NANOSECOND CARBON-DIOXIDE LASER INTERACTION WITH A DENSE HELIUM Z-PINCH PLASMA

A short pulse CO(,2) laser system was constructed to investigate the interaction of intense electromagnetic radiation with dense plasma. The laser was focused perpendicular to the axis of a linear helium Z-pinch plasma and properties of the transmitted beam were monitored. Transmitted beam intensity and spatial distribution were measured as functions of incident intensity and interaction time.; The short pulse laser system consisted of a single-mode oscillator, pulse switch, amplifiers, and focusing optics. The oscillator was a transversely-excited atmospheric pressure (TEA) discharge module having an intracavity CW gain tube for single-mode operation. The pulse selector was a germanium semiconductor reflection switch controlled by a pulse-transmission model (PTM) ruby laser. Switched 10.6 micron pulses were preamplified in a triple-pass double-discharge TEA module and boosted to maximum power in a commercial large aperture amplifier. The laser beam from the final amplifier was focused onto the plasma by a modified Newtonian telescope. The system was capable of producing 4 nanosecond (full width at half maximum) pulses containing up to 2.7 joules. The focused intensity on target is greater than 10('12) W/cm('2) in a 125 micron diameter focal spot.; The plasma was a pulsed linear Z-pinch having a peak density of 4 x 10('19)/cm('3) in a 3 mm column at a temperature of 20 eV. The plasma density is known from holographic interferometry, and the temperature was inferred from visible wavelength spectroscopy and x-ray diagnostics. Depending on the time of laser incidence, the highly collisional plasma provided either an overdense or an underdense target.; Previous work with 40 nanosecond pulses revealed penetration of the critical region of the plasma. The transmitted pulse was strongly modified, and the transmitted spatial distribution was characteristic of diffraction through a hard, circular aperture. No penetration was observed with the 4 nanosecond pulses incident on the overdense plasma. Fresnel diffraction was seen when the 4 nanosecond pulse was incident on the underdense plasma. In addition, measurements of the transmission of the 4 nanosecond pulses in the underdense plasma were performed.; The results of the experiments with the overdense plasma were found to be consistent with plasma hydrodynamic theory. The 40 nanosecond pulse was sufficiently long to "burn" through the plasma, but the 4 nanosecond pulse was not. The 4 nanosecond pulse was long enough to form a local density depression in the underdense plasma and density gradients steep enough to produce Fresnel diffraction, despite the absence of a critical surface. The resultant change in refractive index could cause thermal self-focusing.; The transmission measurement was not found to be consistent with a simple model of inverse bremsstrahlung absorption. At an intensity of 10('12) W/cm('2) there was a sharp decrease in transmission. This suggests the possibility of either increased absorption due to enhanced ionization or increased reflection due to stimulated Brillouin backscatter.