# A Study of Current Controlled Discharge in a Nitrogen Filled Tube
## 1 Introduction  
The basic subject of our interest is the temporal development of plasma excited by an electric discharge in a tube filled with nitrogen, in order to create conditions suitable for recombination excitation of a laser at quantum transitions of hydrogen-like nitrogen [1]. During the pulse discharge development, it is necessary to ionize fully the plasma and then to cool the plasma rapidly. Using the computer modelling method, we have shown that the required conditions can be set in a pinching discharge in a non-ablative capillary with a diameter of 3 mm filled with nitrogen to the pressure of 500 Pa excited by a current pulse with amplitude 50 kA and duration 80 ns. Experiments with parameters corresponding to the formulated requirements were performed in several laboratories, and laser activity was not achieved [2; 3; 4; 5]. We have come to the conclusion that in capillaries with such a high electric current, the wall ablation becomes a significant process [6]. We assume that to suppress the effect of ablation on the dynamics of plasma heating, the capillary diameter, and the current pulse shape should be modified, or additional plasma heating with a laser beam should be applied [7]. In order to suppress the ablation of the wall, we propose to increase the tube diameter by factor 2-4 and to use three consecutive independent current pulses. The first one (so called \"pre-ionization\") will ignite the discharge and prepare uniform and stable plasma column with high percentage of the first ionization state abundance. The second pulse (\"pedestal\") should be high and fast enough to create a magnetic pressure sufficient to force the plasma to detach from the wall, but not too high in order to keep the current below the tube wall ablation threshold. The final fast high current pulse (\"main\") can be then set according to the non-ablative computer modelling. In this paper, we focus on the first (pre-ionization) stage and we present a comparison of laboratoryexperiments and computer simulations of these experiments aimed at determining the spatial distribution of electron density and temperature together with resistance of the plasma-discharge column for slow pre-ionization discharge. The role of small (hundreds of Ampere) pre-ionization current lasting several us was described in several articles. Initially, the small pre-ionization current less than 16 A was used mainly to stabilize the plasma fiber [8; 9]. [PERSON] and [PERSON] investigated [10] the influence of pre-ionization on the shock wave motion in an alumina tube with 6 mm diameter and 10 mm length. The working gas was Ar with the range of initial pressures 80-320  Pa. No visible influence of the pre-ionization was observed for the current less than 50 A. Faster evolution of the shock wave and earlier start of the shock wave were observed for the pre-ionization current with exponentially decaying sinusoidal shape with quarter period of 500 ns, \(I_{max}=2500\) A, and \(t_{d}=4\) us. Computer simulation [11] suggests that pre-ionization current may redistribute plasma mass density to parabolic density profile with maximum along capillary wall, which has an effect on plasma evolution during subsequent main discharge action. It seems that it is possible to change plasma condition for lasing. A similar effect was observed for the argon laser in a few laboratories, the pre-pulse (or pre-ionization) current influences plasma dynamics, subsequently laser intensity, beam profile, divergence, and the length of the laser pulse [12; 13]. (There, it is shown that plasma is escaping through open capillary ends during us times of pre-ionization and its initial mass density is reducing.) [PERSON] et al. [14] demonstrated how time delay between triggering pre-pulse and main pulse improves gain of an argon laser and shifts time of the pinch onset. We optimize the tube discharge system for recombination excitation of the nitrogen laser via computer modelling. We vary electric current, tube diameter and initial pressure.