An experimental investigation of slow-mode shock waves

Abstract

This paper presents an idealized, one-dimensional theoretical model of a novel method of launching a slow-mode hydromagnetic shock wave. The reduction of the transverse magnetic field component, at the boundary of a semi-infinite plasma containing a uniform oblique magnetic field, results in the propagation of a fast-mode expansion wave followed by a slow-mode shock. An experiment designed to test this launching method is described. An initial hydrogen plasma containing axial and azimuthal magnetic field components is set up in a large annular apparatus. The azimuthal field is then reversed at one end of the device, by a fast-rising radial discharge. Diagnosis of the resulting flow, with small magnetic and electric probes, shows the propagation of the fast expansion wave and the slow shock; but there also follows a third wave, having the properties of a ‚piston’. When the theory is extended to include a moving boundary (i.e. ‚piston’), good quantitative agreement is obtained with the experimental results. The annular geometry of the experiment leads to centrifugal effects not considered in the one-dimensional theory; but these are not sufficient to disrupt seriously the main flow pattern. Detailed measurements of the shock indicate a dispersive structure.