B. Yaakobi

Focusing x-ray spectrograph for laser fusion experiments

X-ray spectra from laser fusion targets are normally measured with flat crystal (nonfocusing) spectrographs. We describe here the characteristics of a focusing spectrograph which is capable of recording wide band spectra with significantly higher sensitivity. Measuring spectra in the (10-11) A range from glass microballoons imploded by a two-beam Nd:Glass laser system we find intensity per unit area on film about 100 times higher with a curved mica than with a flat mica spectrograph.

Response model for Kodak Biomax-MS film to x rays

X-ray-sensitive film is used for a variety of imaging and spectroscopic diagnostics for high-temperature plasmas. Replacement film must be found as older films are phased out of production. Biomax-MS is a “T-grain” class of film that is proposed as a replacement for Kodak DEF and a model of its response to x rays is presented. Data from dimensional measurements of the film, x-ray transmission measurements, scanning electron microscopy micrograph images, and x-ray calibration are used to develop this sensitivity model of Biomax-MS film as a function of x-ray energy and angle of incidence. Relative response data provide a check of the applicability of this model to determine the x-ray flux from spectrum data. This detailed film characterization starts with simple mathematical models and extends them to T-grain–type film.

Diagnosis of High-Temperature Implosions Using Low- and High-Opacity Krypton Lines

High-temperature laser target implosions can be achieved by using relatively thin-shell targets, and they can be diagnosed by doping the fuel with krypton and measuring K-shell and L-shell lines. Electron temperatures of up to 5 keV at modest compressed densities (∼1–5 g/cm3) are predicted for such experiments, with ion temperatures peaking above 10 keV at the center. It is found that the profiles of low-opacity (optically thin) lines in the expected density range are dominated by the Doppler broadening and can provide a measurement of the ion temperature if spectrometers of spectral resolution Δλ/λ ≥ 1000 are used. For high-opacity lines, obtained with a higher krypton fill pressure, the measurement of the escape factor can yield the ρRof the compressed fuel. At higher densities, Stark broadening of low-opacity lines becomes important and can provide a density measurement, whereas lines of higher opacity can be used to estimate the extent of mixing.

IMAGING THE COLD, COMPRESSED SHELL IN LASER IMPLOSIONS USING THE Kα FLUORESCENCE OF A TITANIUM DOPANT

Abstract A novel method for imaging the cold, compressed shell in laser-driven implosions, without using backlighting, is presented. A high- Z -doped shell can be imaged using K α line radiation, which fluoresces due to excitation by the intense radiation from the hot core. We show results from titanium-doped target implosions, demonstrating that the K α fluorescence, of significant intensity, is indeed emitted from a shell around the hot core. The one-dimensional spatial profile of the Ti K α radiation yields the average dimensions of the cold shell around the time of peak compression. This result, coupled with the shell areal density determined by the K-edge absorption, yields an estimate of the shell density.

Areal-density measurement of laser targets using absorption lines

Abstract Absorption lines due to 1s-2p transitions in titanium ions are predicted to be observed in the implosion of titanium-doped shells. The measured absorption of these lines can be used to determine the peak areal density, ρΔr, of the doped layer and hence of the total compressed shell. The absorption lines are studied by solving the radiation transport equation using opacity tables and hydrodynamic simulations. The absorption is a function not only of ρΔr but also of the density and temperature of the absorbing layer. However, it is shown that the areal density can be estimated with reasonable accuracy by using the measured intensity of absorption and its distribution over the various absorption lines. The considerations affecting the choice of doping parameters are discussed, as well as the effect of integrating the measured spectrum over time and target volume.

X-ray backlighting imaging of mixed imploded targets

The X-ray image of a compressed shell (enhanced by high-Z doping) was used in recent experiments to diagnose the effect of shell-fuel mixing. The emission ring (due to the limb effect) moves toward smaller radii due to mixing. We show here that using backlighting imaging can greatly enhance future imaging experiments. In addition to the emission ring, a backlighting absorption ring can appear around it. The absorption ring is virtually unaffected by mixing. Thus, the relative position of the two rings constitutes a mixing signature. In other words, the absorption ring delineates the colder part of the shell and is a true signature of the compression, whereas the emission ring reflects the shell material motion due to mixing.

Diagnosis of core-shell mixing using absorption and emission spectra of a doped layer

Abstract We present theoretical analysis of a diagnostic method for detecting core-shell mixing in laser-fusion experiments, based on measuring the emission and absorption spectrum of a Ti-doped layer embedded within the target shell. The doped target layer is placed away from the shell-fuel interface so that the Ti remains relatively cool throughout the implosion. As a result, only absorption lines are observed, of 1 s −2 l transitions in ions with an unfilled L shell. However, for a sufficiently strong mixing, some Ti material migrates to the interface region and emits He- and H-like lines. A particular target experiment is simulated and the expected experimental signature calculated and analyzed.

Target imaging and backlighting diagnosis

The expected backlighting and self‐emission images of a particular CH target to be imploded on the Omega Upgrade are calculated for a variety of experimental parameters. It is shown that to overcome the problem of target self‐emission, the image has to be monochromatized with a diffracting crystal. For the target studied, the two image components are then comparable in intensity and both provide useful information on target behavior. A particularly interesting feature is the appearance in the self‐emission of a circular spike which closely delineates the fuel‐shell interface, but requires high spatial resolution to be observed.

X‐ray spectroscopic methods for the diagnosis of laser‐imploded targets (invited)

Several methods involving x‐ray spectroscopic methods for diagnosing laser‐imploded targets are discussed. The first method involves the recording of absorption lines formed in the target tamper, out of the continuum emitted by a hotter compressed core. This method is applied to ablatively imploded targets having a thin KCl signature layer. The tamper ρΔR is deduced from the area within the absorption lines, whereas the tamper temperature is deduced from the intensity distribution among absorption lines of adjacent charge states. In a second method, doubly diffracting crystals can give two‐dimensional monochromatic images of thin signature layers in spherical targets. Such information is useful in studying stability and mixing. Experimental results relevant to these methods will be shown and the limitations on their application to laser‐target experiments will be discussed.

Diagnosis of high-temperature implosions using low- and high-opacity Krypton lines

High-temperature laser target implosions can be achieved by using relatively thin-shell targets, and they can be. diagnosed by doping the fuel with krypton and measuring K-shell and L-shell lines. Electron temperatures of up to 5 keV at modest compressed densities ({approximately}1-5g/cm{sup 3}) are predicted for such experiments, with ion temperatures peaking above 10 keV at the center. It is found that the profiles of low-opacity (optically thin) lines in the expected density range are dominated by the Doppler broadening and can provide a measurement of the ion temperature if spectrometers of spectral resolution {Delta}{lambda}/{lambda} {ge} 1000 are used. For high-opacity lines, obtained with a higher krypton fill pressure, the measurement of the escape factor can yield the {rho}R of the compressed fuel. At higher densities, Stark broadening of low-opacity lines becomes important and can provide a density measurement, whereas lines of higher opacity can be used to estimate the extent of mixing.