Peng Xiaoshi

Direct drive implosion experiments on SGIII prototype laser facility: Assessing energy coupling efficiency and implosion symmetry

Direct drive implosion experiments were conducted on SGIII prototype laser facility. From the time resolvedx-rayimages, the bright ring and the central bright spot are observed. The radial velocity of the convergent bright ring indicates the shell velocity, and the times when the central bright spot is first seen and becomes most intensive indicate the times of shock convergence and later stagnation, respectively. Radiation hydrodynamic simulations were carried out by changing laser energy deposition factors. When the simulated results are brought close to the measured ones, it is found that the energy coupling efficiency is around 70%. The implosion symmetry is indicated by the core x-ray emission pattern which is pancake when viewing from the equator, and splits into several bright spots when viewing close to the pole. A simple model is developed to understand this asymmetry. It is speculated that the observed implosion asymmetry can be attributed to the laser arrangement which is originally designed for indirect drive experiments. Further improvements of energy coupling efficiency and implosion symmetry in future experiments can be achieved by optimizing target design and laser arrangement.

Reflective-type configuration for monitoring the photobleaching procedure based on surface plasmon resonance

A new scheme to monitor the photoinduced bleaching procedure with ultraviolet light based on surface plasmon resonance (SPR) using attenuated total reflection configuration is presented. The configuration, which is very sensitive to the changes of refractive index and nonlinear optical (NLO) coefficient of polymer, can prevent the effect of the fundamental wave and can be used to determine the nonlinear coefficient d33. With the formulation in this paper, the nonlinear coefficient d33 can be measured by comparing the second harmonic generation intensity generated by SPR from the metal–nonlinear optical (NLO) polymer interface and that from the metal–quartz interface. The remaining second-order NLO coefficients are 78% and 47% for the cross-linked and side-chain polymer, respectively.

Preliminary results from direct-drive cryogenic target implosion experiments on SGIII prototype laser facility

Since ignition target design with layered deuterium and triterium ice had been proposed several decades ago, much effort was devoted to fabricate and implode cryogenic targets. Until recently, direct-drive cryogenic target implosion experiment was carried out on SGIII prototype laser facility. The target consisted of a plastic capsule supported by fill tube. Cryogenic helium gas was used to cool the capsule to a few degrees below the deuterium triple point. The resulting deuterium ice layer was characterized by optical shadowgraph and smoothed by applied temperature gradient. Eight laser beams with total energy of 7 kJ were used to directly drive the implosion. On the path of laser light to the capsule, there were 500 nm sealing film and helium gas of mm length. X-ray pinhole images were analyzed to confirm that the sealing film, and helium gas had little effect on aiming accuracy but caused some loss of laser energy especially when condensation on the sealing film was observed.

Methods of Generation and Detailed Characterization of Millimeter-Scale Plasmas Using a Gasbag Target

Gasbag targets are useful for the research of laser-plasma interactions in inertial confinement fusion, especially in the laser overlapping regime. We report that on the Shengguang-II laser facility, millimeter-scale plasmas are successfully generated by four 0.35 μm laser beams using a gasbag target. Multiple diagnostics are applied to characterize the millimeter-scale plasmas in detail. The images from the x-ray pinhole cameras confirm that millimeter-scale plasmas are indeed created. An optical Thomson scattering system diagnoses the electron temperature of the CH filling plasmas by probing the thermal ion-acoustic fluctuations, which indicates that the electron temperature has a 600 eV flat roof in 0.7–1.3 ns. Another key parameter, i.e. the electron density of the millimeter-scale plasmas, is inferred by the spectrum of the back stimulated Raman scattering of an additional 0.53 μm laser beam. The inferred electron density keeps stable at 0.1nc in early time consistent with the controlled filling pressure and splits into a higher density in late time, which is attributed to the blast wave entering into the SRS interaction region.

Shock-Timing Experiment Using a Two-Step Radiation Pulse with a Polystyrene Target

A shock-timing experiment plays an important role in inertial confinement fusion studies, and the timing of multiple shock waves is crucial to the performance of inertial confinement fusion ignition targets. We present an experimental observation of a shock wave driven by a two-step radiation pulse in a polystyrene target. The experiment is carried out at Shen Guang III Yuan Xing (SGIIIYX) laser facility in China, and the generation and coalescence of the two shock waves, originating from each of the two radiation steps, is clearly seen with two velocity interferometers. This two-shock-wave coalescence is also simulated by the radioactive hydrodynamic code of a multi-1D program. The experimental measurements are compared with the simulations and quite good agreements are found, with relatively small discrepancies in shock timing.

Recent progress of hohlraum physics experiments in indirect-driven ICF in China

In recent years, hohlraum experiments have been performed extensively on Shenguang series laser facilities in the context of laser indirect-drive inertial confinement fusion. Multiple aspects about the hohlraumenergetics, drive symmetry and plasma condition are studied by a variety of methods resolving different photon ranges and multiple viewing areas. To improve the experimental uncertainty, several diagnostics are optimized and calibrated, also the power balance and pointing accuracy of laser beams are evaluated and improved. These works lead a rapid progress on hohlraum experimental capabilities and a series of successful experimental campaigns. In order to further optimize the hohlraum performance, other hohlraum geometry (the spherical hohlram with six LEHs and the cylindrical hohlraum with six LEHs) and hohlraum wall material (depleted Uranium and foam Au) are explored as well. Hohlraum experiments and modeling on Shenguang series laser facilities demonstrated quantitative understanding of the laser conversion, X-ray ablation and plasma motion in different regions.

Laser plasma instability in indirect-drive inertial confinement fusion

In indirect-drive inertial confinement fusion (ICF), the incident laser beam could excite laser plasma instabilities (LPI) such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and two plasmon decay (TPD) besides gently heat the hohlraum through collisional absorption. These instabilities would largely reduce the X-ray conversion and degrade the drive symmetry of the radiation environment. In addition, when the amplitude of parametric instability increases to a certain level, there would be interplay between different instabilities, which makes LPI complicated and unpredictable. Therefore, LPI has become one of the major challenge in achieving ignition. LPI research during recent few years made great strides in identifying, understanding, and controlling instabilities in the context of laser fusion. This paper reviews the progress in this important field according to laser (L), plasma (P), and instability (I). Prospects for the application of our improved understanding for indirect drive ICF and some exciting research opportunities are also discussed.

A Full Aperture Backscattering Light Diagnostic System Installed on the Shenguang-III Prototype Laser Facility

A full aperture backscattering light diagnostic system (FABLDS) implemented on the Shen Guang-III Prototype Laser Facility is described in the paper. FABLDS measures both stimulated brillouin scattering (SBS) and stimulated Raman scattering (SRS) with a series of optical detectors. Energy sensors record the integrated energy, and streak cameras coupled with spectrometers measure the temporal spectrum of the backscattering light. This paper provides an overview of the FABLDS and detailed descriptions of the optical path. Special components, including off-axis parabolic mirror, spatial filter and optical light filters, are incorporated along the beam path for purifying the scattering light. Several hohlraum targets were employed, including C5H12 gas-filled targets and empty targets in the experiments. Results presented in the paper indicate that the fraction of backscatter light has been obviously shrinked when the laser is smoothed by continuous phase plates (CPP).