Ferrite-free closed-loop inductively-coupled low mercury pressure UV lamp

Abstract

Characteristics of ultraviolet (UV) radiation (254 nm) sources employing ferrite-free inductively-coupled low pressure mercury discharge excited in the closed-loop quartz tube with the inner diameter (ID), dt = 16,6 mm and the length, Λpl = 815 mm were experimentally studied. Discharge was maintained at a frequency of 1,7 MHz and RF powers, Plamp = 95–170 W, in the mixture of mercury vapor (~7–8 mTorr) and buffer gas fill (Ar, 30%Ne + 70%Ar) at pressures of 0,7 and 1,0 Torr. A 3-turn induction coil made from low loss Litz wire (resistivity, ρw = 1,4 × 10–4 Ω cm−1, wire diameter, dw = 1,5 mm) was disposed on the lamp surface along the closed-loop tube perimeter. As lamp RF power increased from 95 to 170 W, induction coil power losses, Pcoil, decreased from 7–9 to 3–4 W while coil power efficiency, ηcoil = 1- (Pcoil/Plamp), grew from 92 to 98%. Lamps with buffer gas at pressure of 1,0 Torr, operated at plasma RF power, Ppl = 105–150 W, had high plasma UV radiation generation efficiency, η254pl, max = Φ254/Ppl = 67%–68%, while lamp UV radiation generation maximal efficiency, η254, max = ηcoilη254pl, max, had slightly lower values of 63%–66%.