A Energy Scaling Law for AN Isolated Plasma Produced by a Low Intensity Laser.

An experiment is carried out in which small pellets of solid deuterium in a high vacuum are turned into a plasma with a low power pulsed ruby laser beam. The expansion energy is measured and correlated with the intensity of the laser light. The pellets were formed by cooling a feed gas down to near its triple point using a Joule-Thompson expansion refrigerator. The solid material was extruded through an orifice into a vacuum. The continuous filament was cut into short sections with a very fine oscillating wire. The resulting 50mum pellets were directed into a chamber with a higher vacuum by means of a long collimating tube. Detection electronics triggered the Q-switched laser when a pellet fell close enough to the focal region. The risetime of the laser pulses was measured to be less than 1 ns and the size of the focal region to be around 250 μm in diameter. The range of laser intensities studied extends from about 5 times 10^9 W cm^ {-2}, where reliable breakdown of the solid occurred, to around 10^{11} W cm^{-2}. The energy of the plasma is obtained from time -of-flight data collected with electrostatic probes. It is this expansion energy which is correlated with the intensity of the laser light. The experimental power obtained is E_{plasma} = C * (Phi_{laser}) ^{0.32}.. A discussion of the important physical phenomena in the experiment is presented and several models are compared with the experimental results. Whereas one simple analytical model gives surprisingly good agreement, a much more detailed numerical one is poorer, especially at the low end of the range of laser intensities. Since the plasma parameter Lambda is of the order of unity in the region where a large fraction of the light is being absorbed in the experiment, it is likely that the discrepancy is due to the inapplicability of the usual formulas, which are derived using plasma theory.