Ding Yongkun

Generation and characterization of millimeter-scale plasmas for the research of laser plasma interactions on Shenguang-III prototype

In order to produce millimeter-scale plasmas for the research of laser-plasma interactions (LPIs), gasbag target is designed and tested on Shenguang-III prototype laser facility. The x-ray pinhole images show that millimeter-scale plasmas are produced with the gasbag. The electron temperature inferred from the stimulated Raman scattering (SRS) spectrum is about 1.6 keV. The SRS spectrum also indicates that the electron density has a ∞at region within the duration of 200 ps. The obvious difierences between the results of the gasbag and that of the void half hohlraum show the feasibility of the gasbag target in creating millimeter-scale plasmas. The LPIs in these millimeter-scale plasmas may partially mimic those in the ignition condition because the duration of the existence of a ∞at plasma density is much larger than the growth time of the two main instabilities, i.e., SRS and stimulated Brillouin scattering (SBS). So we make the conclusion that the gasbag target can be used to research the large-scale LPIs.

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.

Rosseland Mean Opacity of a High-Z Mixture Plasma

An eight-beam 0.351-µm laser with pulse duration of about 1.0ns and energy of 260 J per beam was injected into a cylindrical cavity to generate intense x-ray radiation on the Shengguang II high power laser facility. A mixture foil of gold and gadolinium and a gold foil were attached on portion of a diagnostic hole in the mid-plane of the cavity and ablated by the intense x-ray radiation. The propagating time of the radiation heat wave in the mixture and the pure gold foil were measured with soft-x-ray spectrometer and by adopting space- and time-resolved measurement technology, respectively. The results show that the mixture of gold and gadolinium has higher Rosseland mean opacity than the gold sample in our experiment.

Radiation Temperature Scaling Law for Gold Hohlraum Heated with Lasers at 0.35 mm Wavelength

We have carried out the hohlraum experiments about radiation temperature scaling on the Shenguang-II (SG-II) laser facility with eight laser beams of 0.35 μm, pulse duration of about 1.0 ns and total energy of 2000 J. The reradiated x-ray flux through the laser entrance hole was measured using a soft x-ray spectrometer. The measured peak radiation temperature was 170 eV for the standard hohlraum and 150 eV for the 1.5-scaled one. We have derived the radiation temperature scaling law, in which the laser hohlraum coupling efficiency is included. With an appropriate coupling efficiency, the coincidences between experimental and scaling hohlraum radiation temperatures are rather good.

Study of the Performance of a Streaked Optical Pyrometer System for Temperature Measurement of Shocked Materials

A streaked optical pyrometer (SOP) is developed and calibrated for the measurement of the temperature of shocked materials. In order to achieve a higher relative sensitivity, a one-channel scheme is adopted for the system. The system is calibrated with a shocked step-shaped aluminum sample in the SG-III prototype laser facility. The relation between the count number in the detection system and the sample temperature is thus obtained, which can be adopted to infer the temperature of any shocked materials in future experiments.

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.

Emission spectrum from an Al/Mg tracer in the blow-off region of a radiatively ablated capsule

A study of X-ray emissions from Al/Mg tracers buried at two different depths in a plastic shell is presented. The X-rays originating from the K-shell transitions of the Al/Mg ions begin to irradiate after the ablative heating wave has passed through the trace layer and are recorded with a streaked crystal spectrometer. Only emissions from the capsule with the trace layer buried at a smaller depth are observed. Hydrodynamic simulations and a collisional—radiative model including detailed atomic physics are used to investigate the measured spectrum. It is found that the effects of the radiative heating play important roles in the formation of the K-shell emission. The time correlation between the simulations and the measurements is obtained by comparing the measured time profile of the He α emission with the calculated one. The line ratio of Ly α to He α is also calculated and is found to be in fairly good agreement with the experimental data. Finally, the relation between the time profile of the He α emission and the ablation velocity is also discussed.