On the plasma constriction close to the electrodes of high-pressure mercury and sodium lamps

The arc attachment on electrodes is investigated in 50 Hz high-pressure mercury and sodium discharges between 50 and 150 W. In the cathode and also in the anode phase the discharges are constricted towards the electrodes, and in case of mercury a spot is formed at the electrode. For sodium a diffuse attachment is found in both phases. Side-on intensities in the electrode region are measured with an intensified CCD camera connected to a spectrograph. This leads to a spatial resolution of ~10 µm. The plasma temperature is determined from absolute line intensities of optically thin lines. A rapid temperature increase to nearly 11 000 K is found for mercury within 200 µm in front of the cathode. The same tendency is found in case of sodium but the sheath is smaller and the temperatures are lower compared to mercury.