Octahedral Spherical Hohlraum Research Articles

Point design of octahedral spherical Hohlraum with HDC–CH capsule for a predictable inertial confinement fusion at/beyond ignition

Fusion ignition has been successfully achieved at the National Ignition Facility, but the main obstacles of low-mode asymmetries, laser-plasma instabilities (LPIs), and hydrodynamic instabilities (HIs) still remain in the path toward a predictable yield for fusion ignition, especially at high gain. A recently proposed octahedral spherical Hohlraum, i.e., a spherical Hohlraum with six laser entrance holes of octahedral symmetry [Lan et al., Phys. Plasmas 21, 010704 (2014); Phys. Rev. Lett. 127, 245001 (2021)], was demonstrated to have the advantages of a naturally high radiation symmetry without any symmetry tuning technology and a high energy coupling efficiency from the drive laser to the capsule hotspot. In addition, a novel HDC–CH (here, HDC and CH refer to high density carbon and glow discharge plastic, respectively) capsule design was proposed to have the advantages in both low LPIs and low HIs by using two different ablators [Qiao and Lan, Phys. Rev. Lett. 126, 185001 (2021)]. For the first time, here we proposed a point design target composed of an octahedral spherical Hohlraum and an HDC–CH capsule to suppress the above-mentioned obstacles and presented the 2D simulation of the effect of symmetry and hydrodynamic instabilities on implosion performances. Our work provides a novel target design for a more predictable fusion ignition in experiment.

Laser repointing scheme for octahedral spherical hohlraums on the SGIII laser facility

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.

Determining the driving radiation flux on capsule in Hohlraum for indirect drive inertial confinement fusion

In 2021, the fusion yield of 1.35 MJ was produced at NIF by using indirect drive inertial confinement fusion (ICF), indicating that indirect drive ICF has reached ignition. However, the driving radiation flux on capsule inside Hohlraums is still a puzzle in indirect drive ICF studies. The energy deficit at NIF is still neither well understood nor solved. In this paper, we proposed a scheme to determine the driving radiation flux on the capsule by using the combination of the shock wave technique and the reemitted radiation flux measurement. In this scheme, a witness sample is placed in the Hohlraum center as the surrogate of the capsule. The shock velocity in the witness sample is measured by a streaked optical pyrometer from one side, and the temporal reemitted radiation flux is measured by a space-resolved flat response x-ray detector. Then, the peak of the radiation flux is determined by the shock velocity, and the time behavior of the radiation flux is determined by the reemitted flux through the numerical simulation of radiation hydrodynamic code. The rules for designing the witness sample and an example of applying this scheme to determine the driving radiation flux on capsule inside the octahedral spherical Hohlraum are presented in detail.

Determination of laser entrance hole size for ignition-scale octahedral spherical hohlraums

A recently proposed octahedral spherical hohlraum with six laser entrance holes (LEHs) is an attractive concept for an upgraded laser facility aiming at a predictable and reproducible fusion gain with a simple target design. However, with the laser energies available at present, LEH size can be a critical issue. Owing to the uncertainties in simulation results, the LEH size should be determined on the basis of experimental evidence. However, determination of LEH size of an ignition target at a small-scale laser facility poses difficulties. In this paper, we propose to use the prepulse of an ignition pulse to determine the LEH size for ignition-scale hohlraums via LEH closure behavior, and we present convincing evidence from multiple diagnostics at the SGIII facility with ignition-scale hohlraum, laser prepulse, and laser beam size. The LEH closure observed in our experiment is in agreement with data from the National Ignition Facility. The total LEH area of the octahedral hohlraum is found to be very close to that of a cylindrical hohlraum, thus successfully demonstrating the feasibility of the octahedral hohlraum in terms of laser energy, which is crucially important for sizing an ignition-scale octahedrally configured laser system. This work provides a novel way to determine the LEH size of an ignition target at a small-scale laser facility, and it can be applied to other hohlraum configurations for the indirect drive approach.

