COLLISIONLESS INSTABILITY OF THE CATHODE SHEATH IN A HOLLOW-CATHODE DISCHARGE

High-intensity periodic variations of the potential fall across the cathode sheath are observe in a low-pressure magnetic-field-free hollow-cathode discharge. These potential variations cause modulation of the discharge current, reaching a depth of almost 100%. Typical frequencies are tens of MHz, slightly lower than the ion plasma frequency near the cathode-sheath boundary and associated with the ion transit time through the sheath. The measured pressure dependence of the current threshold for the oscillations indicates that the mechanism of the instability is collisionless.