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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.

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.

Escape of α-particles from a burning or an ignited burning deuterium-tritium (DT) hot-spot with temperature up to more than tens of kilo-electron-volts is very important in inertial confinement fusion, which can significantly influence not only the hot-spot dynamics but also the energy gain. In this paper, we study the α-particle escape from a burning or an ignited burning DT hot-spot by considering the modifications, including the α-particle stopping by both DT ions and electrons with their Maxwellian average stopping weights, the relativity effect on electron distribution, and the modified Coulomb logarithm of the DT-α particle collisions. As a result, the escape effect from our modified model is obviously stronger than those from the traditional models. A fitted expression is presented to calculate the escape factor, which can be applied to a burning hot-spot with a temperature of 1–150 keV and an areal density of 0.04–3 g/cm2 with an accuracy within ±0.02. Finally, we discuss the α-particle escape effect on hot-spot dynamics and thermonuclear energy gain by comparing the results with escape factors from different models.

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.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 majority of solid state laser facilities built for laser fusion research irradiate targets with third harmonic light (0.35 μm) up-converted from the fundamental Nd wavelength at 1.05 μm. The motivation for this choice of wavelength is improved laser-plasma coupling. Significant disadvantages to this choice of wavelength are the reduced damage threshold of optical components and the efficiency of energy conversion to third harmonic light. Both these issues are significantly improved if second harmonic (0.53 μm) radiation is used, but theory and experiments have shown lower optical to x-ray energy conversion efficiency and increased levels of laser-plasma instabilities, resulting in reduced laser-target coupling. In this letter, we propose to use a 0.53 μm laser for the laser ignition facilities and use a low density foam wall to increase the coupling efficiency from the laser to the capsule and present two-dimensional radiation-hydrodynamic simulations of 0.53 μm laser light irradiating an octahedral-s...

We coupled the one-dimensional multi-group radiation hydrodynamic code RDMG with the MBDCA atomic physics package, which uses the Matrix-Block Method to solve the coupled rate equations of the Detailed Configuration Accounting (DCA) non-LTE model, and applied the coupled code RDMG-MBDCA with different flux limiters fe to simulate a laser-irradiated CH-tamped Au disk experiment at the SGII laser facility. From our simulations, we found that a higher fe leads to faster laser ablation, earlier x-ray breakout time with a higher maximum x-ray flux, and an x-ray spectrum with a higher intensity. However, for the same fe, the simulation from RDMG with the DCA model shows a slower electron thermal conduction between the laser absorption region and the electron thermal conduction than that with the average-atom model. From our investigation, we can say that it is the lower ionization from DCA in the electron thermal conduction region which causes the slower electron thermal conduction between the two regions. The ...

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 b...

A method is proposed to control the stimulated Raman scattering in the inertial confinement fusion by using auxiliary 2ω light to suppress the stimulated Raman scattering of the 3ω light. In this scheme, inverse bremsstrahlung absorption and parametric instabilities in the 2ω light increase the electron temperature and the plasma-density fluctuation, thus preventing the development of Raman scattering of the 3ω light. This scheme is successfully demonstrated by both one-dimensional kinetic simulations and two-dimensional radiative hydrodynamic simulations. The one-dimensional Vlasov results show that the time-averaged transmissivity of the 3ω light increases from 0.75 to 0.95 under certain conditions. Results obtained using the particle-in-cell method with Monte Carlo collisions show that the electron temperature is greatly increased with the increasing intensity of the 2ω light. The two-dimensional radiative hydrodynamic simulation results show that the electron temperature increases from 3.2 keV to 3.5 ...

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.

Green light (2$\omega$) has the potential to drive ignition target for laser fusion with significantly more energy than blue light (3$\omega$) and a relatively higher damage threshold for the optic components in the final optic assembly, but it has issues of a relatively low laser to x-ray conversion efficiency and a hard x-ray spectrum as compared to 3$\omega$. In this paper, we propose to drive a foam hohlraum wall with an ignition laser pulse by taking a 4$\omega$ laser at the pre-pulse and a 2$\omega$ laser at the main-pulse, called as 4$\omega$ - 2$\omega$ ignition pulse. This novel design has the following advantages: (1) benefiting from 2$\omega$ of its relatively high energy output and low damage threshold during main-pulse, (2) benefiting from foam in its relatively high laser to x-ray conversion efficiency and relatively low M-band fraction in re-emission, (3) benefiting from 4$\omega$ of its low LPI during pre-pulse. From our 1D simulations with Au material, the laser to x-ray conversion in a foam driven by 4$\omega$ - 2$\omega$ pulse has an increase of $28\%$ as compared to a solid target driven by 3$\omega$ with the same pulse shape. The relatively thin optical depth of foam is one of the main reasons for the increase of laser to x-ray conversion efficiency inside a foam target.

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...