Dreaming of predictable, reproducible fusion

Octahedral spherical hohlraum design provides ideal and clear radiation drive, paving the way towards inertial fusion energy power plants.

Dream fusion in octahedral spherical hohlraum

The octahedral spherical hohlraum provides an ideal and practical approach for indirect-drive toward a dream fusion with predictable and reproducible gain and opens a route to the development of a laser drive system for multiple laser fusion schemes. This paper addresses a number of issues that have arisen with regard to octahedral spherical hohlraums, such as how to naturally generate a highly symmetric radiation drive at all times and for all spectra without the use of symmetry tuning technology, how to determine the three-dimensional, temporal, and spectral characteristics of the real radiation drive on a capsule in experiments, and the relative energy efficiency of an octahedral spherical hohlraum compared with a cylindrical hohlraum. A design island for an octahedral spherical hohlraum is presented. Finally, the challenges and future tasks for the path forward are presented.

First Indirect Drive Experiment Using a Six-Cylinder-Port Hohlraum

The new hohlraum experimental platform and the quasi-3D simulation model are developed to enable the study of the indirect drive experiment using the six-cylinder-port hohlraum for the first time. It is also the first implosion experiment for the six laser-entrance-hole hohlraum to effectively use all the laser beams of the laser facility that is primarily designed for the cylindrical hohlraum. The experiments performed at the 100kJ Laser Facility produce a peak hohlraum radiation temperature of ∼222 eV for ∼80 kJ and 2ns square laser pulse. The inferred x-ray conversion efficiency η∼87% is similar to the cylindrical hohlraum and higher than the octahedral spherical hohlraum at the same laser facility, while the low laser backscatter is similar to the outer cone of the cylindrical hohlraum. The hohlraum radiation temperature and M-band (>1.6 keV) flux can be well reproduced by the quasi-3D simulation. The variations of the yield-over-clean and the hot spot shape can also be semiquantitatively explained by the calculated major radiation asymmetry of the quasi-3D simulation. Our work demonstrates the capability for the study of the indirect drive with the six-cylinder-port hohlraum at the cylindrically configured laser facility, which is essential for numerically assessing the laser energy required by the ignition-scale six-cylinder-port hohlraum.

First Inertial Confinement Fusion Implosion Experiment in Octahedral Spherical Hohlraum

In inertial confinement approaches to fusion, the asymmetry of target implosion is a major obstacle to achieving high gain in the laboratory. A recently proposed octahedral spherical hohlraum makes it possible to naturally create spherical target irradiation without supplementary symmetry control. Before any decision is made to pursue an ignition-scale laser system based on the octahedral hohlraum, one needs to test the concept with the existing facilities. Here, we report a proof-of-concept experiment for the novel octahedral hohlraum geometry on the cylindrically configured SGIII laser facility without a symmetry control. All polar and equatorial self-emission images of the compressed target show a near round shape of convergence ratio 15 under both square and shaped laser pulses. The observed implosion performances agree well with the ideal spherical implosion simulation. It also shows limitations with using the existing facilities and adds further weight to the need to move to a spherical port geometry for future ignition laser facilities.

Progress in optical Thomson scattering diagnostics for ICF gas-filled hohlraums

Optical Thomson scattering (OTS) diagnostics have been continuously developed on a series of large laser facilities for inertial confinement fusion (ICF) research in China. We review recent progress in the use of OTS diagnostics to study the internal plasma conditions of ICF gas-filled hohlraums. We establish the predictive capability for experiments by calculating the time-resolved Thomson scattering spectra based on the 2D radiation-hydrodynamic code LARED, and we explore the fitting method for the measured spectra. A typical experiment with a simplified cylindrical hohlraum is conducted on a 10 kJ-level laser facility, and the plasma evolution around the laser entrance hole is analyzed. The dynamic effects of the blast wave from the covering membrane and the convergence of shocks on the hohlraum axis are observed, and the experimental results agree well with those of simulations. Another typical experiment with an octahedral spherical hohlraum is conducted on a 100 kJ-level laser facility, and the plasma evolution at the hohlraum center is analyzed. A discrepancy appears between experiment and simulation as the electron temperature rises, indicating the occurrence of nonlocal thermal conduction.

