On the limitation of RF fields by Alfven's critical ionization velocity phenomenon

Alfven's critical ionization velocity hypothesis and some of the numerous experiments supporting it are briefly reviewed. The critical velocity is associated with a critical electric field, which may limit the transfer of RF power through a cold partially ionized blanket. The underlying mechanism is not fully understood at present, but experimental observations indicate the parameter and frequency ranges in which the phenomenon occurs. The upper frequency limit seems to be higher than the ion cyclotron frequency. For frequencies below the ion-neutral collision frequency, the field limitation becomes less severe.