F. Ze

CHARACTERIZATION OF LASER-PRODUCED PLASMA X-RAY SOURCES FOR USE IN X-RAY RADIOGRAPHY.

We report the absolute conversion efficiency ξx from the incident laser light energy to x‐ray photons for laser‐produced plasmas. Potential x‐ray backlighting (radiography) line sources having photon energies from 1.4 to 8.6 keV are studied as a function of laser wavelength, pulsewidth, and intensity. The laser intensity and pulsewidth range from 1014 to 1016 W/cm2, 100 ps to 2 ns and include incident wavelengths of 1.06, 0.53, and 0.35 μm. We found that K‐shell x‐ray line emission ξx : (1) decreases with increasing x‐ray energy, (2) decreases with increasing laser intensity, (3) decreases rapidly with pulselength, and (4) moderately increases with decreasing laser wavelength. On the contrary, for Au M band emission, at a fixed laser intensity and pulsewidth, ξx significantly increases (∼25×) upon decreasing the laser wavelength from 1.06 to 0.35 μm.

Drive characterization of indirect drive targets on the Nova laser (invited)

The indirect drive method of inertial confinement fusion uses a high‐Z radiation case to convert energy from high‐powered laser beams to x rays which implode fusion capsules. Experiments have been performed on the Nova laser to characterize the x‐ray production in high‐Z cavities for studying the efficiency for x‐ray production using two methods for characterization. One method measures the shock velocity produced in low‐Z materials by the radiation. The shock velocity is measured by observing the optical signal from the rear of a stepped or continuously varying thickness of Al placed over a hole in the cavity wall. The other method measures the reradiated x‐ray flux from the cavity wall viewing through a hole in the cavity. Both methods have been shown to provide a consistent characterization of the x‐ray drive in the cavity target.

A new multichannel soft x-ray framing camera for fusion experiments

Two‐dimensional, time resolved x‐ray imaging is a principal technique used to study hot plasmas produced in laser heated targets. It can be used to study laser energy deposition within the irradiation spot, the spatial and temporal dependence of laser to x‐ray conversion efficiency, electron transport and density profiles, mass ablation rates as well as x‐ray driven implosions in inertial confinement fusion experiments. We have successfully developed a new soft x‐ray framing camera which will allow us to record two‐dimensional images at different times almost simultaneously. It is a broadband diagnostic (100 eV≤Δhν≤400 eV) having three channels which can obtain x‐ray images at four different times from laser driven targets. Its current configuration includes one 500 eV, one 1.0 keV, and one ∼2.5 keV channels. The two low energy channels resulted from pairing transmission filters to grazing x‐ray mirrors. Other channel options can be implemented easily to measure other x‐ray energies. Four different stripl...

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).

Observation of enhanced x-ray emission from long-pulse-width laser-produced plasmas

Experiments investigating the pulse‐width dependence of thermal x‐ray conversion efficiencies (hν<1.5 keV) in laser‐heated gold plasmas are described. The results show that the instantaneous ratio of the emitted x‐ray flux to the laser energy deposition rate increases throughout a 4‐ns laser pulse. The studies were carried out using single arms of the 10‐beam Nova laser facility. The irradiance was ∼4–5×1014 W/cm2 in the target plane as we varied the laser pulses’ FWHM from 2 to 4 ns. The laser pulses were nominally flat‐topped and contained between 1 and 2 kJ of (1)/(3) μm light. Time‐resolved plasma radiation was monitored with a broadband, streaked x‐ray spectrograph that has a roughly 30‐ps time resolution and channels that are roughly 100–150 eV wide. One‐dimensional numerical models run with the lasnex code produce a conversion efficiency that is nearly constant throughout the laser pulse. We discuss various approximations made in the one‐dimensional models and conclude that none of them are a likel...

Angular sensitivity of gated microchannel plate framing cameras

Gated, microchannel-plate-based (MCP) framing cameras have been deployed worldwide for 0.2–9 keV x-ray imaging and spectroscopy of transient plasma phenomena. For a variety of spectroscopic and imaging applications, the angular sensitivity of MCPs must be known for correctly interpreting the data. We present systematic measurements of angular sensitivity at discrete relevant photon energies and arbitrary MCP gain. The results can been accurately predicted by using a simple two-dimensional approximation to the three-dimensional MCP geometry and by averaging over all possible photon ray paths.

Use of thin wall imaging in the diagnosis of laser heated hohlraums

High-Z, laser heated hohlraums can be made thick enough to contain thermal radiation, yet thin enough to let out x rays >∼6 keV produced by hot, relatively dense blow-off plasma. We use such “thin wall hohlraums” to observe the physical location of hot, dense, laser produced hohlraum plasmas. This technique has allowed us to come to some understanding of laser transport/deposition, plasma stagnation, and bulk plasma filling.

A comparative study of x-ray emission from laser spots in laser-heated hohlraums relative to spots on simple disk targets

In this paper we report the results of experiments that compare the x-ray emission from a laser spot in a radiation-filled hohlraum to that from a similar laser spot on a simple disk target. The studies were done using the Nova laser facility [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)] in its 0.35 μm wavelength, 1 ns square pulse configuration. Focal spot intensities were 2–3.5×1015 W/cm2. X-ray images measured x-ray conversion in a hohlraum and from an isolated disk simultaneously. A laser spot inside a hohlraum emitted more x rays, after subtracting the background emission from the hohlraum walls, than a spot on a disk. Numerical models suggest the enhanced spot emission inside the hohlraum is due to an increase in lateral transport relative to the disk. Filamentation in the hohlraum will also increase the spot size. The models agree fairly well with the results on spot spreading but do not explain the overall increase in conversion efficiency.

Investigation of quantum efficiencies in multilayered photocathodes for microchannel plate applications

Microchannel plates (MCPs) are used in many diagnostic systems to study laser-plasma interactions. Typically the front surface of a MCP is coated with some photocathode (PC) material to convert x ray to electron. This is followed by electron multiplication along the microchannels. Materials such as CsI, Al, and Au have been identified as good PC materials, but the overall quantum efficiencies of these materials are low, so that electron multiplication along the channel has been the only path to signal amplification. This approach is known to have some problems (J. D. Wiedwald, University of California UCRL-JC-110906, August 1992). We investigated the effect of pairing transmission PC to standard MC’s and found that CsI and Au foils used in multilayered configuration, significantly enhance the overall quantum efficiency of the combination. The findings could lead to better MCP design and fabrication.

The angular dependence of the absorption of 0.35 μm laser light by high‐Z, laser‐produced plasmas

Experiments to study the angular dependence of laser‐light absorption in long‐scale‐length, planar plasmas are reported. Flat‐topped, 1 nsec pulses of 0.35 μm laser light, with up to 2.5 kJ at average laser intensities of 3×1013 to 6×1015 W/cm2, were used to irradiate thick, gold targets at angles of incidence of 0° to >50°. The scattered light was measured using a number of diagnostics, and the absorption was inferred from these measurements. The absorption decreased somewhat with increasing laser intensity, typically remaining above 90% for laser intensities below 1015 W/cm2, and was very weakly dependent upon the angle of incidence. Modeling of the reflected light was used to constrain the properties of the plasma and of the reflecting mechanism. No simple model of a single mechanism is consistent with all aspects of the data.