RADIATION EFFICIENCY OF HG-AR SURFACE-WAVE DISCHARGES

The radiation efficiency of Hg-Ar low-pressure surface wave (sw) discharges has been measured in a broad range of Ar pressures (40 mTorr-12 Torr), Hg partial pressures (approximately 1-100 mTorr) and excitation frequencies (200-1000 MHz). Gas filling and tube geometry (26 mm outer diameter) were chosen to allow a comparison with the conventionally excited Hg-Ar low-pressure discharge used for general lighting purposes in the well known fluorescent lamp. In contrast to earlier work, the present study focuses on the quantitative measurement of the two Hg uv resonance lines at 185 and 254 nm. This gives a meaningful indication of the power balance of the discharge, because a large fraction of the power input into the plasma column is converted into Hg resonance radiation. The efficiency of the 254 nm line is near 50% and of the 185 nm line near 1 0% for optimized sw discharge conditions. The dependencies on Ar and Hg partial pressures, as well as the absolute values of the efficiency are very close to those found in conventionally excited DC or low-frequency discharges. Therefore, no sign of a reduced trapping of the uv radiation for the case of sw excitation was observed. The efficiencies are independent of the excitation frequency in the measured range. The RF power input into the plasma column extending outside of the used surfatron coupling structure was determined quantitatively. This was done by measuring the power dissipated in the part of the plasma column hidden within the surfatron by a calorimetric method. The measurements of the Hg uv radiation efficiency are supplemented by radial scan experiments of a visible Hg line, and by the study of phenomena occurring at a higher power density such as saturation and standing wave formation.