Modes of low-pressure longitudinal combined discharge

We studied in experiment the longitudinal combined (RF/DC) discharges. We applied across the electrodes the RF voltage and additionally DC voltage Udc simultaneously. Experiments were performed at the nitrogen pressure of p=0.01--5 Torr within the range of amplitude values of the RF voltage Urf $<$ 2000 V, DC voltage Udc $<$ 600V and f=13.56 MHz. The combined RF/DC discharges might exist in three modes. At low RF voltages the additional DC voltage makes a small contribution to the ionization rate of gas molecules within the plasma volume, and we observe the first mode of the combined RF/DC discharge -- ``non self-sustained RF discharge perturbed by the DC voltage''. When the RF voltage and the DC voltage are sufficiently large to induce the breakdown of the ``cathode'' layer, the discharge experiences the transition to the second mode -- ``combined discharge''. Just this mode is of considerable interest for plasma technology. The third mode may be obtained applying a small RF voltage to the burning DC discharge. We will call it conditionally ``non self-sustained DC discharge perturbed by the RF voltage''.