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The novel octahedral spherical hohlraum can provide an ideal and practical approach for the next generation of laser systems to support both direct and indirect drive to achieve predictable and reproducible fusion gain via multiple schemes. To demonstrate its advantage in a naturally high symmetry at a cylindrically configured laser facility, it requires to repoint the laser beams to approach as close as possible the ideal octahedral beam configuration with an injection angle (the angle between a beam and the normal direction of its laser entrance hole (LEH)) ranging from 50° to 60°. We report our investigation and experiment on the optimum repointing scheme at the SGIII facility, which uses 32 beams, with 8 beams entering each polar LEH at 49.5° and 55°, and 4 beams entering each equatorial LEH at 61.5° and 62.1°. It contains residual imbalance between the polar and equatorial beams, leading to an asymmetry dominated by the spherical harmonic Y20 mode, which can be remarkably reduced by the stronger backscatters of equatorial beams. Our experiment demonstrated the feasibility of the 32-beam optimum repointing scheme and generation of 175 eV under 86 kJ inside a 2.4-mm-radius octahedral hohlraum with 0.7-mm-radius LEHs, which provided a strong support for the later experiment on proof-of-concept of octahedral spherical hohlraum [Lan et al., Phys. Rev. Lett. 127, 245001 (2021)]. 2D simulations on LEH closure agree well with the observations. This work opens a novel way of realization of a quasi-spherical irradiation at a cylindrically configured laser facility without supplementary symmetry control.

Space-resolving flux detection is an important technique for the diagnostic of the radiation field within the hohlraum in inertial confinement fusion, especially for the radiation field diagnostic in the novel spherical hohlraum with octahedral six laser entrance holes (LEHs), where localized measurements are necessary for the discrimination of the radiation flux from different LEHs. A novel space-resolving flux detector (SRFD) is developed at the SG-III laser facility for the radiation flux measurement in the first campaign of the octahedral spherical hohlraum energetics experiment. The principle and configuration of the SRFD system is introduced. The radiation flux from the wall of a gas-filled octahedral spherical hohlraum is measured for the first time by placing the SRFD system at the equatorial position of the SG-III laser facility, aiming at the hohlraum wall through one of the six LEHs. The absolute radiation flux from the re-emission area on the hohlraum wall is measured, and good consistency is found between the experimental data and the calculated data from a three-dimensional view factor analysis.

As usual cylindrical hohlraum with double laser ring cones may lead to serious laser-plasma interaction, such as the simulated Raman scatter and cross-beam energy transfer effect, spherical hohlraum with octahedral 6 Laser Entrance Holes (LEHs) and single cone laser beams, was investigated and reported to have a consistent high radiation symmetry during the whole implosion process. However, it has several potential challenges such as the smaller space left for diagnosis and the assembly of centrally located capsule. In this paper, based on the view-factor model, we investigate the radiation symmetry and the drive temperature on the capsule located in the spherical hohlraum with tetrahedral 4 LEHs and single cone laser beams, since there is more available space for laser disposition and diagnosis. Then, such target is optimized on the laser beam pointing direction to achieve a high radiation performance, i.e., the radiation symmetry and drive temperature on the capsule. Finally, an optimal spherical hohlraum with optimal laser beam pointing has been demonstrated and compared with the spherical hohlraum with octahedral 6 LEHs. The resulting radiation symmetry and the drive temperature shows that it has almost a similar radiation symmetry (the radiation asymmetry variation is no more than 0.2%), and higher drive temperature (the temperature has been increased by 1.73%, and an additional 133 kJ energy of 2 MJ energy for fusion can be saved).

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...

The hohlraum is very crucial for indirect laser driven Inertial Confinement Fusion. Usually, its shape is designed as sphere, cylinder, or rugby with some kind of fixed functions, such as ellipse or parabola. Recently, a spherical hohlraum with octahedral 6 laser entrance holes (LEHs) has been presented with high flux symmetry [Lan et al., Phys. Plasmas 21, 010704 (2014); 21, 052704 (2014)]. However, there is only one shape parameter, i.e., the hohlraum to capsule radius ratio, being optimized. In this paper, we build the hohlraum with octahedral 6LEHs with a unified free-form representation, in which, by varying additional shape parameters: (1) available hohlraum shapes can be uniformly and accurately represented, (2) it can be used to understand why the spherical hohlraum has higher flux symmetry, (3) it allows us to obtain a feasible shape design field satisfying flux symmetry constraints, and (4) a synthetically optimized hohlraum can be obtained with a tradeoff of flux symmetry and other hohlraum performance. Finally, the hohlraum with octahedral 6LEHs is modeled, analyzed, and then optimized based on the unified free-form representation. The results show that a feasible shape design field with flux asymmetry no more than 1% can be obtained, and over the feasible design field, the spherical hohlraum is validated to have the highest flux symmetry, and a synthetically optimal hohlraum can be found with closing flux symmetry but larger volume between laser spots and centrally located capsule.

The x-ray drive on a capsule in an inertial confinement fusion setup is crucial for ignition. Unfortunately, a direct measurement has not been possible so far. We propose an angular radiation temperature simulation to predict the time-dependent drive on the capsule. A simple model, based on the view-factor method for the simulation of the radiation temperature, is presented and compared with the experimental data obtained using the OMEGA laser facility and the simulation results acquired with VISRAD code. We found a good agreement between the time-dependent measurements and the simulation results obtained using this model. The validated model was then used to analyze the experimental results from the Shenguang-III prototype laser facility. More specifically, the variations of the peak radiation temperatures at different view angles with the albedo of the hohlraum, the motion of the laser spots, the closure of the laser entrance holes, and the deviation of the laser power were investigated. Furthermore, the time-dependent radiation temperature at different orientations and the drive history on the capsule were calculated. The results indicate that the radiation temperature from “U20W112” (named according to the diagnostic hole ID on the target chamber) can be used to approximately predict the drive temperature on the capsule. In addition, the influence of the capsule on the peak radiation temperature is also presented.

The hohlraum shape attracts considerable attention because there is no successful ignition method for laser-driven inertial confinement fusion at the National Ignition Facility. The available hohlraums are typically designed with simple conic curves, including ellipses, parabolas, arcs, or Lame curves, which allow only a few design parameters for the shape optimization, making it difficult to improve the performance, e.g., the energy coupling efficiency or radiation drive symmetry. A novel free-form hohlraum design and optimization approach based on the non-uniform rational basis spline (NURBS) model is proposed. In the present study, (1) all kinds of hohlraum shapes can be uniformly represented using NURBS, which is greatly beneficial for obtaining the optimal available hohlraum shapes, and (2) such free-form uniform representation enables us to obtain an optimal shape over a large design domain for the hohlraum with a more uniform radiation and higher drive temperature of the fuel capsule. Finally, a hohlraum is optimized and evaluated with respect to the drive temperature and symmetry at the Shenguang III laser facility in China. The drive temperature and symmetry results indicate that such a free-form representation is advantageous over available hohlraum shapes because it can substantially expand the shape design domain so as to obtain an optimal hohlraum with high performance.