Octahedral spherical Hohlraum for Rev. 6 NIF beryllium capsule

We have designed an octahedral spherical hohlraum with 6 laser entrance holes (LEHs) for the Rev. 6 Be ignition capsule [Simakov et al., Phys. Plasmas 21, 022701 (2014)]. With a Au spherical hohlraum of 4400 μm in radius and six LEHs of 1200 μm in radius, a laser pulse of 2.15 MJ energy and 630 TW peak power is required to deliver the radiation drive designed for the Rev. 6 Be ignition capsule. Both our 1D and 2D simulations have shown that the expansion of the Be capsule is very close to or even slightly slower than that of the CH capsule under the same radiation drive inside the spherical hohlraum, in spite of the higher ablation rate of Be. The reason why the Be and CH capsules have similar expansions is due to their similar exhaust velocities, and the reason why CH has a slightly faster expansion is that the CH capsule has a higher opacity which causes the hotter ablated plasmas and then a faster expansion of the CH ablated plasmas. The large volume of the octahedral spherical hohlraum, together with the incident angle of 55° in its laser arrangement, leaves enough room for the laser transportation, thus avoiding the laser being absorbed by Be ablated plasmas and, consequently, avoiding the high risk of laser plasma instabilities. It means that the higher mass ablation rate of Be does not affect the hohlraum energetics and the naturally high symmetry of radiation inside an octahedral spherical hohlraum. Thus, the superior ablation properties of the Be capsule can be fully exploited and hence have a higher opportunity to achieve ignition inside the octahedral spherical hohlraum.

Comparison of three hohlraum configurations with six laser entrance holes for indirect-drive inertial confinement fusion

The radiation drive asymmetry and laser-plasma instabilities (LPIs) inside the conventional cylindrical hohlraum configuration are two daunting issues in indirect-drive inertial confinement fusion. Recently, an octahedral spherical hohlraum (SH) (Lan et al 2014 Phys. Plasmas 21 010704), a novel three-axis cylindrical hohlraum (TACH) (Kuang et al 2016 Sci. Rep. 6 34636), and an advanced three-axis elliptical hohlraum (TAEH) (Jing et al 2017 arXiv:1703.01579) with six laser entrance holes (LEHs) were proposed to mitigate these issues. In this paper, the performance of these three new hohlraum configurations is compared. Preliminary simulations indicate that the TAEH (with a case-to-capsule ratio, CCR = 2.8) could provide excellent radiation symmetry, comparable to those inside the SH (CCR = 5.1) and TACH (CCR = 2.2). The filling time of plasma affecting the LPIs is between those of the SH and TACH, and about 1.5 times the one in the ignition hohlraum Rev5-CH (300 eV) of the National Ignition Campaign (Haan et al 2011 Phys. Plasmas 18 051001). The energy coupling efficiency of the TAEH is about 29% and 17% greater than those inside the SH and TACH, respectively. Moreover, all three configurations have robust symmetry with respect to laser beam pointing errors and capsule offset, with the SH being the most insensitive.

Application of the space-resolving flux detector for radiation measurements from an octahedral-aperture spherical hohlraum.

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.

A novel superconducting magnetic levitation method to support the laser fusion capsule by using permanent magnets

A novel magnetic levitation support method is proposed, which can relieve the perturbation caused by traditional support methods and provide more accurate position control of the capsule. This method can keep the perfect symmetry of the octahedral spherical hohlraum and has the characteristics in stability, tunability and simplicity. It is also favorable that all the results, such as supporting forces acting on the superconducting capsule, are calculated analytically, and numerical simulations are performed to verify these results. A typical realistic design is proposed and discussed in detail. The superconducting coating material is suggested, and the required superconducting properties are listed. Damped oscillation of the floating capsule in thin helium gas is discussed, and the restoring time is estimated.

