Experimental verification of a zero-dimensional model of the ionization kinetics of XeCl discharges

An improved 0-dimensional model for XeCl high-pressure glow discharges is presented. Calculated discharge voltages are compared with precise measurements at a small, very homogeneous discharge. Excellent agreement in a wide parameter field demonstrates that this model may serve as a reference for simpler models describing the ionization kinetics.     

Verification of a XeCl* laser model by measurement of the plasma conductivity

Measurements of current, voltage, and laser emission of an X-ray preionized selfsustained XeCl^* laser discharge using Ne:Xe:HCl gas mixtures are compared with predictions of spatially homogeneous model calculations. As a prerequisite an accurate and detailed model of the electrical circuit is developed. For current and voltage measurements electro-optical methods are used. Corrections to the measurements and the reliability of the final data are discussed in detail. Using collision cross sections from the recent literature and stepwise vibrational excitation of HCl up tov=3 reproduces the electrical conductivities measured in a large field of parameters. The reliability and numerical stability of the model calculations depend on the way in which the e-HCl collision terms of the Boltzmann equation and the e-HCl collision rates are evaluated. The predicted total laser pulse energies are too small, but the shape and timing of the pulse correspond to the experimental values.     

Performance improvement of an x-ray preionized XeCl laser of very small discharge volume

Increasing the preionization electron density is found to be an effective method for improving the output performance of discharge-pumped excimer lasers at high gas pressures when the power-supply voltage becomes a limiting factor. In a small volume (2 cm³), x-ray preionized discharge, orders of magnitude improvement in XeCl laser power output (up to 400 kW) and pulse energy (up to 26 mJ) over previously reported results have been found possible using this method. Modifications to the low energy x-ray source and discharge system leading to these improvements are described. Various laser output characteristics are also presented and discussed.     

Spectroscopic investigation of the ionization kinetics in XeCl laser discharges by Xe* density measurements

Time and spatially resolved absorption measurements of Xe^* particle number densities performed at an X-ray preionized self-sustained XeCl^*-laser discharge were used to investigate the reaction kinetics of such discharges and to test the validity of the commonly-made assumption that they are fit well by spatially homogeneous numerical models. It turned out that the Xe^* density distribution over the discharge cross section remains homogeneous for less than 100 ns only. Comparing the experimental results of the homogeneous discharge phase with zero-dimensional model calculations, we conclude that induced emission modeled using two excited xenon levels must be taken into account for the quantitative interpretation of the absorption measurements. For the interpretation of the Xe 6s-6p absorption measurements even a multilevel model will be necessary. Further we conclude from the relative courses and the absolute values of the Xe^* particle number densities that realistic HCl kinetics should contain three vibrationally excited levels and stepwise excitation processes as proposed by Dem'yanov et al. [8].     

Compact,wide aperture X-ray preionized XeCl laser with high specific optical power

An X-ray preionized, discharge-pumped XeCl laser with a variable beam cross-section of up to 6×6 cm² is described. It uses flat electrodes and the beam width is determined by X-ray collimation. Its operation characteristics concerning reduced electric field strength (E/p) and X-ray dose are discussed in detail. The inductance of the discharge loop is minimized using a water capacitor arrangement. A very high specific optical power (90 MW/l) is achieved in an active volume of 1.2 l. The pulse energy exceeds 5 J in a 45 ns pulse (FWHM). Komatsu Ltd., Manda 1200, Hiratsuka-shi, Kanagawa 254, Japan Institute of Optics and Fine Mechanics, Department of Plasma Physics, Academia Sinica, Shanghai, P.R. China     

Attachment kinetics and life time of perionization electrons in rate-gas halogen laser mixtures

Experiments on homogeneous initiation of pulsed avalanche discharges in typical XeCl laser mixtures show that the x-ray preionization pulse remains effective after a time delay of the order of 20 s even at very high gas pressures (15 atm). Detailed kinetics analyses indicate that dissociative attachment of HCl is relatively slow during and after the preionization period so that the long effective delay time cannot be attributed to the storage effect of Cl^– ions. Instead, the observed phenomenon can be attributed to the long survival time of the free electrons due to relatively slow diffusion and electron-ion recombination. Some problems arising from quantitative interpretation of the experimental results are also noted and discussed.Paper partially presented at the 7th Intern. Conf. on Lasers and Applications, San Francisco, CA, USA (November 26–30, 1984)     

High efficiency discharge-pumped XeCl laser

Received: 19 March 1997/Revised version: 7 August 1997     

Spectroscopic diagnostics of the onset of discharge instabilities in a XeCl phototriggered laser

