M Grozeva

A flexible platform for simulations of sputtering hollow cathode discharges for laser applications

The Plasimo modelling platform, extended with a cathode wall sputtering module is used to study the discharge processes and to optimise the design parameters of a sputtering hollow cathode discharge (HCD). We present Plasimo simulations of a HCD used for laser applications. A time dependent sub-model is used to describe the behaviour of the plasma species. The sputtering yield at the metallic boundaries is calculated using an empirical formula. Copper is chosen as the cathode material and the discharge operates in helium with a small admixture of argon for more efficient sputtering. The optimal conditions for lasing of the infra-red (IR) copper ion line (780.8 nm) that have been determined experimentally are used as input conditions for the simulation model. Calculations are made for various gas mixtures. The observed quantities are compared with the experimental data obtained for the same discharge geometry and operating conditions. The agreement between the measured data and the results from modelling indicates that the main reactions in the model are correctly described. Therefore, it is believed that this model can be used as a design tool in optimizing discharge studies for various applications based on the sputtering of the cathode material.

Geometrical features in longitudinal sputtering hollow cathode discharges for laser applications

Longitudinal sputtering hollow cathode discharge (HCD) used as active medium for lasing is studied by means of numerical modelling. Due to the longitudinal non-uniformities of the discharge, the laser operation could be strongly affected. The non-uniformity of the discharge is mainly influenced by the dimensions of the hollow cathode, in particular by the aspect ratio length/radius. To study the geometrical features, a simulation model for HCDs has been constructed using the Plasimo modelling platform. The model allows in-depth studies of the plasma mechanisms and enables finding the optimum under the working conditions of the HCDs. The model is validated by comparing the results with the experimental observations. The spatial distribution of the plasma density and potential as well as an in-depth discussion of the results and the trends revealed by the model are presented. The proper understanding of the essential geometrical features allowed defining the optimal aspect ratio length/diameter for stable and uniform discharge with high excitation efficiency.

A hollow cathode discharge for laser applications: influence of the cathode length

The influence of the geometry of a longitudinal hollow cathode discharge (HCD) excited in a Cu cathode and He–Ar mixture is studied experimentally and theoretically. Special attention is devoted to the optimization of the HC length to obtain a stable and uniform laser medium with a high excitation efficiency. The influence of the cathode length is demonstrated experimentally by the behaviour of the 780.8 nm Cu ion line. The dependence of the laser power and gain as a function of the cathode length segments is measured. The Plasimo modelling platform is used to construct a model allowing more profound studies of the plasma processes and plasma behaviour under different conditions. Calculations at different cathode lengths are made and typical results such as spatial potential and plasma density distributions are presented and discussed. It is demonstrated that when the cathode length is increased the plasma density at the centre of the cathode decreases and the discharge tends to separate into two independent parts causing axial non-uniformity and reducing the discharge efficiency. The results also suggest that there exist a lower limit of the cathode length. Below this limit an inversion of the axial electric field occurs, which can be regarded as a transition between the conventional and high-voltage HCD under the conditions under study.

Monte Carlo simulation of a sputtering hollow-cathode discharge for laser applications

We report on a kinetic model that computes the electron behaviour in a hollow cathode discharge. It is a part of the PLASIMO toolkit and is based on a Monte-Carlo technique. The model is tested by varying the input parameters and by comparing the output with the output obtained by the freeware Boltzmann equation solver BOLSIG+. The results show that the Monte-Carlo model gives reliable information about the behavior of the electrons in the discharge. The Monte-Carlo module is applied to the case of a hollow cathode discharge for laser applications. Analysis of the output data and its adequateness is done. Future developments of the model are discussed.

Monte Carlo simulation of electron kinetics in a hollow cathode discharge

A kinetic model is reported computing the electron behavior in a hollow cathode discharge based on the Monte Carlo technique. It is a part of the PLASIMO modelling toolkit. The model allows the electrons to be closely followed while they travel and undergo collisions in the discharge. The Monte Carlo modulewas applied to the case of a HCD used as an excitation medium of atoms obtained by laser ablation. Results are obtained on the electron energy distribution function and the mean electron energy under typical discharge conditions. The output data and future development of the model and its applications are analyzed and discussed.

