Observation of ‘switch-on’ shocks in a magnetized plasma

The Charybdis experiment is designed to produce shock waves propagating into a magnetized hydrogen plasma in directions parallel and almost parallel to the initial field and, in particular, the ‘switch-on’ shock. An initial plasma is produced in a pyrex chamber (0.46 m dia., 125 m long) containing an axial magnetic field ( B z up to 0.28 T): a magnetic piston is produced by a fast-rising radial discharge between a short central and an annular outer electrode at one end of the chamber; the Lorentz force on the current sheet causes axial propagation, and under certain conditions a curved shock front propagates ahead of the piston. The Alfvén Mach number of the flow is varied by changing the axial field strength: at high Mach number, where a parallel shock should be gas dynamic, no separated shock is found; at low Mach number, where theory predicts‘switchon’ shock behaviour, a clearly separated curved shock front is found which passes through a point of parallel propagation. Magnetic and electric probe measurements at the point of parallel propagation show the presence of a ‘switchon’ shock. It is suggested that the ‘switch-on’ and adjacent oblique shocks are matched together by an intermediate wave propagating behind the latter and merging with the former.