Fusion alpha particle momentum deposition in thermonuclear burn dynamics

Abstract

In inertial confinement fusion, the DT fusion alpha particles carry not only energy but also appreciable momentum that is typically neglected in models of thermonuclear burn. In the central hotspot ignition scheme, the hotspot must self-heat and propagate thermonuclear burn before disassembly. Using radiation hydrodynamics simulations with a Monte Carlo alpha particle transport model, we investigate the effect of alpha momentum deposition across sub-ignition to robustly igniting regimes by hydrodynamic scaling of current central hotspot ignition designs from the National Ignition Facility (NIF). We find that the effective alpha particle ram pressure accelerates the shell at burn, reducing hotspot compression, increasing the rate of disassembly and decreasing yield. This causes a notable (~ 30%) reduction in yield at current NIF scale, with a persistent (~ 10%) penalty at larger hydrodynamic scales. These results demonstrate that alpha momentum deposition is a significant effect for present ignition-scale implosions, necessitating its inclusion in ignition criteria, burn models, and designs for high-gain inertial confinement fusion.

Figures (4)

• Figure 1: (Top) The yield amplification (ratio of simulated yields with and without alpha heating) from different hydrodynamic scales. Scale = 1 represents current designs at the NIF (in this work, N210808). Shown in blue and red symbols are calculations . (Bottom) The ratio of yields from full alpha particle transport to alpha energy transport only.
• Figure 2: (Left) The maximal value of the alpha momentum-to-thermal force ratio, $\beta$, as a function of hydrodynamic scale. (Middle) The alpha momentum driven pressure gradients and peak alpha momentum deposition rate as a function of scale. The alpha momentum driven pressure gradients, $\Delta dP/dr$, are found by taking the difference between simulations with and without alpha momentum transport. The pressure gradients and alpha momentum deposition rates are calculated at the hotspot radius (defined as the radius enclosing 98% of neutron production). (Right) Time series from the scale = 1 calculation with the full alpha transport model, where $t_{BT}$ is the nuclear bang time. The change in shell velocity $\Delta u_{\mathrm{shell}}$ is calculated by taking the difference between fuel areal density averaged velocities from simulations with and without alpha momentum transport.
• Figure 3: Trajectories in $\rho R_{HS}-\langle T_i \rangle$ space from scale = 1 calculations, full alpha transport in red and energy only in blue. The opacity of the points are coloured by the fraction of burn. Conditions at the time of peak neutron production are shown with crosses and at 10% of burn with empty circles. Also shown, as a black dashed line, is the ignition boundary as given in Atzeni and Meyer-Ter-VehnAtzeni2004
• Figure 4: Radial profiles of mass density ($\rho$), pressure ($P$), velocity ($v$), momentum deposition rate ($S_{\rho v}$), local unburnt DT fraction ($\Phi_{DT}$) for the energy only and full alpha transport models, at 10% of burn. Shown in the dot-dashed lines are ratios of the hydrodynamic quantities where the ratio is (full alpha transport result)/(energy only transport result).