Plasma Production by a High-Power <i>Q</i>-Switched Laser

Abstract

Studies have been made of the confinement in a magnetic mirror of plasmas produced by focusing a Q-switched laser beam on aluminum disks, foils, and spheres. High speed photographs show that, in the absence of a confining magnetic field, the plasma generated from ball targets is roughly symmetrical whereas one component of the plasma generated from flat targets is asymmetric and moves toward the laser with a velocity of about 107 cm/sec. A second component which is more nearly symmetric expands with a velocity of approximately 2 × 106 cm/sec. The expansion velocities are not appreciably reduced in a 14–6-14 kG mirror field. However, the duration of the luminosity from a slower constituent of the plasma increases from 1 μsec to about 5 μsec and microwave interferometer measurements show an electron density of about 5 × 1011 cm−3 at times as late as 50 μsec. Electrostatic probes verify that the maximum expansion velocity corresponds to an ion kinetic energy of about 1.4 keV. Time integrated spectra of the plasma show lines from Al III. Microwave attenuation measurements lead to an apparent electron temperature of the order of 0.03 eV at times later than 1 μsec. Analysis of microwave data shows that radial density gradients probably exist in some plasmas.