Electric field and ionization-gradient effects on inertial-confinement-fusion implosions

The generation of strong, self-generated electric fields (10(8)-10(9) V m(-1)) in direct-drive, inertial-confinement-fusion capsules has been reported (Li et al 2008 Phys. Rev. Lett. 100 225001). Various models are considered herein to explain the observed electric field evolution, including the potential roles of electron pressure gradients near the fuel-pusher interface and plasma polarization effects that are predicted to occur across shock fronts (Zel'dovich and Raizer 2002 Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena (Mineola, NY: Dover) p 522). In the latter case, strong fields in excess of 10(10) V m(-1) and localized to 10-100 nm may be consistent with the data obtained from proton radiography. Such field strengths are similar in magnitude to the criterion for runaway electron generation that could lead to plasma kinetic effects and potential shock-front broadening. The observed electric field generation may also be partly due to plasma ionization gradients localized near the fuel-pusher interface. A model is proposed that allows for differing electron-and ion-density gradient scale lengths in the presence of ionization gradients while preserving overall charge neutrality. Such a redistribution of electrons compared with standard, charge-neutral, single-fluid radiation-hydrodynamics modelling may affect the interpretation of imploded-core x-ray diagnostics as well as alter alpha particle deposition in the thermonuclear fuel.