Shock-ignited Targets Research Articles

Fast-electron maintaining a high shock-ignition gain with a significant decrease in the laser pulse energy

The effect of energy transfer by laser-accelerated fast electrons on thermonuclear gain of a shock-ignited ICF target at different powers and durations of the high-intensity part of the laser pulse (spike) responsible for igniting shock wave generation has been investigated on the basis of hydro-kinetic numerical simulations. The key result of these studies is that the fast-electron energy transfer is able to provide a great contribution to igniting shock wave pressure to maintain a high thermonuclear gain with a significant decrease in the energy of the igniting part of the laser pulse. Calculations were performed for the 2nd harmonic Nd-laser pulse in order to justify shock-ignition experiments at the Megajoule-class facility, which is currently under construction in Russia. Spike energy conversion to fast electron energy and its temperature were selected in the ranges, which are discussed in the literature. It has been found that fast electrons with a temperature of 50–70 keV, whose energy contains 20%–40% of spike energy, make such a large contribution to the pressure of the igniting shock wave that the gain factor retains its value of 70–80 with spike energy decrease by 1.5–2 times.

Optimal design of shock ignition target in order to stop generated hot electrons within the cold fuel

AbstractGeneration of hot electrons (HEs) within ignitor pulse interaction with pre‐compressed fuel is an important challenge in the shock ignition approach. Target optimization in order to prevent the destructive effects of HE is the main goal of the present work. In the first stage, the spectrum of electron energy generated during the interaction of ignitor pulse at different widths with the HiPER pre‐compressed target has been estimated by applying particle simulation tool. Then, by changing the thickness of the cold fuel in the range of 185–225 μm, the corresponding areal densities are calculated using 1D hydrodynamic simulations. Finally, in order to assess the energy fusion yield, the iso‐gain curves are obtained for different ignitor time windows as well as target thicknesses. Simulation results indicate that by decreasing the baseline, target thickness leads to a 17–70% increase in the fuel areal density. Subsequently, it has been demonstrated that by properly adjusting the parameters of ignitor pulse launch time and its width and employing a target with areal density high enough to stop the HEs, energy gain above 140 can be achieved. Optimal areas for shell thickness and ignitor time window are identified.

Hot-electron deposition and implosion mechanisms within electron shock ignition**Project supported by the National Natural Science Foundation of China (Grant No. 11775203) and the Presidential Foundation of China Academy of Engineering Physics (Grant No. YZJJLX 2016007).

A hot-electron driven scheme can be more effective than a laser-driven scheme within suitable hot-electron energy and target density. In our one-dimensional (1D) radiation hydrodynamic simulations, 20× pressure enhancement was achieved when the ignitor laser spike was replaced with a 60-keV hot-electron spike in a shock ignition target designed for the National Ignition Facility (NIF), which can lead to greater shell velocity. Higher hot-spot pressure at the deceleration phase was obtained owing to the greater shell velocity. More cold shell material is ablated into the hot spot, and it benefits the increases of the hot-spot pressure. Higher gain and a wider ignition window can be observed in the hot-electron-driven shock ignition.

The role of fast electron energy transfer in the problem of shock ignition of laser thermonuclear target

The role of energy transfer by fast electrons, which is responsible for the positive effect of increasing the ablative pressure and the negative effect of preheating, on the implosion and thermonuclear gain of the target designed for shock ignition is investigated in comparison with the target designed for traditional spark ignition. On the base of one-dimensional hydrodynamic simulations with kinetic description of fast electron transfer it is shown, that depending on the characteristics of fast electron flux, in shock ignition target are manifested to varying degree both effects the positive and negative ones. This is a distinguishing feature of a shock ignition target compared to a traditional spark ignition target, in which only the negative effect of fast electron energy transfer takes place. In a shock ignition target up to temperatures of 50–60 keV, the positive effect is dominant and provides high gain. With an increase in the fast electron temperature, the role of the negative preheating effect increases, that ultimately leads to the ignition failure as temperature exceeding 90–100 keV.

Preliminary results from the LMJ-PETAL experiment on hot electrons characterization in the context of shock ignition

In the Shock Ignition scheme, the spike pulse intensity is well above the threshold of parametric instabilities, which produce a considerable amount of hot electrons that could be beneficial or detrimental to the ignition. To study their impact, an experiment has been carried out on the LMJ-PETAL facility with a goal to generate a strong shock inside a plastic layer under plasma conditions relevant to full-scale shock ignition targets. To evaluate the effect of hot electrons on the shock characteristics, laser temporal smoothing was either switched on or off, which in turns varies the quantity of hot electrons being generated. In this paper, we present preliminary results obtained during the experiment dedicated to the hot electron characterization. We present also calculations for the second part of the experiment, scheduled in 2020 and focused on the shock characterization.

