Power balance of HID lamp electrodes under HF operation

Operating HID lamps with high frequencies is not yet a commonly used method. It is assumed that the efficacy of the whole lamp system including the lamp driver can be improved with higher operating frequencies as it has already been done for low pressure lamps. Indeed the increasing frequency will lead to changes in the electrode physics which may cause a different lamp behavior Thus the Bochum model lamp is used to investigate the changes in the lamp electrode behavior while increasing the frequency. The model lamp is an easy system which allows a fundamental research of the electrodes and the near electrode region without the occurrence of acoustic resonances. For the investigation phase resolved pyrometry and spectroscopy combined with U/I measurements (ESV) are used. It can be shown that for an operation of the lamp with high frequencies the behavior during the anodic and cathodic phase changes. The presented results describe changes in the electrode temperature and the electrode power balance depending on frequency. With increasing frequency the course of voltage as well as the course of the electrode sheath voltage (ESV(t)) adjust to the sinusoidal current waveform. This causes a rise in the average input power into the discharge and the power input into the electrode boundary layers. Also very remarkable is the sinusoidal waveform of the ESV(t). It indicates a change of the mechanism of current transfer between the electrode and the bulk plasma in the cathodic phase. It means that the present model of the stationary boundary layer has to be revised. For a better understanding a comparison between the power input into the cathodic and anodic boundary layer is made. This is possible since ESV(t) = U_cathode(t) + U_anode(t) can be measured and V_cathode(t) can be estimated.