Yongkun Ding

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.

First Investigation on the Radiation Field of the Spherical Hohlraum.

The first spherical hohlraum energetics experiment is accomplished on the SGIII-prototype laser facility. In the experiment, the radiation temperature is measured by using an array of flat-response x-ray detectors (FXRDs) through a laser entrance hole at four different angles. The radiation temperature and M-band fraction inside the hohlraum are determined by the shock wave technique. The experimental observations indicate that the radiation temperatures measured by the FXRDs depend on the observation angles and are related to the view field. According to the experimental results, the conversion efficiency of the vacuum spherical hohlraum is in the range from 60% to 80%. Although this conversion efficiency is less than the conversion efficiency of the near vacuum hohlraum on the National Ignition Facility, it is consistent with that of the cylindrical hohlraums used on the NOVA and the SGIII-prototype at the same energy scale.

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.

Radiation flux study of spherical hohlraums at the SGIII prototype facility

An octahedral spherical hohlraum is a promising candidate in target design for inertial confinement fusion study, because of its potential superiority in uniform radiation and efficient coupling [Lan et al., Phys. Plasmas 21, 010704 (2014)]. Before the experimental investigation for octahedral spherical hohlraum, an energetics experiment is accomplished on the Shenguang-III prototype laser facility by using spherical hohlraums with two cylindrical laser entrance holes. Time evolution of the radiation temperature is obtained with flat response X-ray diode detectors at four different viewing angles with demonstrated repeatability of the measurements. The experimental observations are successfully explained by using a phenomenological model which considers not only the radiation flux contributed from the laser ablated and radiation ablated plasma from hohlraum wall, but also that contributed from the filling plasma inside the hohlraum. This method proves to be a simple but effective way to interpret the time...

Direct measurement of x-ray flux for a pre-specified highly-resolved region in hohlraum

A space-resolving flux detector (SRFD) is developed to measure the X-ray flux emitted from a specified region in hohlraum with a high resolution up to 0.11mm for the first time. This novel detector has been used successfully to measure the distinct X-ray fluxes emitted from hot laser spot and cooler re-emitting region simultaneously, in the hohlraum experiments on SGIII prototype laser facility. According to our experiments, the ratio of laser spot flux to re-emitted flux shows a strong time-dependent behavior, and the area-weighted flux post-processed from the measured laser spot flux and re-emitting wall flux agrees with that measured from Laser Entrance Hole by using flat-response X-ray detector (F-XRD). The experimental observations is reestablished by our two-dimensional hydrodynamic simulations and is well understood with the power balance relationship.

Neutron Generation by Laser-Driven Spherically Convergent Plasma Fusion

We investigate a new laser-driven spherically convergent plasma fusion scheme (SCPF) that can produce thermonuclear neutrons stably and efficiently. In the SCPF scheme, laser beams of nanosecond pulse duration and 10^{14}-10^{15}  W/cm^{2} intensity uniformly irradiate the fuel layer lined inside a spherical hohlraum. The fuel layer is ablated and heated to expand inwards. Eventually, the hot fuel plasmas converge, collide, merge, and stagnate at the central region, converting most of their kinetic energy to internal energy, forming a thermonuclear fusion fireball. With the assumptions of steady ablation and adiabatic expansion, we theoretically predict the neutron yield Y_{n} to be related to the laser energy E_{L}, the hohlraum radius R_{h}, and the pulse duration τ through a scaling law of Y_{n}∝(E_{L}/R_{h}^{1.2}τ^{0.2})^{2.5}. We have done experiments at the ShengGuangIII-prototype facility to demonstrate the principle of the SCPF scheme. Some important implications are discussed.

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...

Demonstration of enhancement of x-ray flux with foam gold compared to solid gold

Experiments have been conducted to compare the re-emission from foam gold with a 0.3 g cc−1 density and solid gold in a SGIII prototype laser facility. Measurements of the re-emission x-ray flux demonstrate that emission is enhanced by the low density foam gold compared to the solid gold under the same conditions. The emission fraction increases with time and is concentrated on soft x-ray flux between 0.1–1 keV. The simulation results with Multi 1D agree with the experimental results. There are potential advantages to using foam walls for improving the emission and soft x-ray flux in hohlraums.

Uranium hohlraum with an ultrathin uranium–nitride coating layer for low hard x-ray emission and high radiation temperature

An ultrathin layer of uranium nitrides (UN) has been coated on the inner surface of depleted uranium hohlraum (DUH), which has been proven by our experiment to prevent the oxidization of uranium (U) effectively. Comparative experiments between the novel depleted uranium hohlraum and pure golden (Au) hohlraum are implemented on an SGIII-prototype laser facility. Under a laser intensity of 6 × 1014 W cm−2, we observe that the hard x-ray (hν keV) fraction of the uranium hohlraum decreases by 61% and the peak intensity of the total x-ray flux (0.1 keV~5.0 keV) increases by 5%. Radiation hydrodynamic code LARED is used to interpret the above observations. Our result for the first time indicates the advantages of the UN-coated DUH in generating a uniform x-ray source with a quasi-Planckian spectrum, which should have important applications in high energy density physics.

Measurement and Analysis of Electromagnetic Pulse from Laser-Target Interaction at ShenGuang II Laser Facility

Abstract Electromagnetic pulses (EMPs) generated from lasers interacting with solid targets at the ShenGuang II laser facility were measured and analyzed in this work. The EMP radiations were related to the target geometries, where the strongest EMP signal with a magnitude of 103 V and duration of several dozens of nanoseconds resulted from the monopole flat coil and Au foil targets. The EMPs detected inside the laser facility were seriously affected by the chamber wall, which could reflect EMPs and prolong the signals with several typical pulsed peaks. This study was expected not only to provide basic information to interpret physical processes caused by laser irradiating targets but also to offer a path for electromagnetic interference shielding designs and protect the diagnostics from damage in inertial confinement fusion.