Jiyan Zhang

Opacity Studies of Silicon in Radiatively Heated Plasma

Measurements of the opacity of silicon at high temperature and high density are reported. A silicon dioxide foam was heated by eight nanosecond laser beams while a backlighter X-ray source was produced with a picosecond laser. Absorptions of the 1-2 transitions of Si XII through Si VI were observed in the wavelength range from 6.6 to 7.1 A. The experimental results are simulated with theoretical calculations under local thermodynamic equilibrium using a detailed level accounting model and can be reproduced in general when the effects of the oxygen in the SiO2 are taken into account.

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.

Characterization of a multi-keV x-ray source produced by nanosecond laser irradiation of a solid target : The influence of laser focus spot and target thickness

The influence of focus spot and target thickness on multi-keV x-ray sources generated by 2ns duration laser heated solid targets are investigated on the Shenguang II laser facility. In the case of thick-foil targets, the experimental data and theoretical analysis show that the emission volume of the x-ray sources is sensitive to the laser focus spot and proportional to the 3 power of the focus spot size. The steady x-ray flux is proportional to the 5∕3 power of the focus spot size of the given laser beam in our experimental condition. In the case of thin-foil targets, experimental data show that there is an optimal foil thickness corresponding to the given laser parameters. With the given laser beam, the optimal thin-foil thickness is proportional to the −2∕3 power of the focus spot size, and the optimal x-ray energy of thin foil is independent of focus spot size.

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.

Study of x-ray radiant characteristics and thermal radiation redistribution in CH foam filling cylindrical cavities

Experiments are presented, which demonstrate the properties of x-ray radiation and redistribution of radiant thermal energy in high Z cylindrical cavities filled with low Z CH foam. Time integrated spectra records were obtained by a calibrated space-resolved transmission grating spectrometer. The x-ray radiation became weaker in intensity and was changed to a softer near-Planckian radiation light after a 1500 μm long transport in the foam filling cavity. The experimental redistribution of the radiant thermal energy was plotted and compared to the numerical results of a simplified model. Good agreements have been achieved.

Precise measurement technology of soft-x-ray spectrum using dual transmission grating spectrometer

A novel transmission grating spectrometer, called a dual-transmission grating spectrometer, has been designed and established. This spectrometer adopts two transmission gratings to disperse x rays twice, which suppresses the overlap of the grating higher-order diffraction, cuts out the transfer of experimental errors in unfolding the measured x-ray spectrum effectively, and increases the x-ray spectrum measurement precision. It was shown by experiment that the dual-transmission grating spectrometer was applicable to precisely measure the soft-x-ray spectrum from laser-created plasmas.

The M-band transmission flux of the plastic foil with a coated layer of silicon or germanium

Silicon (Si) and Germanium (Ge) can be used as the dopant in the ablator material for the purpose of reducing preheating in indirect-drive inertial confinement fusion. Their performances in reducing preheating are quite different. A method to evaluate the difference of these two kinds of dopants has been presented in this letter. In the Shenguang-II high power laser facility, the M-band (1.6–4.4 keV) transmission flux of Si-coated plastic (CH) and Ge-coated plastic (CH) has been measured by using the M-band x-ray diode. In the experiment, we find that the Si-coated CH can absorb more M-band x-rays and thus reduce the preheating of the fuel in our experiment condition. By using the radiation hydrodynamic code MULTI-1D, we got the simulation result which was well suited for the experiment. The comparison of their opacities (Te = 60–100 eV and ρ = 0.1–0.5 g/cm3) also shows that the opacity of Si is higher than that of Ge almost in the whole range of 1.6–4.4 keV.

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.