Guohong Yang

Opacity measurement of a gold plasma at Te = 85 eV

The opacity of a gold plasma at the temperature of 85 eV and density of 0.02 g/cm3 was measured over the energy range from 150 eV to 1200 eV. The gold sample was heated by thermal x-ray radiation generated with a foam-baffled gold cavity. The sample transmission was obtained from the backlight, absorption and self-emission spectra measured by a time-gated, spatially resolved grating spectrometer, with the backlight and absorption spectra being measured simultaneously in a single shot and the self-emission in another shot. The temperature and density of the gold absorber were determined by the hydrodynamic simulation with Multi-1D code, which was partially tested by the reemission radiative flux measurements of the heated sample. This work permits the first test of opacity models over the photon energy range that dominates the Rosseland mean opacity at the temperature of interest for the inertial confinement fusion.

L- and M-shell absorption measurements of radiatively heated Fe plasma

Measurements of iron-plasma absorption spectrum over 150–1200 eV photon energy range were reported at temperature T = (72 ± 4) eV. The electron temperature was diagnosed with the absorption spectrum of aluminum mixed with iron. The density was not diagnosed directly but obtained from a radiative hydrodynamic simulation with the Multi-1D code. The broad photon energy range enables simultaneous observation of the L-shell and M-shell transitions that dominate the radiation transport at this temperature. The spectrally resolved transmission data were compared to the detailed-configuration-accounting model calculations and reasonable agreement was found.

Efficient K-shell x-ray sources produced with titanium foils

The conversion efficiency of titanium K-shell x rays is experimentally investigated in the Shenguang II laser facility. For comparison, Ti foils with the thickness of 3.5 and 5.8μm are irradiated under the same laser condition. The conversion efficiency with the thinner foils reaches about 3.5% and is about two times of that obtained with the thicker foils. The experiments show that the enhancement of the conversion efficiency should be due to the larger size of hot underdense plasmas generated with the thinner foil.

K-shell transition absorption measurement of radiatively heated Al plasma

High temperature aluminum plasmas have been produced by irradiating the layered Au–Al foils with the smoothed high power laser at the Xingguang II laser facility. High-resolution transmission spectrum of the Al plasma has been measured by using penta-erythritol tetrakis (hydroxymethy) methane C(CH2OH)4 crystal spectrometer. Absorption lines of the aluminum ion transition 1s−np(n=3,4,5) in the region of 0.61–0.70 nm, have been observed and identified. The unresolved transition array model has been introduced to calculate the transmission spectra of aluminum plasma. The measured transmission spectrum has been compared with those calculated.

Calibration of a gated flat field spectrometer as a function of x-ray intensity

We present an experimental determination of the response of a gated flat-field spectrometer at the Shenguang-II laser facility. X-rays were emitted from a target that was heated by laser beams and then were divided into different intensities with a step aluminum filter and collected by a spectrometer. The transmission of the filter was calibrated using the Beijing Synchrotron Radiation Facility. The response characteristics of the spectrometer were determined by comparing the counts recorded by the spectrometer with the relative intensities of the x-rays transmitted through the step aluminum filter. The response characteristics were used to correct the transmission from two shots of an opacity experiment using the same samples. The transmissions from the two shots are consistent with corrections, but discrepant without corrections.

One-dimensional space resolving flat-field holographic grating soft x-ray framing camera spectrograph for laser plasma diagnostics

A 1D space resolving x-ray spectrum diagnostic system has been developed to study the radiation opacity of hot plasma on SG-II laser facility. The diagnostic system consists of a 2400 lines/mm flat-field holographic grating and a gated microchannel plate coupled with an optical CCD and covers the wavelength range of 5-50 Å. The holographic grating was compared with a ruled one by measuring the emission spectra from a laser-produced molybdenum plasma. The results indicate that the holographic grating possesses better sensitivity than the ruled grating having nearly similar spectral resolution. The spectrograph has been used in radiative opacity measurement of Fe plasma. Simultaneous measurements of the backlight source and the transmission spectrum in appointed time range in one shot have been accomplished successfully with the holographic grating spectrometer. The 2p-3d transition absorption of Fe plasma near 15.5 Å in has been observed clearly.

X-ray ultraviolet absorption measurements of laser-driven Au/CH/Al multilayers

X-ray ultraviolet absorption measurements of aluminum plasma at high temperature and high density are reported. A sample plasma was created by direct laser irradiation of a multilayered foil consisting of Au, CH, and Al. Observations were made using the method of self-backlighting spectroscopy. Simulations were performed with one-dimensional radiation-hydrodynamics code to compute the backlight profile as well as the time history of the density and temperature in the sample. By comparing the measured absorption spectra with detailed-term-accounting calculations at Te=120eV and ρ=0.1g∕cm3, it is found that the major spectral features predicted by calculations have been observed. Even better agreement between the experiment and the simulated absorption spectra was obtained by temporally averaging the radiation-hydrodynamics result over the backlight profile. This experiment shows that it is possible to measure high density and high temperature opacity by a laser-driven multilayer experiment.

Spectroscopic absorption measurement of a low-Z plasma

Low-Z plasmas have been produced by x-ray radiative heating of a low-Z sample on the “Xingguang II” laser facility. High-resolution transmission spectra of the low-Z plasma (CHO) have been measured by using a flat field grating spectrometer. Absorption lines of oxygen and carbon ions in the region from 1.6 nm to 5.0 nm have been observed clearly and identified. The unresolved transition array model (UTA) has been introduced to calculate the transmission spectra of the CHO plasma. The measured transmission spectra have been compared with the calculated ones and the results of other works.

Different approaches to precise wavelength calibration of a flat-field grating spectrometer for laser-produced plasmas

Accurate mathematical models called one-line calibration and parameter fitting are presented for wavelength calibration of a flat-field grating spectrometer. The models precisely establish the relationship between wavelength and pixel position of the detector, since geometry parameters and the grating equation of the spectrometer are taken into account. Compared with the commonly used polynomial fitting, the models presented here provide more reliable calibration results, especially in the extended region away from the calibration points. In addition to the high precision of calibration, the parameter fitting procedure provides a helpful way to obtain the actual parameters of the spectrometer.

High intensity x-ray line emission from aluminum plasmas generated by a 120 TW, 30 fs laser pulse

The K-shell x-ray spectra from the aluminum plasmas generated by the interaction of the 120TW, 30fs laser with aluminum targets have been measured by defocusing the laser beam. Laser energy-normalized intensity of the He-a line increases with the laser intensity approximately as a power law e∝ILγ with the much smaller exponent of γ=0.062 compared to γ=0.2∼0.5 in the previous experiments, which is caused by the stronger radial thermal diffusivity in the target for the smaller laser spot. Laser-to-He-a line conversion efficiency of up to 1.9×10−3 and as high as about 3×1013 photons/2π Sr aluminum He-a line x-ray source have been achieved for a single shot due to the preplasma effect and relatively large laser spot and energy for the single shot. The x-ray spectra as a function of the laser intensity are also analyzed to get the electron temperature and density.