Experimental Study of the Propagation of an Ionizing Wave in a Coaxial Plasma Gun

A coaxial plasma gun experiment is described. The gun has an azimuthal bias magnetic field, which is strong compared to the field from the discharge current. The discharge voltage is shown to depend linearly on the bias field, thus defining a velocity that is found to be almost independent of the pressure and the discharge current. This velocity is close to the ``critical velocity'' which has been found in rotating-plasma experiments. The velocity of the current layer is also measured; it is always smaller than or equal to the critical velocity and it decreases with increasing pressure and decreasing magnetic field.