K. G. Tirsell

The interaction of 1.06 μm laser radiation with high Z disk targets

Gold disks have been irradiated with 1.06 μm laser light at intensities between 7 × 1013 and 3 × 1015 W/cm2, and pulse lengths between 200 and 1000 psec. Due to the high Z and long pulse, inverse bremsstrahlung becomes an important absorption mechanism and competes strongly with resonance absorption and stimulated scattering. In addition to measured absorptions, data on the temporal, spatial, angular, and spectral characteristics of the x‐ray emission are presented. Temporally and spectrally resolved back‐reflected light, and polarization‐dependent sidescattered light are detected, providing estimates for the amount of stimulated scattering and of the coronal electron temperature. Inhibited electron thermal conduction and nonlocal thermodynamic equilibrium ionization physics play key roles in bringing numerical simulations of these experiments into agreement with all of the above‐mentioned data.

Laser irradiation of disk targets at 0.53 μm wavelength

Results and analyses are presented for laser irradiation of Be‐, CH‐, Ti‐, and Au‐disk targets with 0.53 μm light in 3–200 J, 600–700 psec pulses, at nominal incident intensities from 3×1013 to 5×1015 W/cm2. The measured absorptions are higher than observed in similar 1.06 μm irradiations, and are largely consistent with modeling which shows the importance of inverse‐bremsstrahlung and Brillouin scattering. Observed red‐shifted back‐reflected light shows that Brillouin scattering occurs at low to moderate levels. Backscattering fractions up to 30% were observed in the f/2 focusing lens. The measured fluxes of multi‐keV x rays indicate hot‐electron fractions of 1% or less, with temperatures of 6 to 20 keV which are consistent with resonance absorption or perhaps 2ωpe. Measurements show 30%–50% efficient conversion of absorbed light into sub‐keV x rays, with time‐, angular‐, and spatial‐emission distributions which are generally consistent with non‐local‐thermodynamic‐equilibrium modeling using inhibited th...

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.

Characteristics of lateral and axial transport in laser irradiations of layered‐disk targets at 1.06 and 0.35 μm wavelengths

Results and analysis are presented for Be‐on‐Al disk target irradiations at 1.06 and 0.35 μm laser wavelengths with 600–700 psec pulses, 240 μm spot diameter, and 1×1014 W/cm2 absorbed intensity. Absorptions of 32%–39% (1.06 μm) and 90% (0.35 μm) are largely due to inverse bremsstrahlung. The hard x‐ray spectra indicate low hot‐electron fractions of 10−2 (1.06 μm) and 10−4 (0.35 μm). Backreflected light shows strong hot spots for 0.35 μm irradiations. Multiple absolute and relative x‐ray measurements are compared with one‐ and two‐dimensional computer hydrodynamics calculations. Only weak indications of lateral transport are found and limits are set from x‐ray imaging and spectral data from targets with and without a surrounding Ti shield. Axial transport appears strongly inhibited at 1.06 μm and mildly inhibited at 0.35 μm wavelength. Measured shock‐wave transit times and velocities imply ablation pressures of 7 Mbar (1.06 μm) and 11 Mbar (0.35 μm).

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.

Absolute detection efficiency of a microchannel plate detector to x rays in the 1-100 KeV energy range

There is little information in the literature on the performance of working micro-channel plate (MCP) detectors at high x-ray energies. We have measured the absolute efficiency of a microchannel-plate-intensified, subnanosecond, one dimensional imaging x-ray detector developed at LLNL in the 1 to 100 keV range and at 1.25 MeV. The detector consists of a gold photocathode deposited on the front surface of the MCP (optimized for Ni K(alpha ) x rays) to convert x rays to electrons, an MCP to amplify the electrons, and a fast In:CdS phosphor that converts the electrons kinetic energy to light. The phosphor is coated on a fiber-optic faceplate to transmit the light out of the vacuum system. Electrostatic focusing electrodes compress the electron current out of the MCP in one dimension while preserving spatial resolution in the other. The calibration geometry, dictated by a recent experiment, required grazing incidence x rays (15.6 degree(s)) onto the MCP detector in order to maximize deliverable current. The experiment also used a second detector made up of 0.071 in. thick BC422 plastic scintillator material from the Bicron Corporation. We compare the absolute efficiencies of these two detectors in units of optical W/cm2 into 4 (pi) per x ray W/cm2 incident. At 7.47 keV and 900 volts MCP bias, the MCP detector delivers approximately 1400 times more light than the scintillator detector.

Novel, movable, multianode x-ray source for imaging detector calibrations

We have developed a moveable, multi-anode, electron-induced x-ray source for calibrating x-ray imaging detector systems. The source features four water-cooled, copper-based anodes that are easily replaced and interchangeable under vacuum. A spherically symmetric filament and anode configuration yields very symmetrical source spot sizes variable from 1 to 5 mm diam. The source is coupled to vacuum by means of a triple-axis translator with x and z motions computer controlled to better than +/- 13 micrometers step accuracy. Absolute source flux is continuously monitored with a proportional counter. We characterize the source spot in detail by aiming the anode towards a pin-hole imaging x-ray detector system. To date we have used Co, Fe, Mg, and Ge vapor-deposited on copper anodes. When filtered with Ni or Mg foils, these emitting materials provide sets of four moderately pure x-ray lines below 1.5 keV. High deposition purity as well as 1E-7 torr source chamber pressures help reduce source degradation. Source translation and data acquisition are computer controlled using a Macintosh/LabVIEW software system. We discuss the application of our source configuration to spatial calibrations of extended pinhole and slit imaging detector systems. In addition, our multi-anode capability is useful for calibrating x-ray spectrograph response versus wavelength.