Radiation efficiency of ac-excited micro-hollow cathode discharges

In previous works we reported about non-linear self-sustained oscillations in the usual 'standard' micro-hollow cathode discharges (MHCD) for a high-pressure argon gas without special cautions for its purity. These strong non-linear oscillations are observed into the whole pressure and discharge current range in which the MHCDs are sustained. The induced current and light intensity modulations against their averaged values reach to 20% and 5% for the current and discharge voltage, respectively. However, for the conditions near to the strong negative slope on the static I-V characteristic, which corresponds to the transition towards the micro hollow mode, the modulation degree of the discharge current and light intensity can increase up to 100%. These current and light spikes are well correlated with the spreading out of the negative glow upon the outer plane surface of the micro hollow cathode, a phenomenon which is in full agreement with recent predictions about the MHCD static I-V characteristics. Thus, the aim of this contribution is to prove if a resonant energy transfer can be sustained by the ac-excitation of such self-oscillations with the help of a low-frequency power supply for the above 'standard' MHCD. This device consists of 0.2 mm thick molybdenum electrodes separated by a 0.25 mm thick alumina insulator. The discharge channel with an internal diameter of 0.2 mm was drilled through this sandwich. The VUV radiation of our argon gas was investigated by the same experimental set-up shown elsewhere. For the ac-excitation we use ac currents in the frequency range from 40 kHz to 140 kHz, which are rectified but not filtered to preserve the micro cathode function of one MHCD-electrode. The so-obtained time functions for the current and voltage over the MHCD device are non-linear and the corresponding power input shows a flat maximum at about 50 kHz. Then, for our argon gas we estimate relative radiation efficiency in the VUV spectral range which, in fact, refers to the resonant line of an oxygen impurity at 130.5 nm. This VUV efficiency, defined by the ratio between the relative VUV radiation output and the electrical power input, displays a more pronounced maximum at the same frequency but this value remains still comparable with those of the MHCD device supplied by dc currents. Finally, it must be noted that recently appears also the first paper about the ac-excitation of an array with a very large number of small silicon micro cavities. (author)