Study of the output power of a XeCl phototriggered laser at high energy loading has shown a multipulse operating mode. Above a critical value giving the maximum output energy, an increase of the applied electric field induces a dramatic decrease of the power emitted in the later peaks as well as the onset of a strong increase of the densities of various excited states, while such an increase is not observed for ionic states. Continuous-Wave (cw) laser-absorption spectroscopy is proved a very sensitive diagnostic tool to study the dynamics of these processes.On leave from Centre Técnico Aerospacial, S.J. Campos, Sao Paulo, Brazil     

Numerical modeling of a XeCl laser excited by microwave discharge

12 , 169 (1987). Reasonably good agreements in the peak output power and laser efficiency have been achieved. Model calculations also predict that an efficiency as high as 2.7% can be obtained once the conditions of the above-mentioned experiments have been optimized. From the consideration that the skin depth effectively limits the absorption length of the microwave pumping and hence the excitable volume, it is concluded that high input power densities (>2 MW/cm³) and higher gas pressures (between 3 and 10 atm) are the preferable conditions to achieve higher efficiency. Preliminary calculations on CCl₄ containing XeCl gas mixtures show that improvement in laser efficiency by several folds may be achieved as a result of the higher intrinsic efficiency of excimer formation. Received: 23 September 1996 / Revised version: 25 March 1997     

XeCl laser excitation by high power microwave pulses

Considerable progress of microwave pumping of excimer laser gas mixtures has been achieved. The present measurable best values for the pulse energy lie in the mJ range. A microwave pulse compression technique is applied using a resonantly charged microwave cavity. The stored energy is extracted by igniting a high density plasma in a quartz tube which acts as a switch and as a laser amplifier yielding a high energy laser pulse.Presented at LASERION '91, June 12–14, 1991, Müchen (Germany)     

Space and time resolved discharge evolution of a large volume X-ray triggered XeCl laser system

The spatial distribution and the temporal development of the net gain have been measured in a ten liter active volume switchless discharge XeCl laser. The experimental results are compared with both zero-dimensional and two-dimensional kinetic code predictions. The comparison between the results of the kinetic codes and of some measurements relevant to the time-dependent discharge homogeneity allows a deeper insight into the influence of the streamer evolution on the discharge characteristics.ENEA StudentENEA Guest     

Investigation of XeCl vibrational and quenching kinetics: Numerical simulation of gain and laser spectra in discharge-pumped oscillators

Model calculations of the small-signal gain and laser spectra and of laser output energies have been performed for various discharge-pumped XeCl laser systems. From comparison between model and experiment, limits have been set for the rate coefficients for XeCl(B,C) quenching by Ne, Ne + Xe and electrons. The influence of non-stationarity and electron-electron collisions in the solution of the Boltzmann equation on the estimation of these rate coefficients has been investigated. Some rough estimates of the XeCl(X,v) dissociation rate coefficients are given. It is shown, that due to the inclusion of bound-free emission from the XeCl(B, = 1) level it is not necessary to include strong variations of these rate coefficients with the vibrational quantum number to reproduce the intensity ratios of the two laser lines observed in free-running XeCl-excimer lasers.     

Investigation of XeCl vibrational and quenching kinetics: Measurements of gain and laser spectra in a discharge-pumped oscillator module

The small signal gain, amplified spontaneous emission and laser spectra of a transverse discharge-excited XeCl laser have been measured. Several gas mixtures, total gas pressures and electron densities have been investigated. From these measurements it is concluded, that bound-free transitions and transitions to high-lying vibrational levels of the ground state contribute significantly to the gain and laser emission. For the upper laser level a vibrational population ratio [XeCl(B, =1)]/[XeCl(B, =0)] corresponding to a temperature of about 370 K has been determined. The intensity ratio of about 1 observed for the two laser lines may be explained by the vibrational and quenching kinetics of the lower laser level together with the upper state kinetics. A table summarizing the result of the small gain measurement has been included for model comparison.     

High-pressure behaviour of an X-ray preionized discharge pumped XeCl laser

The output characteristics are described of an X-ray preionized discharge pumped XeCl laser, fed by a low-impedance pulse forming line (PFL), at pressures up to 12 bar. The influence of a multichannel rail gap placed between the PFL and the laser head on the output energy was studied. We found an increase of output energy with increasing pressure up to 8 bar. At higher pressures a saturation behaviour was found. The maximum output energy per unit volume was 6.5 J/l.     

