Peter Amendt, Lawrence Livermore National Laboratory

The Potential Role of Electric Fields and
Plasma Barotropic Diffusion for Inertial Confinement Fusion

Abstract:
The generation of strong, self-generated electric fields (~GV/m) in 
direct-drive, inertial-confinement-fusion capsules has been reported
[1]. These data motivate the question whether such fields can have observable 
effects on target performance. Two anomalies in the inertial confinement 
fusion database are well known: (1) an observed ‰2 greater-than-expected 
deficit of neutrons in an equimolar D3He fuel mixture compared with 
hydrodynamically equivalent DD [2] mixtures, and (2) a similar shortfall of 
neutrons when trace amounts of argon are mixed with DD fuel in indirect-drive 
implosions [3]. 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 Omega-scale implosions the (low Mach number) return shock has 
an appreciable scale length over which the lighter
DD ions can diffuse away from fuel center. The depletion of DD fuel is 
estimated and found to yield a corresponding reduction in neutrons, 
consistent with the anomalies observed in experiments for both argon-doped 
DD fuels and D3He equimolar mixtures. The reverse diffusion of the heavier 
ions towards fuel center from diffusive mass flux conservation also increases 
the pressure, potentially resulting in lower stagnation pressures and larger 
imploded cores in agreement with gated self-emission x-ray imaging data. The 
role of plasma temperature gradients on diffusion (or "thermodiffusion") is 
also introduced, borrowing from the concept of adiabatic lapse rates familiar 
to atmospheric physics. 

[1] Rygg et al., Science 319, 1223 (2008), Li et al., 
PRL 100, 225001 (2008); 
[2] Rygg et al., PoP 13, 052702 (2006); 
[3] Lindl et al., PoP 11, 339 (2004).

Prepared by LLNL under Contract DE-AC52-07NA27344.