Progress in octahedral spherical hohlraum study

In this paper,we give a review of our theoretical and experimental progress in octahedral spherical hohlraum study.From our theoretical study,the octahedral spherical hohlraums with 6 Laser Entrance Holes(LEHs)of octahedral symmetry have robust high symmetry during the capsule implosion at hohlraum-to-capsule radius ratio larger than 3.7.In addition,the octahedral spherical hohlraums also have potential superiority on low backscattering without supplementary technology.We studied the laser arrangement and constraints of the octahedral spherical hohlraums,and gave a design on the laser arrangement for ignition octahedral hohlraums.As a result,the injection angle of laser beams of 50°-60°was proposed as the optimum candidate range for the octahedral spherical hohlraums.We proposed a novel octahedral spherical hohlraum with cylindrical LEHs and LEH shields,in order to increase the laser coupling efficiency and improve the capsule symmetry and to mitigate the influence of the wall blowoff on laser transport.We studied on the sensitivity of the octahedral spherical hohlraums to random errors and compared the sensitivity among the octahedral spherical hohlraums,the rugby hohlraums and the cylindrical hohlraums,and the results show that the octahedral spherical hohlraums are robust to these random errors while the cylindrical hohlraums are the most sensitive.Up till to now,we have carried out three experiments on the spherical hohlraum with 2 LEHs on Shenguang(SG)laser facilities,including demonstration of improving laser transport by using the cylindrical LEHs in the spherical hohlraums,spherical hohlraum energetics on the SGIII prototype laser facility,and comparisons of laser plasma instabilities between the spherical hohlraums and the cylindrical hohlraums on the SGIII laser facility.

First demonstration of improving laser propagation inside the spherical hohlraums by using the cylindrical laser entrance hole

The octahedral spherical hohlraums have natural superiority in maintaining high radiation symmetry during the entire capsule implosion process in indirect drive inertial confinement fusion.While,in contrast to the cylindrical hohlraums,the narrow space between the laser beams and the spherical hohlraum wall is usually commented.In this Letter,we address this crucial issue and report our experimental work conducted on the SGIII-prototype laser facility which unambiguously demonstrates that a simple design of cylindrical laser entrance hole(LEH)can dramatically improve the laser propagation inside the spherical hohlraums.In addition,the laser beam deflection in the hohlraum is observed for the first time in the experiments.Our 2-dimensional simulation results also verify qualitatively the advantages of the spherical hohlraums with cylindrical LEHs.Our results imply the prospect of adopting the cylindrical LEHs in future spherical ignition hohlraum design.

First experimental comparisons of laser-plasma interactions between spherical and cylindrical hohlraums at SGⅢ laser facility

We present our recent laser-plasmas instability(LPI)comparison experiment at the SGIII laser facility between the spherical and cylindrical hohlraums.Three kinds of filling are considered:vacuum,gas-filling with or without a capsule inside.A spherical hohlraum of 3.6 mm in diameter,and a cylindrical hohlraum of 2.4 mm?4.3 mm are used.The capsule diameter is 0.96 mm.A flat-top laser pulse with 3 ns duration and up to 92.73 kJ energy is used.The experiment has shown that the LPI level in the spherical hohlraum is close to that of the outer beam in the cylindrical hohlraum,while much lower than that of the inner beam.The experiment is further simulated by using our 2-dimensional radiation hydrodynamic code LARED-Integration,and the laser back-scattering fraction and the stimulated Raman scatter(SRS)spectrum are post-processed by the high efficiency code of laser interaction with plasmas HLIP.According to the simulation,the plasma waves are strongly damped and the SRS is mainly developed at the plasma conditions of electron density from 0.08 n_(c) to 0.1 n_(c) and electron temperature from 1.5 keV to 2.0 keV inside the hohlraums.However,obvious differences between the simulation and experiment are found,such as that the SRS back-scattering is underestimated,and the numerical SRS spectrum peaks at a larger wavelength and at a later time than the data.These dif-ferences indicate that the development of a 3D radiation hydrodynamic code,with more accurate physics models,is mandatory for spherical hohlraum study.

Target alignment in the Shen-Guang Ⅱ Upgrade laser facility

The Shen-Guang II Upgrade(SG-Ⅱ-U) laser facility consists of eight high-power nanosecond laser beams and one shortpulse picosecond petawatt laser. It is designed for the study of inertial confinement fusion(ICF), especially for conducting fast ignition(FI) research in China and other basic science experiments. To perform FI successfully with hohlraum targets containing a golden cone, the long-pulse beam and cylindrical hohlraum as well as the short-pulse beam and cone target alignment must satisfy tight specifications(30 and 20 μm rms for each case). To explore new ICF ignition targets with six laser entrance holes(LEHs), a rotation sensor was adapted to meet the requirements of a three-dimensional target and correct beam alignment. In this paper, the strategy for aligning the nanosecond beam based on target alignment sensor(TAS) is introduced and improved to meet requirements of the picosecond lasers and the new six LEHs hohlraum targets in the SG-II-U facility. The expected performance of the alignment system is presented, and the alignment error is also discussed.