THE BOHM CRITERION IN THE PRESENCE OF RADIOFREQUENCY FIELDS

In d.c. discharges the Bohm condition is the condition for sheath form ation, the ion velocity at the plasma boundary having to satisfy the c ondition 1/2Mv2 1/2k(B)T(e), if the spread of velocities is ignored. T he model is a good one if the Debye distance is small compared with th e relevant scale length. As there is much current interest in radio-fr equency (RF) discharges, the question arises as to whether the criteri on remains valid in the presence of RF fields. The question has been s tudied recently by Riemann, following earlier work by Allen who compar ed the plasma boundary to a sonic surface. Riemann considered a wider range of frequencies and obtained a dispersion relation that was a qua rtic equation. The present paper examines the complex roots of this eq uation, using a method developed by Briggs. The results substantiate R iemann's statement that the Bohm criterion is still valid in the prese nce of RF fields.