render the model tractable.