Core conditions for alpha heating attained in direct-drive inertial confinement fusion

Abstract

It is shown that direct-drive implosions on the OMEGA laser have achieved core conditions that would lead to significant alpha heating at incident energies available on the National Ignition Facility (NIF) scale. The extrapolation of the experimental results from OMEGA to NIF energy assumes only that the implosion hydrodynamic efficiency is unchanged at higher energies. This approach is independent of the uncertainties in the physical mechanism that degrade implosions on OMEGA, and relies solely on a volumetric scaling of the experimentally observed core conditions. It is estimated that the current best-performing OMEGA implosion [Regan et al., Phys. Rev. Lett. 117, 025001 (2016)] extrapolated to a 1.9 MJ laser driver with the same illumination configuration and laser-target coupling would produce 125 kJ of fusion energy with similar levels of alpha heating observed in current highest performing indirect-drive NIF implosions. DOI: 10.1103/PhysRevE.94.011201 Inertial confinement fusion (ICF) [1] uses lasers or other drivers such as pulsed-power devices or particle accelerators to implode a shell of cryogenic deuterium and tritium (DT). Laser light can be directly incident on the capsule surface (direct drive) or converted into x-rays (indirect drive) through a high-Z enclosure (hohlraum). The shell is imploded to high velocities of hundreds km/s to achieve high central temperatures and areal densities [2]. The hot spot (∼5-10 keV) is a low-density (30-100 g/cm 3 ) core and is surrounded and tamped by a cold (200-500 eV) near Fermi-degenerate dense (300-1000 g/cm 3 ) fuel layer. Fusion alphas produced in the hot spot deposit their energy primarily through collisions with plasma electrons further enhancing the temperature and fusion reaction rate (alpha heating). Under certain conditions of pressure, temperature, and confinement time, alpha heating initiates a burn wave in the surrounding dense shell, leading to fusion energy outputs greatly exceeding the thermal and kinetic energy supplied to the DT fuel by the implosion [2]. Alpha heating is essential for ignition and energy gain in nuclear fusion.In this Rapid Communication we show that recent direct drive OMEGA implosions [3] have achieved core conditions that would lead to significant levels of alpha heating if reproduced at scales typical of the National Ignition Facility (NIF) [4]. At NIF scale, these direct drive targets would yield about 125 kJ of fusion energy, 5× the highest fusion output achieved to date in ICF. The level of alpha heating and yield amplification is predicted to be similar to that achieved with the 1.9 MJ indirect-drive NIF [4] high foot (HF) implosions [5]. These HF implosions have achieved record fusion yields of nearly 1016 neutrons or about 26 kJ of fusion energy [6], demonstrating significant levels of alpha heating. Based on analytic models and detailed numerical simulations [7,8], it was estimated that alpha-particle heating has led to a ∼2-2.5× enhancement of the fusion yield [6,8]. In the absence of alpha heating, the fusion ...

Demonstration of Fuel Hot-Spot Pressure in Excess of 50 Gbar for Direct-Drive, Layered Deuterium-Tritium Implosions on OMEGA

“…This is nearly half of the ignition-threshold P hs ¼ 120-150 Gbar for direct-drive ICF scaled to NIF energies. These hydrodynamically scaled OMEGA implosions achieved an energy-scaled [14,15], generalized Lawson criterion without alpha heating of 60% [15], similar to indirect drive [6]. Relative to symmetric, one-dimensional (1D) simulations, the inferred P hs is approximately 40% lower.…”

“…A comparison of the measured target performance and the 1D simulations for implosions achieving P hs > 50 Gbar is presented in Table I. A generalized Lawson criterion without alpha heating [6,15] Table II, similar performance was achieved with the single-picket and triple-picket laser drives. The directdrive implosion performance achieved on OMEGA has been extrapolated to NIF: A detailed estimate of the fusion yield amplification due to alpha heating and the total fusion …”

“…[15] for a NIF direct-drive ignition target. Three-dimensional hydrodynamics simulations from the code ASTER [32] are used as a guide to determine possible sources of target performance degradation.…”

Simulation and analysis of time-gated monochromatic radiographs of cryogenic implosions on OMEGA

“…Recent implosion experiments on OMEGA have reached a record core pressure of 56±7 GB for direct-drive ICF cryogenic DT implosions [10]. This quality of performance, hydrodynamically scaled to the energy of the National Ignition Facility (NIF) [12], is equivalent to ~60% of the value of the Lawson parameter required for thermonuclear ignition [13,14] and similar to indirect-drive [15] implosion performance [16,17]. This measured core pressure is ~40% lower than simulated spherically symmetric 1-D performance.…”

“…The important questions associated with this parameter estimation method are whether or not the areal-density estimate obtained in this way corresponds to its radial integral definition in Eq. (14) and whether or not the estimate is accurate.…”