S. A. Yampolskaya

Two-dimensional simulation of initiation and evolution of a plasma channel in the XeCl laser pumping discharge

We present two-dimensional simulation results of the formation and evolution of a diffuse plasma channel in the XeCl pumping discharge. Channel formation was initiated by metallic convexity at the cathode surface with a characteristic dimension of ∼0.1 cm. Two pumping regimes were considered. Initial voltage in the second regime was increased 1.6 times and impedance was decreased 2 times in contrast with the first regime. Evolutions and spatial distributions are presented for electron density, electric field, and frequencies of the main process. In the first regime, the diffuse plasma channel forms. The main processes causing development of the plasma channel are inhomogeneous distribution of the electric field, increase of the stepwise ionization frequency at n e ∼ 10 15 cm −3 , and depletion of HCl. Increasing of the initial voltage and decreasing of the impedance in the second regime results in a quick rise of the discharge current and electron density in the discharge gap, and uniform discharge takes place during the current pulse.

Formation of pumping discharge of XeCl laser by means of semiconductor opening switch

Results of experimental and theoretical investigations of XeCl with discharge formation by means of inductive energy storage and semiconductor opening switch have been presented. Main pumping of active medium has been performed by capacitive energy storage. Measured laser output energy is 0.8 J Laser efficiency calculated from energy stored in capacitive energy storage is 2.3%. Simulations predict that the optimization of pumping conditions may increase these parameters in two times.

Two-dimensional simulation of the development of an inhomogeneous volume discharge in a Ne/Xe/HCl gas mixture

The kinetic processes accompanying plasma column formation in an inhomogeneous discharge in a Ne/Xe/HCl gas mixture at a pressure of 4 atm were investigated by using a two-dimensional model. Two cathode spots spaced by 0.7 cm were initiated by distorting the cathode surface at local points, which resulted in an increase in the field strength in the cathode region. Three regimes differing in the charging voltage, electric circuit inductance, and electric field strength at the local cathode points were considered. The spatiotemporal distributions of the discharge current; the electron density; and the densities of excited xenon atoms, HCl(v = 0) molecules in the ground state, and HCl(v > 0) molecules in vibrational levels were calculated. The development of the discharge with increasing the electron density from 104 to 1016 cm−3 was analyzed, and three characteristic stages in the evolution of the current distribution were demonstrated. The width of the plasma column was found to depend on the energy deposited in the discharge. The width of the plasma column was found to decrease in inverse proportion to the deposited energy due to spatiotemporal variations in the rates of electron production and loss. The calculated dependences of the cross-sectional area of the plasma column on the energy deposited in the discharge agree with the experimental results.

Research of short pulse discharge XeCl laser

Results of experimental and theoretical investigations of pump discharge and generation of XeCl laser with 0.21 J laser energy, 2.7% electric efficiency nad 20 ns (FWHM) pulse duration are presented. The influence of step ionization, dissociative attachment and recombination processes on an active volume spatial uniformity and radiation parameters is shown.

Effect of rate of current rise on the discharge uniformity: a two-dimensional simulation

Summary form only given, as follows. We report results of computer simulations of the development of a high-pressure discharge with several plasma channels. In simulations hot spots were initiated by small metal convexities at the cathode surface with a characteristic dimension of /spl sim/0.1 cm. The convexities cause the local electric field to be non-uniform. The convexity size determines degree of such a non-uniformity. It was shown that if the neighboring convexities have different size the total current switches to a larger plasma channel. Other channels vanish. With the increasing rate of the current rise all diffuse channels develop simultaneously. This takes place due to a fast increase of the electron density and depletion of the HCl molecules in the whole channel volume. Thus the improvement of the discharge uniformity can be achieved by an appropriate choice of the pumping regime.

Effect of pre-ionization border on development of inhomogeneities in electrical discharge pumped XeCl laser

Effect of the border of preionization on discharge uniformity and quality of laser radiation is studied theoretically in the framework of 2D model of discharge pumped XeCl laser. Calculations show that near the border between preionized and unpreionized regions a local enhancement of the electric field occurs. This phenomenon is induced by a divergence of the X-ray beam. The local enhancement of the electric field promotes formation of cathode spots and, finally, leads to formation of discharge inhomogeneities. A decrease of sharpness of the border of preionization region reduces the local enhancement of electric field and improves characteristics of the laser radiation.

Optimization of initial parameters of XeCl laser pumping by changing of pulse duration using computer modeling

This paper presents simulation results for three modes of pumping of a XeCl-laser under radiation pulse duration of 10, 30, and 50 ns. Temporal evolutions of pump power and laser radiation, energy of pumping and lasing, densities of electrons and HCI(i) molecules in the ground and vibrationally excited states have been simulated for each mode. Rate dependences ofthe processes of XeCl** excimer molecules formation, such molecules quenching by electrons and heavy particles of plasma N(i), and induced radiation rate vs. time have been also obtained. For two modes of pumping with radiation durations of 30 ns and 50 ns, simulation data are compared with experimental results taken from the literature available. The simulated temporal evolutions of discharge current, radiation power, and lasing energy are in good agreement with the experimental results. There are no literature data regarding a laser with a pulse duration of 10 ns. The properties of laser operation at decreased pumping duration up to 10 ns are presented. Kinetic processes effect on radiant energy and laser efficiency has been analyzed. It follows from the analysis of calculation results that formation of excimer molecules takes place with high efficiency. Quenching processes of excimer molecules by electrons and heavy particles is the main factor leading to decrease in radiant energy, and therefore laser efficiency.

Peculiarities of short pulse electrical discharge XeCl laser

This paper reports on experimental results and simulation of XeCl-laser with short laser pulse duration. Experimental investigation was aimed at obtaining of maximal lasing power and efficiency. The processes taking place in plasma and in resonator were simulated. Temporal evolutions of plasma particle density as well as simulation rates of processes of ionization, recombination, attachment and formation of XeCl molecules in excited states were obtained. Simulation dependencies have been analyzed; mechanisms of initial parameter effect on lasing power have been established.

Influence of Q-factor value of optical resonator on spatial discharge structure and output characteristics of long pulse XeCl laser

Present paper reports the results of theoretical investigations of influence of Q-factor value of optical resonator on spatial properties and output characteristics of long pulse discharge pumped XeCl laser. It has been shown that high value of Q-factor do not influence homogeneity of discharge plasma. When the value of Q-factor is low an inhomogeneous distribution of intensity of laser photon flux occurs already at the beginning of generation. Photoionization of excited Xe atoms leads to an enhancement of electron density near the output mirror in comparison with the rest discharge volume. Further development of inhomogeneity is caused by instability of discharge plasma. Inhomogeneity of pumping discharge initiated by interaction between laser photon flux and discharge plasma leads to significant changes of characteristics of laser radiation.

The pumping discharge for XeCl lasers

This paper presents the results of experimental studies on the homogeneity of a discharge in aNe−Xe−HCl gas mixture at a pressure of 2 atm in relation to the discharge current density, the cathode material (Al, Cu, Ti), and the mode of preconditioning of the cathode. With freshCu electrodes, a discharge of current density j∼50 A/cm2 with no cathode spot has been generated. Upon prolonged preconditioning ofAl andCu electrodes, a homogeneous discharge with j>100 A/cm2 and a high density of cathode spots has been realized. The results of numerical calculations based on a plasma model which allows for more than 300 plasma chemical reactions agree well with experiment. The plasma particle densities and the rates of death and birth of charged species are presented as functions of time. The physical processes occurring in the discharge plasma are analyzed.