Ponderomotive force exerted on a plasma by an infrared laser beam

The continuum and single particle approaches to the ponderomotive force in an underdense plasma are presented. The force density tends to expel the plasma out of a focused laser beam. Transient and steady-state responses of the plasma to the ponderomotive force are studied, and the importance of α = (ωp2/ω2) 〈ε0(E2/2) 〉 /2neKT is emphasized. Experimental verifications are presented, obtained with a pulsed CO2 laser-amplifier system (500 MW) irradiating an argon arc plasma (ne = 3×1014 cm−3, T=1.4 eV). Density changes are monitored by studying light emitted from the focal region, and the measured decrease in light intensity is consistent with the plasma density decrease expected for a value of α=0.37.