The effect of sheared axial flow on nonlinear Z-pinch dynamics

A two dimensional Eulerian fluid code has been used to study three problems related to Z-pinch and laser produced plasmas. a) The nonlinear evolution of a localised m=0 MHD mode neck is studied in order to extract some scaling laws for the size and form of the artificial neck. We examine whether the ubiquitous m=0 instability could be beneficially used to assist in the formation of a transient localised dense plasma. The results obtained were in satisfactory agreement with experiments and other theoretical work where available. b) The development of the m=0 instability on a Z-pinch although beneficial in the previous case, is detrimental from a stability point of view and thus to the utilisation of the device as a fusion reactor by itself. This is because the timescales of the instability development are faster than the confinement time needed for fusion to occur. Sheared axial flow is a proposed mechanism for the non-linear saturation of this particular instability. Indeed the linear growth rate also can be substantially reduced. It is hoped that it can inhibit the growth of the instabilities or at least delay their development sufficiently for fusion to take place. The numerical study of the effect of sheared axial flow on the nonlinear dynamics of the Z-pinch carried out, demonstrates that sheared flow with velocity u_z <1 Alfven speed has rather weak stabilisation effects. Above this limit, flow becomes significant. However after u_z >4 Alfven speed other modes, of the Kelvin-Helmholtz type, are excited which take over from the fastest growing mode in the static case. c) The expansion of the ablated plasma in laser-solid interactions is an important phenomenon for a plethora of reasons one of which is ICF. The simulations were in direct agreement with previous experimental work regarding the bulk properties of the ablation surface. They also provided justification for some assumptions made during the analysis of the observations and helped to confirm the calibration of the diagnostics timewise. The most striking feature of the experiments, namely the density dip on the laser axis although present in the simulations when radiation pressure was taken into account, is not as pronounced as expected. This can be accounted for by the absence of magnetised transport effects in the code such as the non-entropy producing Righi-Leduc heat flux and Nernst and Ettingshausen effects as well as the absence of fast electrons which carry their energy deep into the target. Furthermore results from a 1-D Lagrangian fluid code modeling a compressional pinch were presented and compared to experiments with good agreement. It is shown that the plasma is not in equilibrium with the confining magnetic field and does not exactly satisfy the Bennett relation as previously assumed. It is further shown how it is possible in a compressional Z-pinch to reach large ion Larmor radius collisionless conditions at stagnation. (author)Available from British Library Document Supply Centre-DSC:DXN051822 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo