Physical evolution of metal surface layers exposed to pulsed megagauss magnetic fields

Metal surfaces exposed to pulsed high magnetic fields in vacuum can experience phase transitions within the metal, vapor over the surface and the development of various dynamic and thermal phenomena that can adversely affect performance. The desired performance may include implosion of a liner without deleterious effects of perturbation growth on the outer surface, or liner compression of buffer magnetic flux surrounding plasma without penetration by high-Z metal vapor. The complexity of interactions and processes has made this a long-standing problem for both theoretical modeling and experimental diagnosis. As a guide for further work, the present paper steps through the physical evolution of the several portions of the surface layer from early heating by skin currents and vapor production, to the possible transition of this vapor into significant plasma. With additional heating, after the onset of nonlinear diffusion, the current density in the metal near the surface becomes roughly uniform and continued resistive heating allows transition from solid to liquid state. As a liquid subject to acceleration, perturbations can grow exponentially. Such growth, however, is restrained for perturbations with wavelengths not small compared to the thickness of the liquid layer. Similar restrained growth of perturbations can occur in the vapor/plasma layer, which may also be thinner than wavelengths of concern (e.g., thickness of region of buffer flux). Experimental attempts to understand the evolution described here suffer due to severe variations of material properties from the metal surface through vapor and plasma, with opportunities for inhomogeneities and nonequilibrium in many forms, and associated uncertainties in transport properties and observed radiation.