Breaking the CIV speed limit in a rotating plasma device*

The critical ionization velocity (CIV) was first proposed by Hannes Alfven [1954] as a mechanism for ionization in a neutral gas streaming through a magnetized plasma. The newly born pick-up ions act as momentum loading and restrain the relative velocity. The velocity and rate of ionization depend on how the momentum can be coupled by currents to distant plasma or to nearby conductors. Maintaining ionization requires power, and increasing the velocity beyond the CIV rapidly increases the power requirement. Thus CIV acts as an effective speed limit. To study this and other aspects of momentum coupling, we have built a cylindrical plasma device with an axial magnetic field. A plasma with a radial electric field is established using hollow cathode electron and ion emitters as virtual electrodes, with glass end caps to support the radial electric field. This device produces an ExB rotating plasma and we observe a rapidly increasing power requirement approaching the CIV velocity. There are however, applications where it is desired to exceed the CIV limit. Thus this experiment will explore the practical dependence of CIV on parameters such as power, magnetic field, momentum coupling, and in particular the neutral gas density.