Randolf Butzbach

Time-resolved ten-channel monochromatic imaging of inertial confinement fusion plasmas

We report on what we believe to be the first use of toroidally bent crystals to record two-dimensional, spatially resolved, monochromatic images of laser-produced fusion plasmas combined with a 34-ps fast x-ray framing camera. An array of five toroidal silicon (311) and five toroidal germanium (311) crystals was developed. The imaging properties of the geometries are checked by a ray-tracing program and are compared with experimental results. The total imaging system (crystal and detector) provides an experimentally measured spatial resolution better than 15 mum. Time histories for the hydrogenlike argon emission and the heliumlike argon emission of fusion pellets driven with the GEKKO XII glass laser system are presented.

Monochromatic x-ray imaging with bent crystals for laser fusion research

Monochromatic x-ray imaging with toroidally bent crystals has been applied to investigate critical issues in laser fusion research in collaboration with Jena University [for example, I. Uschmann et al., Rev. Sci. Instrum. 66, 734 (1995)]. Described in this article is the study on x-ray diagnostics to provide simultaneously time, space, and spectral resolutions for the laser fusion experiments by the use of toroidally bent crystals. A monochromatic x-ray imager was developed, which consists of two sets of identical five bent crystals and a fast x-ray framing camera. The attained spatial resolution of 6 μm, time resolution of 34 ps, and spectral resolution of 10 eV are high enough to observe temporal evaluation and temperature distribution of the laser fusion hot core. In addition, a new x-ray imager with bent crystal was developed to investigate hydrodynamic instabilities occurring in laser-driven planer targets as one of the most critical issues in laser fusion research. The imager was so designed as to m...

Temporal evolution of temperature and density profiles of a laser compressed core (invited)

Temporal evolutions of the electron temperature and density profiles in the inertial confinement fusion core plasma were obtained through a combination of experimental technology to create a uniform core, diagnostics developments, and a data analysis technique to obtain an optimal solution. In the experiment, time- and space-resolved x-ray spectroscopic measurements were carried out; an x-ray streak spectrograph and a monochromatic x-ray framing camera provided temporal evolution of x-ray spectrum and monochromatic emissivities, respectively. The spectrum and emissivity profile were iteratively fitted as a function of electron temperature, density, and space. Then, optimal solutions were successfully extracted with the help of Niched–Pareto genetic algorithm. Temporal behavior between those data is well corrected by comparing spatially and spectrally integrated intensities. The resultant profiles were compared with hydrocode simulations, showing that spatially averaged electron temperature and density evo...

Experimental characterization of short-wavelength Ni-like soft-x-ray lasing toward the water window

We report experimental results of studies of Ni-like x-ray lasing at wavelengths as short as 4.5 nm. The performance of x-ray lasing pumped by various types of pulse train that were composed of 100-ps pulses was investigated with a double-slab target placed in series to double the gain length. Two opposing laser beams separated by an appropriate time difference irradiated the double targets for quasi-traveling-wave pumping. Well-collimated double-target amplification was successfully demonstrated for Yb and Hf lasing at 5.0 and 4.7 nm whose gain–length products were 11 and 6, respectively. The Ni-like lasing lines of Ta have also been observed at 4.5 nm.

Time- and space-resolved X-ray spectroscopy for observation of the hot compressed core region in a laser driven implosion

Abstract Fusion pellet implosion by direct laser irradiation was investigated by means of time- and space-resolved X-ray spectroscopic measurements. Fusion pellets filled with a deuterium fuel gas including a small amount of Ar dopant were irradiated with 12 beams of intense partially coherent light and line emissions from the dopant were observed. There were two type of implosion studies. First, experimental conditions were carefully chosen to provide implosions that were as stable as possible, these we refer to as the “balanced” cases. Second, the energy balance was manipulated so that two specific beams, diametrically opposed to each other, had relatively large energy differences. This imposes additional low-modal non-uniformity on the pellet, and these experiments are called that the “unbalanced” cases. Experimental results were compared with hydrocode simulations, which were post-processed by the aid of X-ray spectrum analysis code. The experimental results in terms of temporal variations of the Ar He-β line and their spatial profiles obtained with an X-ray monochromatic imager were well replicated by one-dimensional (1-D) simulation for the balanced case, whereas those for the unbalanced case showed large discrepancies. Furthermore, a clear difference was found in the emission of Li-like satellite lines between two cases: In the unbalanced case, the satellite lines became much more intense than the He-β line at late time in the implosion. These experimental results suggest that the imposed low-modal non-uniformities assisted in the quenching of hot, compressed core formation.

Diagnostics and applications of laser-produced plasmas

X-ray spectroscopy is one of the most important diagnostics of laser-produced plasmas, finding application in diverse areas such as laser fusion, x-ray lasers, and novel experiments using shot-pulse lasers to probe chemical and biological phenomena on the femtosecond timescale. Depending on the aims of these experiments, either high resolution spectra combined with either spatial or time resolution, or monochromatic x-ray spectrometer was also used here in x-ray diagnostics of 4f yields 3d transitions in Nickel-like transitions of elements with atomic numbers between 70 and 74. The dependence of this x-ray emission on laser energy, spot size, and target materials provides information about ionization degree, electron temperature and density - important parameters for the population inversion of a Ni- like x-ray laser in the water window.

Development of intense short-wavelength Ni-like soft x-ray lasers toward the water window

We report experimental results on Ni-like x-ray laser at the wavelength as short as 4.4 nm. The performance of x-ray lasing pumped by various types of pulse trains which were composed of 100 ps pulses was investigated with a double slab targets which were placed in series to double the gain length. Two opposing laser beams irradiated the double targets with a suitable time difference for quasi traveling wave pumping. The well collimated double target amplification was successfully demonstrated with two beam irradiation for Yb and Hf lasing at 5.0 nm and 4.7 nm whose gain length products were 11 and 6, respectively. The Ni- like lasing line of Ta have been observed at 4.5 nm. Based on these results, we will report the suitable pumping condition for the saturated water window x-ray lasers.

A spectroscopy diagnostic of plasma gradients in inertial confinement fusion imploded cores (abstract)

X-ray spectroscopy of laser-driven imploded inertial confinement fusion (ICF) cores has proven to be a powerful diagnostic of spatially averaged temperature and density plasma conditions at the collapse of ICF implosion experiments. Temperature and density time histories can be extracted from the analysis of time-resolved x-ray line spectra using the temperature and density sensitivity of line intensities and Stark-broadened line shapes. The next step in the spectroscopy of imploded cores is the bracketing of core plasma gradients as a function of time. To this end, we discuss a spectroscopy diagnostic which is based on the self-consistent and simultaneous simulation and analysis of time-resolved x-ray line spectra and x-ray monochromatic images. Abel inversion of x-ray monochromatic images provide line emissivity spatial profiles; this information is critical for the determination of gradients in the core. We apply this technique to the analysis of data recorded in Ar-doped ICF implosion experiments driv...

X-ray spectroscopic analysis of the influence of low-modal nonuniformity on the formation of a hot spark

In the direct-drive scheme implosion of the inertial confinement fusion, the hot spark formation is critically affected by laser irradiation non-uniformities and subsequent hydrodynamic instabilities. Influence of the low- modal irradiation non-uniformities on the hot spark formation was investigated by means of the time- and space- resolved x-ray spectroscopic measurements. Experimental results were compared with post-processed hydro-code simulations by the aid of x-ray spectrum analysis code.