Zhijian Zheng

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.

Cherenkov radiation generated by a beam of electrons revisited

Cherenkov radiation generated by a beam of electrons is theoretically investigated. In the case that the boundary effect is negligible, coherent Cherenkov radiation does not depend on the longitudinal bunch form of the electron beam, which is remarkably different from other kinds of coherent radiation like coherent transition radiation and coherent synchrotron radiation. The reason for this result is ascribed to the criterion of the emission of Cherenkov radiation. The angular distribution of coherent Cherenkov radiation is mainly determined by the transverse bunch form of the beam. The spectral intensity of incoherent Cherenkov radiation is proportional to the velocity distribution function of the electrons in the beams. Based on these results, some methods are suggested to study hot electrons with the measurement of Cherenkov radiation.

Experimental study for angular distribution of the hot electrons generated by femtosecond laser interaction with solid targets

The experimental results of angular distribution of hot electrons in the interaction of a 60 fs, 125 mJ, 800 nm, ∼1017 W cm−2 laser pulse with Al targets are reported. Three obvious peaks of hot electrons emission have been observed, as there is a weak normal component of the laser electric field. These emission peaks are located in the directions of the specular reflection of the laser, the target normal, and the backreflection of the laser, respectively. In the case of the P-polarized laser pulse, which has a strong normal component of the laser electric field, the peak in the backreflection of the laser disappeared, and only two obvious peaks of hot electron emissions existed. It shows that the different directions of hot electrons emission are dominated by different absorption or acceleration mechanisms. The experimental result of the hot electrons energy spectrum at the target normal shows that the effective temperature of hot electrons is about 190 keV, which is consistent with a scaling law of the ...

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.

Laser hohlraum coupling efficiency on the Shenguang II facility

Recently, hohlraum experiments were performed at the Shenguang-II (SG-II) laser facility [Lin et al., Chin. J. Lasers B10, Suppl. IV6 (2001)]. The measured maximum radiation temperature was 170 eV for the standard hohlraum and 150 eV for a 1.5-scaled one. This paper discusses the radiation temperature and laser hohlraum coupling efficiency in terms of a theoretical model [Phys. Plasmas 8, 1659 (2001)] and numerical simulation. A 2D laser–hohlraum coupling code, LARED-H [Chin. J. Comput. Phys. 19, 57 (2002)], gives a satisfactory coincidence with the measured time-resolved radiation temperature. Upon fitting the time-resolved curve, the theoretical model obtains the hohlraum coupling efficiency and, furthermore, the parameter n+s for the hohlraum wall material (Au) can be determined simultaneously, where n, s are the power exponents of temperature for the radiation Rosseland mean-free path and specific internal energy, respectively.

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.

RADIATIVE OPACITY OF IRON STUDIED USING A DETAILED LEVEL ACCOUNTING MODEL

The opacity of iron plasma in local thermodynamic equilibrium is studied using an independently developed detailed level accounting model. Atomic data are generated by solving the full relativistic Dirac-Fock equations. State mixing within one electronic configuration is considered to include part of the correlations between electrons without configuration interaction matrices that are too large being involved. Simulations are carried out and compared with several recent experimental transmission spectra in the M- and L-shell absorption regions to reveal the high accuracy of the model. The present model is also compared with the OPAL, LEDCOP and OP models for two isothermal series at T = 20 eV and T = 19.3 eV. It is found that our model is in good agreement with OPAL and LEDCOP while it has discrepancies with OP at high densities. Systematic Rosseland and Planck mean opacities in the range 10-1000 eV for temperature and 10–5-10–1 g cm–3 for density are also presented and compared with LEDCOP results, finding good agreement at lower temperatures but apparent differences at high temperatures where the L- and K-shell absorptions are dominant.

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.

Thomson scattering from laser-produced gold plasmas in radiation conversion layer

With a 263.3 nm probe laser beam, the Thomson scattering (TS) spectra are measured at the distance from the surface of gold disc targets as close as 50μm. All of the TS spectra become narrower with time, which indicates the overall drop of plasma temperature as the high-Z plasmas blow off. The experimental data show that there are distinct characteristics between the inner region (i.e., 50 and 100μm away from target surface) and the outer region. The gradient of the electron temperature in outer region is toward the target and becomes opposite to that in the inner region. The asymmetries of the TS spectra in the inner region are in good agreement with calculation based on the Spitzer–Harm theory. In the outer region, however, the discrepancies between the experiments and calculations are significant, which may be a hint that the electron energy transport becomes flux limited or nonlocal in this region.

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.