Hydrodynamic studies of high gain shock ignition targets: effect of low- to intermediate-mode asymmetries

Shock-ignition (SI) is a direct-drive laser fusion scheme, in which the fuel is imploded at velocity somewhat smaller than in conventional schemes. The hot spot required for ignition is attained thanks to a converging shock-wave generated by an intense final spike of the laser pulse. Earlier studies show potentials of SI for high gain at driver energy of the order of 1 MJ, provided that laser-plasma instabilities do not degrade laser absorption and do not preheat the fuel. However, also hydrodynamic aspects need investigation. A crucial issue is the design of target-pulse concepts with sufficiently large 1D safety margin (ITF, ignition threshold factor), capable of tolerating unavoidable departures from design specifications. In the present paper we use 2D simulations to investigate the effect of low- to intermediate-mode implosion asymmetries (modes l = 1 − 8) on the performance of targets with different ITF’s. As expected, tolerance to perturbations increases with ITF. For given perturbation amplitude at beginning of deceleration, modes l = 1 − 4 are substantially more damaging than mode l = 8. By comparing simulations with and without alpha-particle heating we find a clear relation between tolerable perturbation amplitudes in simulations with alpha-particle heating and yield degradation in simulations without alpha heating.

The role of hot electrons in the dynamics of a laser-driven strong converging shock

Experiments on strong shock excitation in spherical plastic targets conducted at the Omega Laser Facility are interpreted with the radiation–hydrodynamics code CHIC to account for parametric instabilities excitation and hot-electron generation. The effects of hot electrons on the shock-pressure amplification and upstream preheat are analyzed. It is demonstrated that both effects contribute to an increase in the shock velocity. Comparison of the measured laser reflectivity and shock flash time with numerical simulations makes it possible to reconstitute the time history of the ablation and shock pressures. Consequences of this analysis for the shock-ignition target design are discussed.

Improved robustness study of a shock ignited target, with DUED code including non-local electron transport and 3D laser ray-tracing

Accurate descriptions of laser power coupling to the plasma and electron energy transport are crucial for designing shock-ignition targets and assessing their robustness (in particular with regard to laser and positioning errors). To this purpose, the 2D DUED laser fusion code has been improved with the inclusion of a 3D laser ray-tracing scheme and a model for non-local electron transport. 2D simulations with the upgraded code are presented; the dependence of the fusion yield vs target displacement is studied. Two different irradiation configurations are considered.

Spherical strong-shock generation for shock-ignition inertial fusion

Recent experiments on the Laboratory for Laser Energetics' OMEGA laser have been carried out to produce strong shocks in solid spherical targets with direct laser illumination. The shocks are launched at pressures of several hundred Mbars and reach Gbar upon convergence. The results are relevant to the validation of the shock-ignition scheme and to the development of an OMEGA experimental platform to study material properties at Gbar pressures. The experiments investigate the strength of the ablation pressure and the hot-electron production at incident laser intensities of ∼2 to 6 × 1015 W/cm2 and demonstrate ablation pressures exceeding 300 Mbar, which is crucial to developing a shock-ignition target design for the National Ignition Facility. The timing of the x-ray flash from shock convergence in the center of the solid plastic target is used to infer the ablation and shock pressures. Laser–plasma instabilities produce hot-electrons with a moderate temperature (<100 keV). The instantaneous conversion efficiencies of laser power into hot-electron power reached up to ∼15% in the intensity spike. The large amount of hot electrons is correlated with an earlier x-ray flash and a strong increase in its magnitude. This suggests that hot electrons contribute to the augmentation of the shock strength.

Effects of non-local electron transport in one-dimensional and two-dimensional simulations of shock-ignited inertial confinement fusion targets

In some regions of a laser driven inertial fusion target, the electron mean-free path can become comparable to or even longer than the electron temperature gradient scale-length. This can be particularly important in shock-ignited (SI) targets, where the laser-spike heated corona reaches temperatures of several keV. In this case, thermal conduction cannot be described by a simple local conductivity model and a Fick's law. Fluid codes usually employ flux-limited conduction models, which preserve causality, but lose important features of the thermal flow. A more accurate thermal flow modeling requires convolution-like non-local operators. In order to improve the simulation of SI targets, the non-local electron transport operator proposed by Schurtz-Nicolaï-Busquet [G. P. Schurtz et al., Phys. Plasmas 7, 4238 (2000)] has been implemented in the DUED fluid code. Both one-dimensional (1D) and two-dimensional (2D) simulations of SI targets have been performed. 1D simulations of the ablation phase highlight that while the shock profile and timing might be mocked up with a flux-limiter; the electron temperature profiles exhibit a relatively different behavior with no major effects on the final gain. The spike, instead, can only roughly be reproduced with a fixed flux-limiter value. 1D target gain is however unaffected, provided some minor tuning of laser pulses. 2D simulations show that the use of a non-local thermal conduction model does not affect the robustness to mispositioning of targets driven by quasi-uniform laser irradiation. 2D simulations performed with only two final polar intense spikes yield encouraging results and support further studies.

