M. Olazabal-Loumé

Fast ignitor target studies for the HiPER project

Target studies for the proposed High Power Laser Energy Research (HiPER) facility [M. Dunne, Nature Phys. 2, 2 (2006)] are outlined and discussed. HiPER will deliver a 3ω (wavelength λ=0.35μm), multibeam, multi-ns pulse of about 250kJ and a 2ω or 3ω pulse of 70–100kJ in about 15ps. Its goal is the demonstration of laser driven inertial fusion via fast ignition. The baseline target concept is a direct-drive single shell capsule, ignited by hot electrons generated by a conically guided ultraintense laser beam. The paper first discusses ignition and compression requirements, and presents gain curves, based on an integrated model including ablative drive, compression, ignition and burn, and taking the coupling efficiency ηig of the igniting beam as a parameter. It turns out that ignition and moderate gain (up to 100) can be achieved, provided that adiabat shaping is used in the compression, and the efficiency ηig exceeds 20%. Using a standard ponderomotive scaling for the hot electron temperature, a 2ω or 3ω ...

Shock ignition: modelling and target design robustness

Shock ignition of a pre-compressed deuterium tritium fuel is considered here. When properly timed, a converging shock launched in the target prior to stagnation time strongly enhances the hot spot pressure. This allows ignition to be reached in a nonisobaric configuration. We show in this work that the igniting mechanism is pressure amplification by shock convergence and shock collision. The shock ignition applied to the HiPER target allows one to study the robustness of this method. It is shown that the spike energy is not a critical parameter and that the spike power delivered on the target depends mainly on the shell implosion velocity. Finally, a family of homothetic targets ignited with a shock wave is studied.

Compression phase study of the HiPER baseline target

The European High Power laser Energy Research (HiPER) project aims at demonstrating the feasibility of high gain inertial confinement fusion using the fast ignitor approach. A baseline target has been recently developed by Atzeni et al (2007 Phys. Plasmas 14 052702). We study here the robustness of this target during the compression phase and define pulse shape tolerances for a successful fuel assembly. The comparison between a standard and a relaxation pulse shows that the latter allows one to reduce both the laser power contrast and the growth of perturbations due to Rayleigh?Taylor instability. We have found that with 95?kJ of absorbed laser energy one can assemble the fuel with a peak density around 500?g?cm?2 and a peak areal density of 1.2?g?cm?2. This implies a total target gain of about 60.

Studies on targets for inertial fusion ignition demonstration at the HiPER facility

Recently, a European collaboration has proposed the High Power Laser Energy Research (HiPER) facility, with the primary goal of demonstrating laser driven inertial fusion fast ignition. HiPER is expected to provide 250 kJ in multiple, 3ω (wavelength λ = 0.35 µm), nanosecond beams for compression and 70 kJ in 10–20 ps, 2ω beams for ignition. The baseline approach is fast ignition by laser-accelerated fast electrons; cones are considered as a means to maximize ignition laser–fuel coupling. Earlier studies led to the identification of an all-DT shell, with a total mass of about 0.6 mg as a reference target concept. The HiPER main pulse can compress the fuel to a peak density above 500 g cm−3 and an areal density ρR of about 1.5 g cm−2. Ignition of the compressed fuel requires that relativistic electrons deposit about 20 kJ in a volume of radius of about 15 µm and a depth of less than 1.2 g cm−2. The ignited target releases about 13 MJ. In this paper, additional analyses of this target are reported. An optimal irradiation pattern has been identified. The effects on fuel compression of the low-mode irradiation non-uniformities have been studied by 2D simulations and an analytical model. The scaling of the electron beam energy required for ignition (versus electron kinetic energy) has been determined by 2D fluid simulations including a 3D Monte Carlo treatment of relativistic electrons, and agrees with a simple model. Integrated simulations show that beam-induced magnetic fields can reduce beam divergence. As an alternative scheme, shock ignition is studied. 2D simulations have addressed optimization of shock timing and absorbed power, means to increase laser absorption efficiency and the interaction of the igniting shocks with a deformed fuel shell.

