Spherical expansion of a dense plasma: test of ambipolar field model in Aleph.

appropriate. Close to the piston the magnetic field is perturbed a finite amount which to the lowest order is given by the vacuum dipole field associated with a perfectly conducting sphere in a constant external magnetic field. The linearization procedure permits the explicit solution for all quantities, the magnetic field, pressure, density, and velocity in terms of elementary functions. It is found to lowest significant order that all quantities are continuous across the shock surface. There remains in this limit to solve a boundary layer problein in the vicinity of the piston so as to determine more precisely the nature of the fields and the flows. (auth)