Roger A. Haas

Irradiation of parylene disks with a 1.06 μm laser

Parylene (C8H8) disks have been irradiated with Nd:YAG‐glass laser pulses focused to flux levels in the 1015 to 1017 W/cm2 range. The flux level was varied by changing the pulse length (50–150 psec), the laser energy (5–15 J), and the axial position of the target with respect to the f/1.1 focusing lens. An extensive array of diagnostics was used to measure the temporal and energy distribution of the focused laser light at the target, the temporal and angular distribution of the scattered laser light, the x‐ray spatial and spectral emission characteristics, and the emitted ion and electron energy distributions. The experimental results, together with two‐dimensional numerical simulations imply absorption via collective processes, laser generation of suprathermal electrons, and transport inhibition consistent with the presence of mega‐Gauss level thermoelectric magnetic fields.

Evidence of localized heating in CO2‐laser‐produced plasmas

Polyethylene foils and parylene disks have been irradiated by CO2 (λ∼10.6 μm) and Nd : YAG‐glass (λ∼1.06 μm) laser pulses focused to flux levels in the 1013‐ and 1014‐W/cm2 range. X‐ray pinhole photographs of the CO2‐laser‐produced plasmas exhibit intense localized emission regions whose characteristic dimensions are smaller than the nearly diffraction‐limited focal spot of the laser. Corresponding photographs of the glass‐laser‐produced plasmas show no evidence of localized emission. These experimental results are consistent with theoretical predictions for laser‐beam trapping and/or filamentation in laser‐produced plasmas.

Interaction of 1.06 μm laser radiation with variable Z̃ targets

Parylene (C8H8) and tungsten‐glass (W2O/P2O5) disks have been irradiated with 150–400 psec Nd:YAG‐glass laser pulses focused to diameters of 250–300 μm with flux levels in the 1013–1015 W/cm2 range. An extensive array of diagnostics was used to measure the temporal and energy distributions of the focused laser light at the target, the angular distribution of the scattered laser light, the x‐ray spatial and spectral emission characteristics, and the emitted ion and electron distributions. Analysis of the experimental results indicates that the laser‐plasma interaction was characterized by a variety of collective phenomena which appeared stronger in the tungsten‐glass experiments.

Evidence for profile steepening in laser‐irradiated plasmas

Experimental evidence of a steepened electron density profile near critical density has been obtained by studying the light scattered by targets (10‐μm thick disks and 100‐μm diam glass spherical microshells filled with deuterium and tritium gas) illuminated by linearly polarized, 1.06‐μm light. Scale lengths on the order of 1 μm have been inferred both from the polarization of the reflected light and from the azimuthal asymmetry (asymmetry about the beam axis) of the time‐integrated scattered light with respect to the laser electric field. Azimuthally asymmetric heating of the microshells targets is indicated both by x‐ray micrographs and by the spatial distribution of the plasma blowoff.

Valkyrie, A CO 2 laser system for laser fusion experiments

The Livermore COz laser system, Valkyrie, was constructed to provide an experimental facility capable of delivering 50 J in a diffraction limited beam at 10.6 pm in 1 nsec. This device has met all of its design goals and is in operation as a target irradiation facility. On a typical day, Valkyrie provides 40 J 10 X in 1 nsec at a repetition rate limited by the time required to replace targets. In order to design Valkyrie a CO, amplifier modeling computer program predicted the output of individual amplifier modules. This information was fed into a laser system simulation routine in order to determine amplifier staging. Once specified and manufactured, Valkyries amplifiers were subjected to an exhaustive series of tests. Small signal gain as a function of gas mixture, E/n, pump pulse duration, and spatial position was measured. Valkyries oscillator employs a specially designed UV preionized TFA gain medium (Lumonics 142A), a Ge Brewsters angle acoustooptic mode-locker, and an NaCl etalon as an output mirror. This oscillator generates a burst of 1 nsec duration pulses with peak pulse energies in excess of 50 mJ distributed reproducibly among -six rotational lines centered on the P20 transition of the 10.4 pm hand. A single pulse is selected by a switch-out assembly consisting of a laser triggered spark gap and CdTe Pockels cells. The pulse is then directed through a second Lumonics 142A which serves as a preamplifier and into a 3 x Keplerian beam expander with a spatial filter at its focus. Expended and collimated, the pulse propagates through a nine centimeter aperture UV-preionized TEA amplifier (Lumonics 602A) followed by two eleven centimeter aperture cold-cathode E-beam sustained amplifiers operated at two atmospheres. Emerging in the target area, the pulse contains 50 J in 1 nsec with an approximately Gaussian spatial intensity distribution. Two NaCl beamsplitters which are wedged and anti-reflection

Prepulse extinction-ratio measurements on a CO 2 -laser system

Prepulse extinction-ratio measurements performed on two pulse-selection systems at the input and the output of a CO 2 -amplifier chain are described.

Plasma diagnostics using laser-excited coupled and transmission ring resonators

In this paper a simple two‐level laser model is used to investigate the frequency response of coupled‐cavity laser interferometers. It is found that under certain circumstances, often satisfied by molecular gas lasers, the frequency response exhibits a resonant behavior. This behavior severely complicates the interpretation of coupled‐cavity laser interferometer measurements of rapidly varying plasmas. To circumvent this limitation a new type of laser interferometer plasma diagnostic with significantly improved time response was developed. In this interferometer the plasma is located in one arm of a transmission ring resonator cavity that is excited by an externally positioned laser. Thus, the laser is decoupled from the interferometer cavity and the time response of the interferometer is then limited by the Q of the ring resonator cavity. This improved time response is acquired without loss of spatial resolution, but requires a more sensitive signal detector since the laser is no longer used as a detecto...