Towards a reduced chemistry module of a He-Ar-Cu hollow cathode discharge

This study is aimed at finding a reduced chemistry module for a hollow cathode discharge (HCD) excited in a He–Ar–Cu mixture. This enables us to construct lean and reliable models that can be used as a part of the design tool of HCDs. To this end estimative calculations and numerical simulations are performed under optimal conditions for lasing. An analysis of the species behaviour and reactions is made and as a result the model is simplified by means of reducing the number of species and reactions. The consequences of these reductions are justified by comparing the results of the simplified models with those of a more complete one. This study delivers a model that is chemically lean and thus, much less time consuming. It can be used in optimization studies to find the optimum in the plasma control parameter set of HCDs. The technique developed in this study for HCDs can be applied to glow discharges in general.

Dependence of laser power and gain on the cathode length of a sputtering copper ion laser

The dependence of the laser output power and the small signal gain for the 780.8 nm copper ion transition as a function of the cathode segments length in a sputtering longitudinal hollow cathode discharge are measured. The optimal cathode length in regard of maximum laser power is determined. From one and the same active volume at equal input power a considerable increase of laser output power is observed using the optimal length cathode segments. The results are in good agreement with the previously performed calculations and measurements of axial current and plasma characteristics, showing that the plasma is most intense near the anode ends of the cathodes. The measurements confirm that the highest laser power and excitation efficiency is achieved when the laser active volume comprises a series of anodes and cathodes, each cathode 2 cm long. This report is a part of a series of investigations aimed at optimization of the longitudinal hollow cathode discharge used as excitation medium of cathode sputtered metal ion lasers.

Optical Emission Spectroscopy of Combined Laser Ablation-Hollow Cathode Glow Discharge Plasma Source

The combination of laser ablation and glow discharge (LA-GD) as a spectroscopic source for elemental analysis is an increasing field of study in recent years. This is due to the fact that the laser ablation-glow discharge provides a more stable and reproducible spectra of the sample in comparison with the transient laser ablation. An important advantage of the laser ablation-glow discharge technique is the possibility for independent control of the processes of sample introduction and sample excitation. The combination of laser ablation and glow discharge is also capable of sampling non-conducting or low-sputtering-yield materials which for stand-alone glow discharge usually requires preliminary sample preparation procedures or use of pulsed power supply. Exploiting the characteristics of this technique has resulted in increase in sensitivity and limit of detection (Tereszchuk K, Vadillo J, Lazerna J Appl Surf Sci 255). Here results on one of the realization of the laser ablation-glow discharge technique – combination of laser ablation and hollow cathode discharge are reported (Karatodorov S, Mihailov V, Grozeva M, Open Chem 13). Spatial separation of the laser ablation introduction from the hollow cathode discharge excitation is achieved. The sample analytical line emission intensity dependence on background gas pressure, sample cathode distance and discharge current is studied. This allowed optimization of the technique for increasing its analytical characteristics to be done.

Optical emission spectroscopy of plasma produced by laser ablation of iron sulfide

Optical emission spectroscopy studies are reported of laser ablation plasma from an iron sulphide (FeS) target in vacuum. The plasma emission is characterized with spatial and tempoal resolution. The spectral observations are used to calculate the electron plasma parameters. The electron temperature is calculated by the Boltzmann plot method and the electron number density, by the Stark broadening of the emission lines.

Influence of low-pressure glow discharge on laser-induced plasma spectra

A scheme is reported for spectrochemical analysis combining sample introduction by laser ablation and additional excitation of the ablated material by a hollow-cathode discharge. The optimal conditions allowing spatial separation of the two processes are determined. Under these conditions, enhancement of the analytical lines intensity in comparison with stand-alone laser-induced plasma is observed.