The potential role of electric fields and plasma barodiffusion on the inertial confinement fusion database

Abstract

The generation of strong, self-generated electric fields (GV/m) in direct-drive, inertial-confinement-fusion (ICF) capsules has been reported [Rygg et al., Science 319, 1223 (2008); Li et al., Phys. Rev. Lett. 100, 225001 (2008)]. A candidate explanation for the origin of these fields based on charge separation across a plasma shock front was recently proposed [Amendt et al., Plasma Phys. Controlled Fusion 51 124048 (2009)]. The question arises whether such electric fields in imploding capsules can have observable consequences on target performance. Two well-known anomalies come to mind: (1) an observed ≈2× greater-than-expected deficit of neutrons in an equimolar D3He fuel mixture compared with hydrodynamically equivalent D [Rygg et al., Phys. Plasmas 13, 052702 (2006)] and DT [Herrmann et al., Phys. Plasmas 16, 056312 (2009)] fuels, and (2) a similar shortfall of neutrons when trace amounts of argon are mixed with D in indirect-drive implosions [Lindl et al., Phys. Plasmas 11, 339 (2004)]. A new mechanism based on barodiffusion (or pressure gradient-driven diffusion) in a plasma is proposed that incorporates the presence of shock-generated electric fields to explain the reported anomalies. For implosions performed at the Omega laser facility [Boehly et al., Opt. Commun. 133, 495 (1997)], the (low Mach number) return shock has an appreciable scale length over which the lighter D ions can diffuse away from fuel center. The depletion of D fuel is estimated and found to lead to a corresponding reduction in neutrons, consistent with the anomalies observed in experiments for both argon-doped D fuels and D3He equimolar mixtures. The reverse diffusional flux of the heavier ions toward fuel center also increases the pressure from a concomitant increase in electron number density, resulting in lower stagnation pressures and larger imploded cores in agreement with gated, self-emission, x-ray imaging data.

Hydrodynamic shock wave studies within a kinetic Monte Carlo approach

“…Further investigations found that non-maxwellian ion distributions resulting from shock front kinetic effects that deplete the ion distribution of fast particles [10][11][12][15][16][17][18] may alter the hydrodynamics and thermonuclear yield of ICF capsules. Other studies suggest that capsule yield can be diminished by kinetic effects associated with strong self-generated electric fields which can broaden the shock fronts [19,20]. With that, the physics of ICF capsules being possibly subject to strong non-equilibrium effects provides a strong motivation to develop a kinetic simulation code that is capable to fully resolve shock fronts in all regions of the capsules.…”

Ion Thermal Decoupling and Species Separation in Shock-Driven Implosions

“…Multiple ion species are not treated separately: the ion mass and charge are set as a weighted average of the individual species. Recent experimental and theoretical work has questioned the validity of the average-ion assumption [5][6][7][8][9][10][11][12][13][14][15]. Anomalous reduction of the compression-phase nuclear yield has been observed in implosions filled with multiple fuel species, such as deuteriumhelium-3 (D 3 He) [5], DT [6], and other combinations [7,8].…”

Influence of coupling on thermal forces and dynamic friction in plasmas with multiple ion species

“…[30]. Equation (2) reduces to the familiar expression in the weakly coupled limit, in which Ξ αβ becomes the conventional Coulomb logarithm ln Λ [38].…”

“…In particular, it has been suggested that strong background gradients introduced during the implosion can separate fusion fuel constituents, resulting in yield degradation [1][2][3][4][5][6][7][8][9][10][11][12][13]. The same ion diffusion mechanisms that govern species separation in the fuel underlie mixing at the shell/fuel interface [14][15][16][17][18][19][20], as well as at the inner boundary of the hohlraum [21].…”