First Octahedral Spherical Hohlraum Energetics Experiment at the SGIII Laser Facility.

The first octahedral spherical hohlraum energetics experiment is accomplished at the SGIII laser facility. For the first time, the 32 laser beams are injected into the octahedral spherical hohlraum through six laser entrance holes. Two techniques are used to diagnose the radiation field of the octahedral spherical hohlraum in order to obtain comprehensive experimental data. The radiation flux streaming out of laser entrance holes is measured by six flat-response x-ray detectors (FXRDs) and four M-band x-ray detectors, which are placed at different locations of the SGIII target chamber. The radiation temperature is derived from the measured flux of FXRD by using the blackbody assumption. The peak radiation temperature inside hohlraum is determined by the shock wave technique. The experimental results show that the octahedral spherical hohlraum radiation temperature is in the range of 170-182eV with drive laser energies of 71kJ to 84kJ. The radiation temperature inside the hohlraum determined by the shock wave technique is about 175eV at 71kJ. For the flat-top laser pulse of 3ns, the conversion efficiency of gas-filled octahedral spherical hohlraum from laser into soft x rays is about 80% according to the two-dimensional numerical simulation.

Three-dimensional simulations and analyses of spherical hohlraum experiments on SGⅢ laser facility

A new type of laser fusion indirect drive octahedral spherical hohlraum has been built up by Chinese researchers in recent years. The hohlraum with 6 laser entrance holes (LEHs) has superiority over other hohlraum configurations in both robust inherent high symmetry and high coupling energy efficiency from laser to hotspot for inertial confinement fusion study. Recently, an experimental investigation on radiation emission from the spherical hohlraum with two LEHs has been performed on the SGⅢ laser facility. In this experiment, 32 laser beams (24 beams from the top, 8 beams from the bottom) are injected into the hohlraum within 3 ns, and the total laser energy is 86.4 kJ. The hohlraum radius is 1.8 mm, and the radius of laser entrance hole is 0.6 mm. The experiments are conducted under two conditions:one is that a 0.48-radius capsule is located at the center of the hohlraum, and the other is that nothing is located in the hohlraum. Some flat response X-ray detectors (FXRDs) are installed at different angles on the target wall to collect the radiation energy. We carry out three-dimensional (3D) simulations of the experiment by using our 3D radiation implicit Monte Carlo code IMC3D. This code was developed in recent years based on fleck and Cumming's ideas. The hydrodynamics is not taken into consideration in the simulations, so we deduct 30% laser energy lost to hohlraum wall movements and back scattered by laser plasma instabilities. Based on the approximation, the simulation results are reasonable in principle. As a result, the radiation temperature of the hohlraum with capsule is 230 eV, and the radiation temperature of the hohlraum without capsule is 238 eV. At the end of laser injection, the capsule reflection ratio is 0.83. Compared with the experimental data, most of the simulation data agree well with the detector observations, except the data at 0 angle. The possible reasons for the difference are analyzed. The flux at 0 angle is more sensitive to the wall plasma movements than at the other angles. So if we ignore this phenomenon, then the witch will occur both in experiment and in simulation, yielding obvious differences for those quantities which strongly relate to the hydrodynamics of wall plasma. Finally, the methods of eliminating the difference are proposed and the prospect of IMC3D is presented.

Design of octahedral spherical hohlraum for CH Rev5 ignition capsule

In this paper, we design an octahedral spherical Au hohlraum for CH Rev5 ignition capsule [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)] by using the initial design method and two-dimensional (2D) simulations, and we investigate its laser entrance hole (LEH) closure and laser-plasma instabilities (LPI) by using a spherical hohlraum with two different-size LEHs via 2D simulations. The designed spherical hohlraum with RH=5 RC, RL=1.2 mm, and RL*=2RL requires an ignition laser pulse of 1.92 MJ in energy and 670 TW in peak power, where RH, RC, RL, and RL* are radii of the spherical hohlraum, capsule, LEH, and the cylindrical LEH outer ring, respectively. From 2D simulations, the closure and opening up of LEH are clearly obtained. The LEH closure and its rate are strongly connected to the radiation pulse, while the LEH opening-up and its rate are strongly connected to the laser pulse. The smallest radius of LEH during closure is 0.6 mm before opening up, which leaves enough room for arranging the laser beams with a radius of 0.5 mm in our design. By using a post-process code for LPI, a relatively high stimulated Brillouin scattering fraction and a very low stimulated Raman scattering fraction are predicted, which may be due to the neglection of three-dimensional density gradients of the ablative flow along the laser transportation in 2D simulations. This work provides the energy and power references for the future ignition laser facility which uses octahedral spherical hohlraums as ignition targets.

Comparison of the laser spot movement inside cylindrical and spherical hohlraums

Compared with cylindrical hohlraums, the octahedral spherical hohlraums have natural superiority in maintaining high radiation symmetry during the whole capsule implosion process in indirect drive inertial confinement fusion. However, the narrow space between laser beams and the hohlraum wall may disturb laser propagation inside the spherical hohlraum. In this work, the laser propagation inside the spherical hohlraum and cylindrical hohlraum is investigated experimentally by measuring laser spot movement at the SGIII-prototype laser facility. The experimental results show that the laser propagations inside the spherical hohlraum and the cylindrical hohlraum are totally different from each other due to different hohlraum structures. For the spherical hohlraum, although the laser energy is mainly deposited in the initial position of the laser spot during the whole laser pulse, some laser energies are absorbed by the ablated plasmas from the hohlraum wall. Because the laser beam is refracted by the thin plasmas near the laser entrance hole (LEH) region, the laser spot in the spherical hohlraum moves toward the opposite LEH. In contrast, the laser spot in the cylindrical hohlraum moves toward the LEH along the laser path due to the plasma expansion. When the laser is to be turned off, the accumulated plasmas near the LEH region in the cylindrical hohlraum absorb a majority of laser energy and hinder the laser arriving at the appointed position on the hohlraum wall.

Experimental demonstration of low laser-plasma instabilities in gas-filled spherical hohlraums at laser injection angle designed for ignition target.

Octahedral spherical hohlraums with a single laser ring at an injection angle of 55^{∘} are attractive concepts for laser indirect drive due to the potential for achieving the x-ray drive symmetry required for high convergence implosions. Laser-plasma instabilities, however, are a concern given the long laser propagation path in such hohlraums. Significant stimulated Raman scattering has been observed in cylindrical hohlraums with similar laser propagation paths during the ignition campaign on the National Ignition Facility (NIF). In this Rapid Communication, experiments demonstrating low levels of laser-driven plasma instability (LPI) in spherical hohlraums with a laser injection angle of 55^{∘} are reported and compared to that observed with cylindrical hohlraums with injection angles of 28.5^{∘} and 55^{∘}, similar to that of the NIF. Significant LPI is observed with the laser injection of 28.5^{∘} in the cylindrical hohlraum where the propagation path is similar to the 55^{∘} injection angle for the spherical hohlraum. The experiments are performed on the SGIII laser facility with a total 0.35-μm incident energy of 93 kJ in a 3 nsec pulse. These experiments demonstrate the role of hohlraum geometry in LPI and demonstrate the need for systematic experiments for choosing the optimal configuration for ignition studies with indirect drive inertial confinement fusion.

Radiation flux study of spherical hohlraums at the SGIII prototype facility

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-dependent behaviour of the radiation temperatures at different viewing angles.

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.