A polar-drive shock-ignition design for the National Ignition Facility

Shock ignition [R. Betti et al., Phys. Rev. Lett. 98, 155001 (2007)] is being pursued as a viable option to achieve ignition on the National Ignition Facility (NIF). Shock-ignition target designs use a high-intensity laser spike at the end of a low-adiabat assembly pulse to launch a spherically convergent strong shock to ignite the hot spot of an imploding capsule. A shock-ignition target design for the NIF is presented. One-dimensional simulations indicate an ignition threshold factor of 4.1 with a gain of 58. A polar-drive beam-pointing configuration for shock-ignition experiments on the NIF at 750 kJ is proposed. The capsule design is shown to be robust to the various one- and two-dimensional effects and nonuniformities anticipated on the NIF. The target is predicted to ignite with a gain of 38 when including all anticipated levels of nonuniformity and system uncertainty.

Optimal conditions for shock ignition of scaled cryogenic deuterium–tritium targets

Within the framework of the shock-ignition (SI) scheme, ignition conditions are reached following the separation of the compression and heating phases. First, the shell is compressed at a sub-ignition implosion velocity; then an intense laser spike is launched at the end of the main drive, leading to the propagation of a strong shock through the precompressed fuel. The minimal laser energy required for ignition of scaled deuterium–tritium (DT) targets is assessed by calculations. A semi-empiric model describing the ignitor shock generation and propagation in the fuel assembly is defined. The minimal power needed in the laser spike pulse to achieve ignition is derived from the hydrodynamic model. Optimal conditions for ignition of scaled targets are explored in terms of laser intensity, shell-implosion velocity, and target scale range for the SI process. Curves of minimal laser requirements for ignition are plotted in the energy–power diagram. The most economic and reliable conditions for ignition of a millimeter DT target are observed in the 240- to 320-km/s implosion velocity range and for the peak laser intensity ranging from ∼2 × 1015 W/cm2 up to 5 × 1015 W/cm2. These optimal conditions correspond to shock-ignited targets for a laser energy of ∼250 kJ and a laser power of 100 to 200 TW. Large, self-ignited targets are particularly attractive by offering ignition at a lower implosion velocity and a reduced laser intensity than for conventional ignition. The SI scheme allows for the compression and heating phases of the high power laser energy research facility target to be performed at a peak laser intensity below 1016 W/cm2. A better control of parametric and hydrodynamic instabilities within the SI scheme sets it as an optimal and reliable approach to attain ignition of large targets.

Studies on shock ignition targets for inertial fusion energy

Shock-ignited inertial fusion targets are studied by one dimensional and two-dimensional numerical simulations. Most of the study refers to the simple all-DT HiPER baseline target (imploded mass of 0.29 mg); both the reference laser wavelength λ = 0.35 μm, and λ = 0.25 μm are considered. The target achieves 1D gain about 80 (120) with total laser energy of 260 kJ (180 kJ) at λ = 0.35 μm (0.25 μm). Operating windows for the parameters of the laser ignition spike are described. According to preliminary simulations, gain 80–100 is also obtained by a scaled target (imploded mass of 1.8 mg) driven by 1.5 MJ of green laser light (0.53 μm). Two dimensional simulations indicate robustness to irradiation nonuniformities, and high sensitivity to target mispositioning. This can however be reduced by increasing the power of the ignition spike.

Direct-drive shock-ignition for the Laser MégaJoule

We present a review of direct-drive shock ignition studies done as an alternative for the Laser MegaJoule (LMJ). One and two dimensional systematic analyses of HiPER-like shock-ignited target designs are performed for the fuel assembly irradiation uniformity using the whole LMJ configuration or a part of the facility, and for the uniformity of the ignitor spike. High-gain shock-ignition is shown to be possible with intensity of each quad less than 10 15 W/cm 2 but low modes asymmetries displace the power required in the ignitor spike towards higher powers. Shock-ignition of Direct-Drive Double-Shell non-cryogenic targets is also addressed.

Hydrodynamic modeling and simulations of shock ignition thresholds

The Shock Ignition (SI) scheme (1) offers to reduce the laser requirements by relaxing the implosion phase to sub-ignition velocities and later adding an intense laser spike. Depending on laser energy, target characteristics and implosion velocity, high gains are expected (2, 3). Relevant intensities for scaled targets imploded in the velocity range from 150 to 400km/s are defined at ignition thresholds. A range of moderate implosion velocities is specified to match safe implosions. These conditions for target design are then inferred for relevant NIF and LMJ shock-ignited targets.

