UV-radiation in nitrogen excited by an electrodeless discharge

Abstract

The purpose of the present study is to investigate optical media emitting in the wavelength range from 320 to 400 nm. An optical medium emitting in the above-indicated wavelength range on the second positive system of nitrogen consists of a gas with low reactivity. Previously, nitrogen was not used for fabrication of unsealed electrodeless lamps because of its low emission efficiency. In the present study, discharges initiated in N2 and N2–Ar mixtures filling a cylindrical tube 15 cm long with an inner diameter of 3 cm are investigated. Ring electrodes 2 cm wide surrounded the external tube wall. An alternating voltage was applied to the electrodes. A generator of unipolar impulses with tunable frequency and variable voltage that allowed the average excitation power to be changed was used in our experiments. A ТDS-334 oscillograph with a bandwidth of 0.3 GHz and a sampling rate of 1.25 GS/s (5 points every 2 ns) was used to register signals from a capacitive divider and current shunt. The radiation intensity was measured with a calibrated FEC-22 SPU photodiode using the procedure described in [4]. The energy deposited into the medium was estimated by the method described in [5]. The lamp emission efficiency was determined from the ratio of the emission power to the power deposited into the medium. A digital camera was used to photograph the glow of a discharge. In our experiments, we obtained the following results. Lines of the CПu–BПg transition of the nitrogen molecule, including lines at λ = 316 nm of the (1–0) band, λ = 337.1 nm of the (0–0) band, λ = 358 nm of the (0–1) band, λ = 381 nm of the (0–2) band, and λ = 406 nm of the (0–3) band were the most intensive emission lines of excited nitrogen. The average emission power of excited nitrogen from the external lamp surface was 2 mW/cm in the wavelength range 310–410 nm, and the emission efficiency was 0.7% of deposited energy. The best result was obtained for nitrogen excited at a frequency of 30 kHz (at a pressure of ~18 Torr and an excitation power of ~1 W/cm). The lamp operation mode was optimized by varying the frequency of exciting radiation from 10 to 100 kHz, input power from 5 to 30 W, and pressure in range 1– 30 Torr. A decrease in the exciting pulse repetition rate relative to the optimum frequency at the given voltage hindered a breakdown and resulted in unstable lamp operation. The discharge was not ignited with further decrease of the exciting pulse repetition rate. The decreased efficiency of spontaneous emission on the CПu–BПg nitrogen transition with increase in the exciting pulse repetition rate was due to a decrease in the gap breakdown voltage. A decrease in the pressure relative to the optimum one decreased the emission power and efficiency. We also investigated an optical medium based on a nitrogen and argon mixture for a wide range of conditions. It is well known that the output laser energy generated on the second positive system of nitrogen increases when the argon and nitrogen mixture is excited with an electron beam [7]. However, an admixture of argon to nitrogen for a nitrogen laser pumped by a self-sustained discharge resulted in a decrease of the energy and lasing efficiency [8]. In the present investigation, the best results were obtained with N2: Ar = 1: 70 (1.5 Torr N2 and 105 Torr Ar). A double mixture with a comparatively small nitrogen content allowed the average output power and efficiency of the lamp to be increased several folds. The results of optimization of the frequency and excitation power are shown in Fig. 1. It illustrates the dependence of the specific emission power I and efficiency ν on the specific excitation power Р at frequencies of 30 and 50 kHz. From the dependence obtained it follows that for the same excitation power, the emission power and efficiency for a frequency of 30 kHz are twice greater than for 50 kHz. A decrease in the pulse repetition rate, as in the case of pure N2, hindered a breakdown, and a decrease in the pressure at the same excitation power decreased the emission power and efficiency. The typical emission spectrum of the argon and nitrogen mixture is shown in Fig. 2. The maximum intensity has the line with λ = 337.1 nm.