Wenbing Pei

Simulation study of Hohlraum experiments on SGIII-prototype laser facility

The Hohlraum physics experiments performed on the SGIII-prototype laser facility are simulated by using our two-dimensional radiation hydrodynamic code LARED-H, and the influence of laser intensity on the two-dimensional Hohlraum simulations is studied. Both the temporal radiation temperature and the x-ray spectrum from the simulations agree well with the observations, except that the simulated M-band fraction (greater than 2 keV) is obviously smaller than the observation. According to our study, the coupling efficiency from laser to x-ray is around 70% for SGIII-prototype laser facility Hohlraums.

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.

A clean radiation environment for opacity measurements of radiatively heated material

A clean x-ray radiation environment is essential for detailed measurements of the opacity of high-temperature radiatively heated material. A lot of laser energy is usually needed to heat a large hohlraum to produce such a clean x-ray radiation environment. A type of target is proposed that uses low-density, low-Z foam to provide a passage to radiation while isolating the sample from the disturbance from laser produced, high-temperature, high-Z plasma and heating by reflected laser light. With a smaller hohlraum, less laser energy is needed to produce high-temperature x-ray radiation for sample heating. Experiments have been done to check the proposal. The recorded clean Al self-emission spectra proved there was no gold plasma in the view-way to disturb the measurement. This type of hohlraum can provide a high-quality work-table for opacity measurement even in a relatively small laser facility.

X-ray conversion efficiency and radiation non-uniformity in the hohlraum experiments at Shenguang-III prototype laser facility

The hohlraum radiation properties are studied experimentally by the Shenguang-III prototype laser facility and numerically by the two-dimensional code LARED with the multi-group radiation transfer model. The measured radiation temperature is consistent with the prediction of the simulations in a wide laser energy range, suggesting that the x-ray conversion efficiency is around 75% at the peak radiation temperature. The delicate hohlraum experiments further show that the radiation intensity inside the hohlraum is significantly non-uniform. The measured radiation flux of the hot spot region is over twice higher than that of the re-emitted wall region. Good agreements between the experiments and simulations further demonstrate the validity of the LARED code to study the hohlraum radiation properties.

Indirect-drive ablative Rayleigh-Taylor growth experiments on the Shenguang-II laser facility

In this research, a series of single-mode, indirect-drive, ablative Rayleigh-Taylor (RT) instability experiments performed on the Shenguang-II laser facility [X. T. He and W. Y. Zhang, Eur. Phys. J. D 44, 227 (2007)] using planar target is reported. The simulation results from the one-dimensional hydrocode for the planar foil trajectory experiment indicate that the energy flux at the hohlraum wall is obviously less than that at the laser entrance hole. Furthermore, the non-Planckian spectra of x-ray source can strikingly affect the dynamics of the foil flight and the perturbation growth. Clear images recorded by an x-ray framing camera for the RT growth initiated by small- and large-amplitude perturbations are obtained. The observed onset of harmonic generation and transition from linear to nonlinear growth regime is well predicted by two-dimensional hydrocode simulations.

Radiation-temperature shock scaling of 1 ns laser-driven hohlraums

Simulations of the x-rayablation process of aluminum are performed using a one-dimensional multigroup radiation hydrodynamic code RDMG [F. Tinggui et al., Chin. J. Comput. Phys.16, 199 (1999)]. The scaling relation of the peak temperatures of the x-ray sources with the shock velocities is studied, and its dependence on the temporal profile and the length of the x-ray sources is described and analyzed in this paper. A scaling relation applicable to x-ray sources of 1 ns pulse laser-driven hohlraums is proposed, the dependence of which is studied and found to be negligible. Our scaling relation of radiation temperature versus shock velocity is about 10 eV lower than that proposed by Kauffman et al. [Phys. Rev. Lett.73, 2320 (1994)] for shock velocity of ( 4 – 8 ) × 10 6 cm / s .

Radiation temperature scaling in indirect-drive hohlraums

The hohlraum radiation temperature is studied for Gaussian laser pulses of order of 1 ns. The simulation results show that for this kind of pulse, the self-similar law of the radiation hydrodynamics is also valid in its time-rising phase. The scaling of the hohlraum radiation temperature as a function of laser parameter, hohlraum size, and x-ray conversion efficiency is obtained and the effect of laser entrance hole is included.

Investigating the hohlraum radiation properties through the angular distribution of the radiation temperature

The symmetric radiation drive is essential to the capsule implosion in the indirect drive fusion but is hard to achieve due to the non-uniform radiation distribution inside the hohlraum. In this work, the non-uniform radiation properties of both vacuum and gas-filled hohlraums are studied by investigating the angular distribution of the radiation temperature experimentally and numerically. It is found that the non-uniform radiation distribution inside the hohlraum induces the variation of the radiation temperature between different view angles. The simulations show that both the angular distribution of the radiation temperature and the hohlraum radiation distribution can be affected by the electron heat flux. The measured angular distribution of the radiation temperature is more consistent with the simulations when the electron heat flux limiter fe=0.1. Comparisons between the experiments and simulations further indicate that the x-ray emission of the blow-off plasma is overestimated in the simulations wh...

Preheat of radiative shock in double-shell ignition targets

For the double-shell ignition target, the nonuniform preheat of the inner shell by high-energy x rays, especially the M-band line radiation and L-shell radiation from the Au hohlraum, aggravates the hydrodynamic instability that causes shell disruption. In this paper, for the first time, we propose another preheating mechanism due to the radiative shock formed in the CH foam, and also confirm and validate such preheat of radiative shock by numerical results. We also give an estimate of the improved double-shell in which the CH foam is replaced by the metallic foam to mitigate the hydrodynamic instabilities, and find that the radiative shock formed in the metallic foam produces a much stronger radiation field to preheat the inner shell, which plays a role in better controlling the instabilities. In double-shells, the preheat of radiative shock, as a potential effect on the instabilities, should be seriously realized and underlined.

Inferring the capsule-absorbed radiation energy from experimental hohlraum radiation temperature

For indirect laser fusion, the implosion is driven by the fusion capsule-absorbed radiation energy emitted by laser-produced plasma. However, the absorbed energy could hardly be directly measured experimentally and usually would require numerical simulation. This paper puts forward a method by which the capsule-absorbed radiation energy can be inferred from the measured time-dependent radiation temperature. In the method, the capusle-absorbed radiation energy is seen as an effective radiation energy loss and should be reflected in the experimental radiation temperature. Furthermore, it is not necessary to know what materials the capsule is made of or how it is constructed.