Characteristics of gain in Ne-like Ar 69.8 nm laser pumped by capillary discharge

Abstract

In this paper, the theoretical calculation model of the gain coefficient of Ne-like Ar 69.8 nm laser is established. With the collisional-radiative model, the rate equations for the 46.9 nm and 69.8 nm lasers are built by considering the 4 levels of the 2s2p6 1S0, 2p53p 1S0, 2p53p 3P2, and 2p53s 1P1. The gain coefficients per ion density of 46.9 nm and 69.8 nm lasers are calculated on the basis of the rate equations. The results show that the 46.9 nm laser has potential of higher gain than the 69.8 nm laser at an electron temperature of 200 eV. The gain coefficients per ion density at different electron temperatures are also calculated. Under the same electron density, the higher electron temperature is favorable for increasing the gain coefficients per ion density of the 69.8 nm laser. Meanwhile there is also an optimal electron density corresponding to the maximum gain coefficient per ion density of the 69.8 nm laser at a given electron temperature. Then a one-dimensional cylindrical symmetry Lagrangian magneto-hydrodynamics (MHD) code is utilized to simulate the Z-pinch process. The radial distributions of the electron temperatures, the electron densities and the Ne-like Ar ion densities are calculated with the MHD code at the different initial pressures. According to the rate equations for the 69.8 nm laser and the simulation results of the MHD code, the gain coefficient distribution of 69.8 nm laser in the radial direction of the plasma can be determined when the plasma is compressed to a minimum radius. According to the experimental parameters, the maximum gain coefficient of 69.8 nm laser is calculated to be 0.32 cm-1 when the main pulse current is 12 kA. The relationship between the radial distribution of gain coefficient of 69.8 nm laser and the initial pressure is also simulated. The theoretical results show that the optimal initial pressure is in a range of 12-14 Pa, in which the amplitude of gain coefficient is maximum. The experiments about 69.8 nm laser are conducted with Al2O3 capillary which has an inner diameter of 3.2 mm and a length of 35 cm. A main current of 12 kA with a rise time of 32 ns is produced by the main pulse generator, which consists of a Marx generator and a Blumlein line filled with de-ionized water. The Blumlein line is pulse-charged by a ten-stage Marx generator and discharges through the capillary by a self-breakdown main switch pressurized with N2 gas. To reduce the amplitude of main current, we reduce the charging voltage of the Marx generator and increase the conducting inductance of the main switch. Prior to the operation of the main current pulse, the capillary filled with Ar is predischarged by a current of~20 A. The 69.8 nm laser intensity as a function of initial pressure is measured by a 1-m grazing incidence Rowland spectrograph. The experimental results show that the optimum pressure is 16 Pa which is similar to the theoretical result. In addition, the gain coefficient (0.4 cm-1) measured in experiment is slightly higher than that (0.32 cm-1) of the theoretical calculation.