Magnetised Transport and Instability in Laser Produced Plasmas

Classical transport theory predicts strong coupling between thermal transport and magnetic field dynamics in laser produced plasmas; for example, fields are carried with the thermal flux, via the Nernst effect, while simultaneously deflecting it, giving rise to a Righi-Leduc heat-flow. Coupling between these effects is shown here to drive a new kind of instability-the magnetothermal instability-which is described in detail for the first time. A linear perturbation theory is derived in the absence of density gradients and hydrodynamical effects, and yields unstable solutions which propagate as magnetothermal waves. The theory is compared with full non-linear simulation in the context of a recent nanosecond gas-jet experiment and found to be in good agreement; exhibiting typical growth-rates and characteristic wavelengths of order 10ns-1 and 50µ m respectively. Further incorporation of density gradients and hydrodynamics into the magnetothermal stability analysis is shown to introduce the well-known field generating thermal instability source term, which can either complement or counteract the magnetothermal mechanism. Inequalities for predicting the dominance of each process are given: of the two, the magnetothermal mechanism is found to represent the main-and sometimes only-source of unstable feedback for the conditions considered here. Using super-Gaussian transport theory, the implications of inverse-bremmstrahlung heating for transport in laser-plasmas are also explored. Super-Gaussian modifications are shown to suppress a number of classical processes, by up to ~90% in some cases, while simultaneously introducing new effects, such as advection of magnetic field up density gradients. The combined consequences of these modifications are considered for the field generating thermal instability, and super-Gaussian effects are found to reduce growthrates by [greater than or similar to] 80% compared to predictions from classical transport theory under inertial confinement fusion conditions. The development of a unique code CTC, written to assist the exploration of both classical and super-Gaussian transport phenomena, and the new magnetothermal instability, is also described