A one-dimensional Lagrangian magneto-hydrodynamics code coupled to an atomic kinetics code was developed. In this code, the ablation and ionization of the wall material were taken into account, the coupled rate equations were solved by using the matrix-block method, and the effect of the reabsorption of the resonant lines was calculated by using an approximate expression of the escape-factor in a cylindrically expanding plasma. The numerical results obtained from this code on a discharge-pumped J=0−1 transition 46.9 nm laser line in Ne-like Ar were in good agreement with experimental observations. The physical schemes in the 46.9 nm laser line were studied.

The behavior of Lagrangian cells in expanding recombination plasma from laser-irradiated carbon fiber are studied. Approximate expressions of electron density, electron temperature, and fractional population of bare nuclei, the escape probability of 2-1 transition and gain for 3d5/2–2p3/2 transition in H-like C ions obtained in this paper are in agreement with numerical simulation.

A new hybrid-drive (HD) nonisobaric ignition scheme of inertial confinement fusion (ICF) is proposed, in which a HD pressure to drive implosion dynamics increases via increasing density rather than temperature in the conventional indirect drive (ID) and direct drive (DD) approaches. In this HD (combination of ID and DD) scheme, an assembled target of a spherical hohlraum and a layered deuterium-tritium capsule inside is used. The ID lasers first drive the shock to perform a spherical symmetry implosion and produce a large-scale corona plasma. Then, the DD lasers, whose critical surface in ID corona plasma is far from the radiation ablation front, drive a supersonic electron thermal wave, which slows down to a high-pressure electron compression wave, like a snowplow, piling up the corona plasma into high density and forming a HD pressurized plateau with a large width. The HD pressure is several times the conventional ID and DD ablation pressure and launches an enhanced precursor shock and a continuous compression wave, which give rise to the HD capsule implosion dynamics in a large implosion velocity. The hydrodynamic instabilities at imploding capsule interfaces are suppressed, and the continuous HD compression wave provides main pdV work large enough to hotspot, resulting in the HD nonisobaric ignition. The ignition condition and target design based on this scheme are given theoretically and by numerical simulations. It shows that the novel scheme can significantly suppress implosion asymmetry and hydrodynamic instabilities of current isobaric hotspot ignition design, and a high-gain ICF is promising.

Low-mode asymmetries in the laser-indirect-drive inertial confinement fusion implosion experiments conducted on the National Ignition Facility [G. H. Miller et al., Nucl. Fusion 44, S228 (2004)] are deemed the main obstacles hindering further improvement of the nuclear performance of deuterium-tritium-layered capsules. The dominant seeds of these asymmetries include the P2 and P4 asymmetries of x-ray drives and P2 asymmetry introduced by the supporting “tent.” Here, we explore the effects of another possible seed that can lead to low-mode asymmetric implosions, i.e., the M-band flux asymmetry (MFA) in laser-driven cylindrical gold Hohlraums. It is shown that the M-band flux facilitates the ablation and acceleration of the shell, and that positive P2 MFAs can result in negative P2 asymmetries of hot spots and positive P2 asymmetries of shell's ρR. An oblate or toroidal hot spot, depending on the P2 amplitude of MFA, forms at stagnation. The energy loss of such a hot spot via electron thermal conduction is ...

The random radiation asymmetry in the octahedral spherical hohlraum [K. Lan et al., Phys. Plasmas 21, 0 10704 (2014)] arising from the power imbalance, pointing accuracy of laser quads, and the assemblage accuracy of capsule is investigated by using the 3-dimensional view factor model. From our study, for the spherical hohlraum, the random radiation asymmetry arising from the power imbalance of the laser quads is about half of that in the cylindrical hohlraum; the random asymmetry arising from the pointing error is about one order lower than that in the cylindrical hohlraum; and the random asymmetry arising from the assemblage error of capsule is about one third of that in the cylindrical hohlraum. Moreover, the random radiation asymmetry in the spherical hohlraum is also less than the amount in the elliptical hohlraum. The results indicate that the spherical hohlraum is more insensitive to the random variations than the cylindrical hohlraum and the elliptical hohlraum. Hence, the spherical hohlraum can relax the requirements to the power imbalance and pointing accuracy of laser facility and the assemblage accuracy of capsule.

Our recent works [K. Lan et al., Phys. Plasmas 21, 010704 (2014); K. Lan et al., Phys. Plasmas 21, 052704 (2014)] have shown that the octahedral spherical hohlraums are superior to the cylindrical hohlraums in both higher symmetry during the capsule implosion and lower backscatter without supplementary technology. However, both the coupling efficiency from the drive laser energy to the capsule and the capsule symmetry decrease remarkably when larger laser entrance holes (LEHs) are used. In addition, the laser beams injected at angles > 45° transport close to the hohlraum wall, thus the wall blowoff causes the LEH to close faster and results in strong laser plasma interactions inside the spherical hohlraums. In this letter, we propose a novel octahedral hohlraum with LEH shields and cylindrical LEHs to alleviate these problems. From our theoretical study, with the LEH shields, the laser coupling efficiency is significantly increased and the capsule symmetry is remarkably improved in the spherical hohlraums. The cylindrical LEHs take advantage of the cylindrical hohlraum near the LEH and mitigate the influence of the blowoff on laser transport inside a spherical hohlraum. The cylindrical LEHs can also be applied to the rugby and elliptical hohlraums.

