Absorption of CO2-laser radiation in a hypocycloidal-pinch plasma

Absorption of a 50-J pulsed CO2 laser beam in the plasma produced in a hypocycloidal-pinch apparatus was studied. The apparatus consists of three disk electrodes with a 5-cm hole on the axis. The current sheets are snow-plowed toward the center of the hole to produce a plasma with a density greater than 10 to the 18th/cu cm and a temperature near 1 keV which emits soft X rays for 5 microsec. IR detectors measured the laser radiation incident onto, transmitted through, and scattered from the plasma as functions of time. For a period up to 70 microsec from the onset of X-ray emission, neither transmitted nor scattered laser radiation was observed above the detector threshold of 0.3% of the incident flux. This indicates effective absorption of the laser beam (greater than 99%) and that the particle confinement time extends far beyond the energy confinement time of 5 microsec. Thus, the plasma in this apparatus appears much more amenable to laser heating than that in a theta pinch.