Electrothermal Filamentation of Fusion Plasmas

Electron currents within extremely dense (n > 10³² m^-3) , hot (T > 500 eV) plasmas are susceptible to filamentation via the electrothermal instability, yielding significant temperature and magnetic structures. The mechanism driving this instability has an intuitive picture; supposing that the temperature of an electron current is perturbed in the direction transverse to the current axis, hotter regions have an enhanced conductivity, giving an increased rate of Ohmic heating and thus increased temperature. The system of fluid equations describing a current, return current and background plasma are linearised to obtain the corresponding dispersion relation for the instability. Considering the particular plasma parameters of the cold, dense fuel layer of an imploded ICF capsule, this linear analysis predicts unstable modes for wavelengths larger than 2nm, with growth rates of the order 10¹⁵ s^-1. Particle-in-cell simulations are then employed to benchmark the growth rates, providing also a kinetic prediction for the non-linear growth and eventual saturation of the instability. The resulting temperature filaments are a candidate explanation for the non-Maxwellian nature of the fusing ions observed at the NIF.