Design of a deuterium and tritium-ablator shock ignition target for the National Ignition Facility

Shock ignition presents a viable path to ignition and high gain on the National Ignition Facility (NIF). In this paper, we describe the development of the 1D design of 0.5 MJ class, all-deuterium and tritium (fuel and ablator) shock ignition target that should be reasonably robust to Rayleigh-Taylor fluid instabilities, mistiming, and hot electron preheat. The target assumes “day one” NIF hardware and produces a yield of 31 MJ with reasonable allowances for laser backscatter, absorption efficiency, and polar drive power variation. The energetics of polar drive laser absorption require a beam configuration with half of the NIF quads dedicated to launching the ignitor shock, while the remaining quads drive the target compression. Hydrodynamic scaling of the target suggests that gains of 75 and yields 70 MJ may be possible.

2D analysis of direct-drive shock-ignited HiPER-like target implosions with the full laser megajoule

AbstractWe present a 2D analysis of direct-drive shock ignition for the laser Megajoule. First, a target design is chosen in the HiPER-like target family generated by scale up and down of the original HiPER target. A first analysis is done considering the 1D fuel assembly and 2D shock ignition by means of the ring at polar angle of 33°2. The intensity profile is top-hat and calculations are done for several different radii. It is shown that larger the radius, lower the minimum spike power is. In addition, the intensity in each quad can stay below 4 × 1014 W/cm2 and is considered non crucial for parametric instabilities such as two plasmons. A 2D analysis of the fuel assembly is done in a second step by considering the two rings located at 49° and 59°5 and their symmetric by the equatorial plane symmetry. It is shown that low mode asymmetries are important at the stagnation and can significantly affect the areal density obtained. Finally, full 2D calculations of shock ignition is done, using all the beams of the LMJ and show that the spike power needed for ignition and gain is increased by a factor greater than 3 regarding the power needed in perfectly isotropic fuel assembly. This increase is mainly due to high level low mode asymmetries generated during fuel assembly.

Studying ignition schemes on European laser facilities

Demonstrating ignition and net energy gain in the near future on MJ-class laser facilities will be a major step towards determining the feasibility of Inertial Fusion Energy (IFE), in Europe as in the United States. The current status of the French Laser MégaJoule (LMJ) programme, from the laser facility construction to the indirectly driven central ignition target design, is presented, as well as validating experimental campaigns, conducted, as part of this programme, on various laser facilities. However, the viability of the IFE approach strongly depends on our ability to address the salient questions related to efficiency of the target design and laser driver performances. In the overall framework of the European HiPER project, two alternative schemes both relying on decoupling target compression and fuel heating—fast ignition (FI) and shock ignition (SI)—are currently considered. After a brief presentation of the HiPER project's objectives, FI and SI target designs are discussed. Theoretical analysis and 2D simulations will help to understand the unresolved key issues of the two schemes. Finally, the on-going European experimental effort to demonstrate their viability on currently operated laser facilities is described.

The role of spatial and temporal radiation deposition in inertial fusion chambers: the case of HiPER

The first wall armour for the reactor chamber of HiPER will have to face short energy pulses of 5 to 20 MJ mostly in the form of x-rays and charged particles at a repetition rate of 5–10 Hz. Armour material and chamber dimensions have to be chosen to avoid/minimize damage to the chamber, ensuring the proper functioning of the facility during its planned lifetime. The maximum energy fluence that the armour can withstand without risk of failure, is determined by temporal and spatial deposition of the radiation energy inside the material. In this paper, simulations on the thermal effect of the radiation–armour interaction are carried out with an increasing definition of the temporal and spatial deposition of energy to prove their influence on the final results. These calculations will lead us to present the first values of the thermo-mechanical behaviour of the tungsten armour designed for the HiPER project under a shock ignition target of 48 MJ. The results will show that only the crossing of the plasticity limit in the first few micrometres might be a threat after thousands of shots for the survivability of the armour.

Shock ignition target design for inertial fusion energy

Continuing work in the design of shock ignition targets is described. Because of reduced implosion velocity requirements, low target adiabats, and efficient drive by short wavelength lasers, these targets produce high gain (>100) at laser energies well below 1 MJ. Effects of hydrodynamic instabilities such as Rayleigh–Taylor or Richtmyer–Meshkov are greatly reduced in these low-aspect ratio targets. Of particular interest is the optimum ratio of ignitor to compression pulse energy. A simple pellet model and simulation-derived coupling coefficients are used to analyze optimal fuel assembly, and determine that shock ignition allows enough control to create theoretically optimum assemblies. The effects on target design due to constraints on the compression and ignitor pulse intensities are also considered and addressed. Significant sensitivity is observed from low-mode perturbations because of large convergence ratios, but a more powerful ignitor can mitigate this.