Experimental and Theoretical Study of Magnetohydrodynamic Ionizing Fronts

Abstract

An experimental and theoretical description of ionization fronts propagating into hydrogen with a bias magnetic field in the plane of the front is presented. The experimental evidence showed that the absolute light intensity behind the front corresponds to a fully ionized plasma. The density ratio was obtained by measuring the magnitude of the current in the front, the front speed, and using the equation of mass and momentum. The front speeds are small compared to the Alfvén velocity and are characterized by a critical velocity U_c, where U_c is the velocity a molecule must have so the kinetic energy equals the energy necessary to ionize it. A theory for the ionization front was developed which neglects the plasma pressure and abandons the condition E = V × B both ahead and behind the front. The theory predicts the front velocity and the rather unusual variation of the density ratio across the fronts with the front speed.