No abstract

No abstract

It is shown that inertial confinement fusion targets designed with low implosion velocities can be shock-ignited using laser-plasma interaction generated hot electrons (hot-e's) to obtain high energy gains. These designs are robust to multimode asymmetries and are predicted to ignite even for significantly distorted implosions. Electron shock ignition requires tens of kilojoules of hot-e's which can be produced only at a large laser facility like the National Ignition Facility, with the laser-to-hot-e conversion efficiency greater than 10% at laser intensities ∼10^{16}  W/cm^{2}.

A record fuel hot-spot pressure P hs ¼ 56 AE 7 Gbar was inferred from x-ray and nuclear diagnostics for direct-drive inertial confinement fusion cryogenic, layered deuterium-tritium implosions on the 60-beam, 30-kJ, 351-nm OMEGA Laser System. When hydrodynamically scaled to the energy of the National Ignition Facility, these implosions achieved a Lawson parameter ∼60% of the value required for ignition [A. Bose et al., Phys. Rev. E 93, LM15119ER (2016)], similar to indirect-drive implosions [R. Betti et al., Phys. Rev. Lett. 114, 255003 (2015)], and nearly half of the direct-drive ignition-threshold pressure. Relative to symmetric, one-dimensional simulations, the inferred hot-spot pressure is approximately 40% lower. Three-dimensional simulations suggest that low-mode distortion of the hot spot seeded by laserdrive nonuniformity and target-positioning error reduces target performance. DOI: 10.1103/PhysRevLett.117.025001 The spherical concentric layers of a direct-drive inertial confinement fusion (ICF) target nominally consist of a central region of a near-equimolar deuterium and tritium (DT) vapor surrounded by a cryogenic DT-fuel layer and a thin, nominally plastic (CH) ablator. The outer surface of the ablator is uniformly irradiated with multiple laser beams having a peak overlapped intensity of <10 15 watts=cm 2 . The resulting laser-ablation process causes the target to accelerate and implode. As the DT-fuel layer decelerates, the initial DT vapor and the fuel mass thermally ablated from the inner surface of the ice layer are compressed and form a central hot spot, in which fusion reactions occur. ICF relies on the 3.5-MeV DT-fusion alpha particles depositing their energy in the hot spot, causing the hotspot temperature to rise sharply and a thermonuclear burn wave to propagate out through the surrounding nearly degenerate, cold, dense DT fuel, producing significantly more energy than was used to heat and compress the fuel. Ignition is predicted to occur when the product of the temperature and areal density of the hot spot reach a minimum of 5 keV × 0.3 g=cm 2 [1-3]. Currently, the 192-beam, 351-nm, 1.8-MJ National Ignition Facility (NIF) [4] is configured for indirectdrive-ignition experiments using laser-driven hohlraums to accelerate targets via x-ray ablation. Approximately 26 kJ of thermonuclear fusion energy has been recorded on the NIF using indirect-drive ICF targets [5], where alpha heating has boosted the fusion yield by a factor of ∼2.5 from that caused by the implosion system alone [6,7]. The indirect-drive NIF implosions have achieved over 60% of the Lawson parameter Pτ required for ignition, where P is the pressure and τ is the confinement time [6]. Here P and τ are estimated without accounting for alpha heating to assess the pure hydrodynamic performance. The goal of achieving laboratory fusion and progress made with direct-drive ICF over the last decade motivate direct-drive implosions on NIF [8,9]. Hot-spot formation for spherically symmetric, direct-drive, DT-layered implosions is st...

The theory of ignition for inertial confinement fusion capsules [R. Betti et al., Phys. Plasmas 17, 058102 (2010)] is used to assess the performance requirements for cryogenic implosion experiments on the Omega Laser Facility. The theory of hydrodynamic similarity is developed in both one and two dimensions and tested using multimode hydrodynamic simulations with the hydrocode DRACO [P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)] of hydro-equivalent implosions (implosions with the same implosion velocity, adiabat, and laser intensity). The theory is used to scale the performance of direct-drive OMEGA implosions to the National Ignition Facility (NIF) energy scales and determine the requirements for demonstrating hydro-equivalent ignition on OMEGA. Hydro-equivalent ignition on OMEGA is represented by a cryogenic implosion that would scale to ignition on the NIF at 1.8 MJ of laser energy symmetrically illuminating the target. It is found that a reasonable combination of neutron yield and areal density for OMEGA hydro-equivalent ignition is 3 to 6 × 1013 and ∼0.3 g/cm2, respectively, depending on the level of laser imprinting. This performance has not yet been achieved on OMEGA.

The effect of asymmetries on the performance of inertial confinement fusion implosions is investigated. A theoretical model is derived for the compression of distorted hot spots, and quantitative estimates are obtained using hydrodynamic simulations. The asymmetries are divided into low (ℓ&lt;6) and intermediate (6&lt;ℓ&lt;40) modes by comparison of the mode wavelength with the hot-spot radius and the thermal-diffusion scale length. Long-wavelength modes introduce substantial nonradial motion, whereas intermediate-wavelength modes involve more cooling by thermal losses. It is found that for distorted hot spots, the measured neutron-averaged properties can be very different from the real hydrodynamic conditions. This is because mass ablation driven by thermal conduction introduces flows in the Rayleigh–Taylor bubbles that results in pressure variations, in addition to temperature variations between the bubbles and the neutron-producing region. The differences are less pronounced for long-wavelength asymmetries since the bubbles are relatively hot and sustain fusion reactions. The yield degradation—with respect to the symmetric case—results primarily from a reduction in the hot-spot pressure for low modes and from a reduction in burn volume for intermediate modes. A general expression is found relating the pressure degradation to the residual shell energy and the flow within the hot spot (i.e., the total residual energy).

No abstract

Experiments were performed with CH, Be, C, and SiO2 ablators interacting with high-intensity UV laser radiation (5 × 1015 W/cm2, λ = 351 nm) to determine the optimum material for hot-electron production and strong-shock generation. Significantly more hot electrons are produced in CH (up to ∼13% instantaneous conversion efficiency), while the amount is a factor of ∼2 to 3 lower in the other ablators. A larger hot-electron fraction is correlated with a higher effective ablation pressure. The higher conversion efficiency in CH is attributed to stronger damping of ion-acoustic waves because of the presence of light H ions.