Analysis of the Liner Stability in Various Experiments

The main problem occurring at cylindrical liner acceleration by azimuthal magnetic field is its motion distortion caused by the combination of the Rayleigh-Taylor (RT) and the sausage instabilities, which, in principle, can lead to complete liner destruction. The strength of the material can inhibit the development of such instabilities; however, as far as melted liner section thickness increases due to ohmic heat, the instabilities start to develop fast. The experiments have shown, that even similar acceleration conditions may result in significantly different states of liners with different degrees of their distortion. Thus, in similar conditions of the LD, HF and NTLX experiments, the X-ray images for the HF and NTLX experiments showed liners´ homogeneity, suggesting that their motion was comparatively stable, while the X-ray images made for the LD experiments showed that the liners external surfaces were seriously distorted due to the perturbations growth. In this work 2D MHD computations are presented for liner LD, HF and NTLX implosions with consideration of perturbation growth. Initial perturbations were considered as random with characteristic values determined by the resolution of used computational grid. Computation results have shown, that, in fact, the conditions of the HF and NTLX experiments lead to smaller perturbation growth, than those of the LD experiments, if the value of perturbation growth is determined over the perturbation development on the internal surface. Comparison of the computed shape of the external surface for HF and LD showed, that in the computations, as well as in the experiments, the HF liner remained weakly perturbed, while the LD liner was significantly distorted. However, for the LD and NTLX the external surfaces of the liners remained qualitatively similar and rather perturbed even at extremely refined grid. Hence, the issue of greater perturbation development in the LD experiments may be thought of as explainable in some deg- ree by 2D computations, but the degree of computation and experimental data agreement requires additional refinement. The 2D computations results for the liners driven by 40-cm-diameter 15-module DEMG show that in these conditions the perturbations growth can be significant for the liners with aspect ratio (AR) higher than 20 and therefore it is necessary to study the liner stability for the liner DEMG experiments´ designing.