A recent publication [K. Lan et al., Phys. Plasmas 21, 010704 (2014)] proposed a spherical hohlraum with six laser entrance holes of octahedral symmetry at a specific hohlraum-to-capsule radius ratio of 5.14 for inertial fusion study, which has robust high symmetry during the capsule implosion and superiority on low backscatter without supplementary technology. This paper extends the previous one by studying the laser arrangement and constraints of octahedral hohlraum in detail. As a result, it has serious beam crossing at θL≤45°, and θL=50° to 60° is proposed as the optimum candidate range for the golden octahedral hohlraum, here θL is the opening angle that the laser quad beam makes with the Laser Entrance Hole (LEH) normal direction. In addition, the design of the LEH azimuthal angle should avoid laser spot overlapping on hohlraum wall and laser beam transferring outside hohlraum from a neighbor LEH. The octahedral hohlraums are flexible and can be applicable to diverse inertial fusion drive approaches...

The radiation temperature TR and M-band fraction fM inside the vacuum Au hohlraum have been experimentally determined by a shock wave technique and a broadband soft x-ray spectrometer (SXS) on the SGIII-prototype laser facility. From the results of the shock wave technique, TR is about 202 eV, and fM is about 9% for the hohlraums driven by a 1 ns flattop pulse of 6 kJ laser energy. The Continuous Phase Plate (CPP) for beam smoothing is applied in the experiment, which increases TR to 207 eV while has almost no influence on fM . Comparisons between the results from the two kinds of technologies show that TR from the shock wave technique is lower than that from SXS whether with CPP or not. However, fM from the shock wave technique is consistent with that from SXS without CPP, but obviously lower than the SXS's result with CPP. The preheat effect on exterior surface of witness plate is reduced by thicker thickness of witness plate designed for higher laser driven energy.

We propose a spherical hohlraum with octahedral six laser entrance holes at a specific hohlraum-to-capsule radius ratio of 5.14 for inertial fusion study, which has robust high symmetry during the capsule implosion and low backscatter without supplementary technology. To produce an ignition radiation pulse of 300 eV, it needs 1.5 MJ absorbed laser energy in such a golden octahedral hohlraum, about 30% more than a traditional cylinder. Nevertheless, it is worth for a high symmetry and low backscatter. The proposed octahedral hohlraum is also flexible and can be applicable to diverse inertial fusion drive approaches.

An electron transportmodel based on the non-Maxwellian distribution f 0 ∝ e - ν m (NM model), caused by the inverse bremsstrahlung heating, is used in 1-D plane target and 2-D hohlraum simulations. In the NM model, the electron heat flux depends not only on the gradient of electron temperature T e but also on the gradients of electron number density and the index m. From 1-D simulations, the spatial distribution of T e is dune-like and T e decreases obviously in the flux-heated region, which is very different from the flat profile obtained by using the flux limit model (FL model) but similar to the experimental observations [Gregori et al., Phys. Rev. Lett. 92, 205006 (2004)] and the nonlocal results [Rosen et al., High Energy Density Phys. 7, 180 (2011)]. The reason which causes the dune-like profile of T e is discussed in the paper. From 2-D hohlraum simulations, the NM results of the plasma status, the emission peak and profile inside hohlraum are very different from the FL model results. Finally, it is hard to use an average flux limiter in the FL model to obtain the same hohlraumplasma status and emission with those under the NM model.

The Hohlraum physics experiments performed on the SGIII-prototype laser facility are simulated by using our two-dimensional radiation hydrodynamic code LARED-H, and the influence of laser intensity on the two-dimensional Hohlraum simulations is studied. Both the temporal radiation temperature and the x-ray spectrum from the simulations agree well with the observations, except that the simulated M-band fraction (greater than 2 keV) is obviously smaller than the observation. According to our study, the coupling efficiency from laser to x-ray is around 70% for SGIII-prototype laser facility Hohlraums.

Simulations of the x-rayablation process of aluminum are performed using a one-dimensional multigroup radiation hydrodynamic code RDMG [F. Tinggui et al., Chin. J. Comput. Phys.16, 199 (1999)]. The scaling relation of the peak temperatures of the x-ray sources with the shock velocities is studied, and its dependence on the temporal profile and the length of the x-ray sources is described and analyzed in this paper. A scaling relation applicable to x-ray sources of 1 ns pulse laser-driven hohlraums is proposed, the dependence of which is studied and found to be negligible. Our scaling relation of radiation temperature versus shock velocity is about 10 eV lower than that proposed by Kauffman et al. [Phys. Rev. Lett.73, 2320 (1994)] for shock velocity of ( 4 – 8 ) × 10 6 cm / s .