Jiang Shao-En

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.

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.

Concentric-ring structures in an atmospheric pressure helium dielectric barrier discharge ⁄

This paper performs a numerical simulation of concentric-ring discharge structures within the scope of a twodimensional difiusion{drift model at atmospheric pressure between two parallel circular electrodes covered with thin dielectric layers. With a relative high frequency the discharge structures present difierent appearances of ring structures within difierent radii in time due to the evolvement of the fllaments. The spontaneous electron density distributions help understanding the formation and development of self-organized discharge structures. During a cycle the electron avalanches are triggered by the electric fleld strengthened by the feeding voltage and the residual charged particles on the barrier surface deposited in the previous discharges. The accumulation of charges is shown to play a dominant role in the generation and annihilation of the discharge structures. Besides, the rings split and unify to bring and annihilate rings which form a new discharge structure.

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.

Experiments and analysis of gold disk targets irradiated by smoothing beams of Xingguang II facilities with 350 nm wavelength

Gold disk targets were irradiated using focusing and beam smoothing methods on Xingguang (XG-II) laser facilities with 350 nm wavelength, 0.6 ns pulse width and 20–80 Joules energies. Laser absorption, light scattering and X-ray conversion were experimentally investigated. The experimental results showed that laser absorption and scattered light were about 90% and 10%, respectively, under focusing irradiation, but the laser absorption increased 5%–10% and the scattered light about 1% under the condition of beam smoothing. Compared with the case of focusing irradiation, the laser absorption was effectively improved and the scattered light remarkably dropped under uniform irradiation; then due to the decrease in laser intensity, X-ray conversion increased. This is highly advantageous to the inertial confinement fusion. However, X-ray conversion mechanism basically did not change and X-ray conversion efficiency under beam smoothing and focusing irradiation was basically the same.

Progress and plans of X-ray temporal and spatial diagnosis technology of Shenguang facilities

In the field of indirect-drive inertial confinement fusion, temporal and spatial diagnosis of X-ray is very important to the imploded process researches and the simulate procedure verification. According to the temporal, spatial and spectral characteristics of X-ray radiation, many X-ray imaging diagnosis devices were successfully developed. Along with the development of ICF research, especially the Shenguang III facility establishment, our X-ray imaging diagnosis capability has been increasingly powerful. Some of the novel diagnosis equipment even have more excellent characteristics than that kind of diagnosis equipment abroad.

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.

Progress of indirectly driven implosion experiments

Achieving ignition via inertial confinement approach is of strong application value and important science significance. It is also a project of large scale that depends on the national strength. Involving theory, target fabrication, diagnostics, and laser facilities, integrated implosion experiment plays an important role in the research of ICF. Since the first successful neutron production with directly driven implosion in the 80’s, the quasi one-dimensional implosion performance dominated by shock compression was demonstrated in the period of 1990–2010, and the quasi one-dimensional implosion performance dominated by inertial compression was achieved in recent years. All of these facts definitely show the progress on improving the performance of integrated implosion. With the completion of SG-III laser facility, and in the situation of facing frustration in the US ignition program, there is grand opportunity and challenge when we are exploring toward the ignition parameter space with higher convergence ratio and lower adiabat.