Hydrodynamic and symmetry safety factors of HiPER's targets

Hydrodynamics and robustness of three high yield targets within the HiPER project are presented. Using realistic illumination nonuniformity configuration, hydrodynamic perturbations sensitivity analysis is carried out. A rather simple hydrodynamic perturbation modeling sequence is validated thanks to 2D simulations. 1D simulations post-processed with such a modeling sequence provide a good estimation of the thermonuclear burn. First estimates of hydrodynamic safety factor are given.

Radiation hydrodynamic theory of double ablation fronts in direct-drive inertial confinement fusion

The one-dimensional theory of double ablation fronts is developed for direct-drive inertial confinement fusion targets. The theory is based on the subsonic ablation front approximation and includes the effects of both radiation and electron heat fluxes. It is found that the structure of the ablation front is determined by two dimensionless parameters: the Boltzmann number and the effective mean free path. The Boltzmann number represents the ratio of the convective thermal and radiation energy fluxes, while the effective mean free path is the ratio between the characteristic plasma temperature gradient conduction scale length and the radiation mean free path. The development of a double ablation front is determined based on the range of the above dimensionless parameters. Temperature and density profiles in double ablation fronts are derived from a simplified analytic model and compared with the results of numerical simulations.

Dynamics and stability of radiation-driven double ablation front structures

The dynamics of double ablation front (DAF) structures is studied for planar targets with moderate atomic number ablators. These structures are obtained in hydrodynamic simulations for various materials and laser intensities and are qualitatively characterized during the acceleration stage of the target. The importance of the radiative transport for the DAF dynamics is then demonstrated. Simulated hydrodynamic profiles are compared with a theoretical model, showing the consistency of the model and the relevant parameters for the dynamics description. The stability of DAF structures with respect to two-dimensional perturbations is studied using two different approaches: one considers the assumptions of the theoretical model and the other one a more complete physics. The numerical simulations performed with both approaches demonstrate good agreement of dispersion curves.

Experimental evidence of foam homogenization

The propagation of an ionization wave through a subcritical foam is studied under inertial confinement fusion conditions. Independent measurements of the ionization wave velocity are compared with hydrodynamic simulations and analytical models. It is shown that simulations of a homogeneous material at equivalent density strongly overestimate the front velocity. The internal foam structure can be accounted for with a simple model of foam homogenization that allows improving agreement between experiment and calculations.

Progress in indirect and direct-drive planar experiments on hydrodynamic instabilities at the ablation front

Understanding and mitigating hydrodynamic instabilities and the fuel mix are the key elements for achieving ignition in Inertial Confinement Fusion. Cryogenic indirect-drive implosions on the National Ignition Facility have evidenced that the ablative Rayleigh-Taylor Instability (RTI) is a driver of the hot spot mix. This motivates the switch to a more flexible higher adiabat implosion design [O. A. Hurricane et al., Phys. Plasmas 21, 056313 (2014)]. The shell instability is also the main candidate for performance degradation in low-adiabat direct drive cryogenic implosions [Goncharov et al., Phys. Plasmas 21, 056315 (2014)]. This paper reviews recent results acquired in planar experiments performed on the OMEGA laser facility and devoted to the modeling and mitigation of hydrodynamic instabilities at the ablation front. In application to the indirect-drive scheme, we describe results obtained with a specific ablator composition such as the laminated ablator or a graded-dopant emulator. In application to ...

Reduced entropic model for studies of multidimensional nonlocal transport in high-energy-density plasmas

Hydrodynamic simulations of high-energy-density plasmas require a detailed description of energy fluxes. For low and intermediate atomic number materials, the leading mechanism is the electron transport, which may be a nonlocal phenomenon requiring a kinetic modeling. In this paper, we present and test the results of a nonlocal model based on the first angular moments of a simplified Fokker-Planck equation. This multidimensional model is closed thanks to an entropic relation (the Boltzman H-theorem). It provides a better description of the electron distribution function, thus enabling studies of small scale kinetic effects within the hydrodynamic framework. Examples of instabilities of electron plasma and ion-acoustic waves, driven by the heat flux, are presented and compared with the classical formula.