Influence of cathode roughness on discharge homogeneity of a high-pulse repetition frequency long-pulse XeCl laser

Received: 17 October 1997/Revised version: 3 March 1998     

Diagnostics of a Compact Discharge-Pumped XeCl Laser with BCl3 Halogen Donor

The results of a comprehensive study of a compact UV-preionized XeCl laser are presented. The subjects of this study were: discharge voltage and current measurements, dye laser probing of the active medium, and the mass spectrometry of gas mixture degradation products. It is shown that the gas lifetime was significantly improved when the laser was operated with BCl₃ as a halogen donor instead of commonly used HCl. By the dye laser absorption and gain probing, the temporal and spatial dependences of the densities for several plasma components, Ne*, Xe*, Xe⁺*, Cl^–, XeCl* and of ground state boron atoms were measured. Some aspects of plasma kinetics for uniform and constricted phases of the discharge are discussed. By the mass spectrometry of gas mixture degradation products on long-term operation of the laser device several gaseous (N₂, O₂, CO₂, H₂O, C₂H₄) and solid (NiCl₂, H₃BO₃) products were detected in the laser chamber. NH₄Cl was determined to be a stable fraction of the deposits on optics surfaces. The reasons for the improvement of gas lifetime for BCl₃-containing gas mixtures are discussed.     

Rapid discharge-pumped wide aperture X-ray preionized KrF laser

A wide aperture X-ray preionized discharge-pumped KrF excimer laser has been constructed. A flat plate pulse-forming line (36 nF, 340 kV) charges a peaking capacitor (6 nF) through a rail-gap to facilitate a rapid discharge in the laser head. Collimated X-ray preionization is employed to obtain a wide and uniform discharge. The laser is intended to be used as a short pulse amplifier and results are presented when characterized as an oscillator. The active cross-section of the laser beam is 10×8 cm² with 50 cm effective electrode length. The laser pulse energy exceeds 4.7 J in a 28 ns pulse (FWHM).     

Experimental verification of a zero-dimensional model of the kinetics of XeCl* discharges by XeCl*(B)-, XeCl*(C)-, and Xe2Cl*-density measurements

Absolutely calibrated emission spectroscopy has been used to determine the particle number densities of XeCl^*(B), XeCl^*(C), and Xe₂Cl^* in a small scale Ne/Xe/HCl discharge with well-defined current and voltage pulses for a wide range of parameters. The measured particle number densities could be reproduced quite well by numerical model calculations using the rate-coefficient values of Quiñones et al. [1] for the quenching of XeCl^*(B,C) by Ne, Xe, and 2Xe, but 3.0 × 10^–31 cm⁶/s for the formation of Xe₂Cl^* by (Ne + Xe)-quenching. For the electron quenching, we recommend a rate coefficient value of 3.2 × 10^–8 cm³/s. From the equilibrium ratio of the particle number densities of XeCl^*(C) and XeCl^*(B), the energy separation between these states has been estimated to be 72 ± 33 cm^–1 with the B state placed above the C state.     

Optimization of the UV CuII laser

The cw output power of the uv CuII laser has been optimized with respect to the hollow cathode geometry, the discharge current, the fill gas pressure and the resonator mirror parameters. A maximum laser output power of 900 mW for multiline operation at 248.6, 259.1, 260.0, and 270.3 nm was achieved with 100 A discharge current, 260 V voltage and 16 mbar fillgas pressure, when a hollow cathode of 1.2 m length and 2×6 mm² cross section was employed. The single-pass pain gl has been estimated to 7%. A hollow cathode cross section of 1.5×4.5 mm² is suggested as an optimum geometry. In addition, some investigations on the mechanism of the laser power decay in Ne–Cu-discharges are presented.     

Population densities and optical gain of VUV transitions of Cu II in Cu−He hollow cathode discharges

Population densities of excited Cu II-levels between 16 and 25 eV in Cu–He hollow cathode discharges were determined by emission spectroscopy. Population inversion was detected for several 6s-4p transitions. Investigation of the enhancement of Cu II vuv lines in He compared to Ne discharges showed that the excitation of the 6s levels by charge transfer is up to 100 times higher in He than in pure Ne discharges. Using the population densities and known transition probabilities, a single pass gain of 55% m^–1 at 780.8 nm and 1.2% m^–1 at 154.17 nm at a current density of 0.4 A cm^–2 was calculated.This paper is dedicated to Prof. Dr. H. Gobrecht on the occasion of his 75th birthday