Fast cavity collapse and Hall magnetic fields*

Laboratory experiments with lasers to model the sub-Alfvenic interaction of expanding plasma cloud with magnetic field remain of interest in magnetospheric physics. Despite numerous previous works a number of unanswered questions remains, one of which is anomalously fast cavity collapse at the condition of finite ion gyroradius. In recent experiment at KI-1 facility we created a spherical plasma cloud by four-beam tetrahedron ablation of spherical pellet and reviewed the main aspects of interaction in the parameter range of ion gyroradius comparable to radius of stopping by magnetic field. The main new finding is a fast cavity collapse proceeding like a transport with a sharp front, and that it is accompanied by generation of Hall magnetic fields and currents. We have measured the spatial structure of the Hall fields which has quadrupole character similar to that known in the reconnection studies. We propose and illustrate by hybrid numerical simulations in comparison with measurements that the Hall currents carried by electrons may be responsible for the transport of magnetic field and fast cavity collapse.