On the cathode sheath in microhollow cathode discharges

An experimental study has been made on the potential fall of cathode sheaths in microhollow cathode discharge (MHCD) with open planar geometries and closed cathode cavities. The corresponding hole diameters D and electrode gaps DAC were between 0.1 and 0.8 mm. Therefore, we have investigated the static current-voltage (I-V) characteristics of the MHCD for discharge currents up to 15 mA and argon pressures up to 900 mbar. For the planar geometry, a scaling law of the microhollow cathode effect was found as function of the reduced pressure pD. On the contrary, for the closed cathode geometry a solely positive slope over the entire I-V characteristics is obtained. This result can be very important for the parallel operation of identical MHCD without ballast resistors. We have also shown that in the whole existence domain of the self-sustained MHCD, a strong low-frequency instability occurs for both the microhollow cathode geometries. This can be explained by a non-stationary extending of the negative regions of the MHCD from the inside of the cathode hole to the larger plane area on the outside of the same microhollow cathode. (author)