The structure of a microwave torch plasma jet was investigated by taking pictures of the jet with a digital camera and processing them next with the aid of software for astronomical imaging and data visualization. The commonly accepted idea of the structure of such a plasma jet is that the jet consists of two regions: a core and an envelope around it. It can be assumed that the core is that region where the microwave power dissipation occurs. Its extent depends on the intensity and geometry of the electric field, which is concentrated close to the nozzle tip. The remaining region of the jet is filled with recombining (afterglow) plasma, not held in position by the electric field, and forming an envelope around the core. The results reported here helped to clarify that picture. The plasma jet was produced in argon at atmospheric pressure by a pulsed 2.45 GHz microwave field applicator of nozzle type. The gas flow rate was equal to 3 l/min and the maximum peak power amounted to 1 000 W. Photographs taken with short exposure times showed that for a turbulent jet the core, rather than being immersed in the envelope, is surrounded by erratically wandering tongues of excited gas leaving the core region. Thus, the regular envelope does actually not exist as such, but is observed in long-exposure photographs or by the naked eye as a superposition of images of the excited gas tongues. The electron concentration in the core of the microwave torch plasma jet was estimated based on the core radius. The value obtained, 4×10 21 m −3 , has the correct order of magnitude for the discharge under consideration.

This work concerns the modeling of the plasma of a pulsed microwave discharge in nitrogen. The goal is to determine the temporal variations of the densities of the respective plasma species. The ultimate goal is to adopt the methods developed for these calculations for discharges in polluted air or flue gas. A global numerical model is formulated. The object modeled is the plasma of a microwave torch operating at atmospheric pressure. The set of equations for the model consists of particle balance and power balance equations. The most important reactions between the plasma species are only taken into account. The rate coefficients for processes involving electrons are calculated based on a solution of the Boltzmann equation for the electrons. Diffusion and heat transfer are modeled with the aid of terms reflecting also the discharge geometry.

The effect of a weak rotating magnetic field on an ECR discharge plasma in argon has been investigated. The field was produced by an arrangement of bars surrounding the discharge vessel and carrying harmonic currents appropriately shifted in phase. Optical probes, Thomson scattering, Li beam diagnostics and a fast CCD camera were used for the investigations. A significant modulation of the electron density and temperature at the plasma edge was observed. This can be assigned to an increase of the transport of electrons to the discharge vessel walls where the lines of the total B field cross them. Alteration of the transport conditions results in perturbations, which diffuse radially inwards at a velocity corresponding to that of the Bohm diffusion. A pattern of light emission develops, which rotates synchronously with the rotating field. Correlation between the total light emission from different plasma volumes was determined based on measurements made with optical probes. This shows fluctuations propagating azimuthally, anticlockwise with respect to the direction of the main B field, with a wavelength of about (7-10) × 10-2 m. These fluctuations exist independently of the presence of the rotating magnetic field. Further investigations are still needed to answer two key questions: does the plasma rotate as a whole or is the observed effect a rotation of the plasma state only, and how does the rotating field affect fluctuations at the plasma edge?

The behaviour of , the saturation intensity averaged over the optical cavity width w of a transverse-flow laser, is discussed based on a numerical analysis of a laser of that type. It is shown that for any w the mean saturation intensity is almost independent of the cavity position x along the flow and remains nearly unaffected by the optical power extraction upstream of the cavity. Thus, similarly as for non-flow lasers, it can be used, in conjunction with the small-signal gain coefficient averaged over the cavity width, for prediction of the optical power available from a transverse-flow laser and optimization of the optical cavity position and width. For axial-flow lasers the available optical power can be predicted in the same way if the beam path along the discharge is taken for w.

A narrow-band BOXCARS system has been used for spatially resolved diagnostics of a microwave discharge in an axial-flow CO2 laser module. Axial and transverse distributions of the vibrational and rotational temperatures of N2 and CO as well as those of the CO2 dissociation degree have been determined. They have been used as a reference for a vibrational kinetic model for the laser medium. This allowed determination of an approximate electron density distribution along the discharge tube. A CCD camera has been used for visualizing the intensity distribution in the beam overlap region and checking its effect on the CARS saturation behaviour.

Concerns experimental investigations of the glow to arc transition in a transverse-flow CO2 laser discharge in a pressure range of 30-100 hPa. Constriction development was recorded with a high-speed film camera and storage oscilloscope, and the maximum density of the electric power deposited in the glow discharge below the instability threshold was determined. Spatially resolved diagnostics of the discharge prior to the instability onset were carried out with an electric probe and a boxCARS set-up. Constrictions start to develop in discharge regions characterized by high plasma density or density gradient, large pressure pulsations or high density of negative ions. They propagate towards the electrodes at velocities in the range of 100-800 m s-1. The electric field inside a constriction is of the order of 104 V m-1. The probable mechanism of their development involves avalanche ionization at the constriction head without thermalization in the filament. The influence of discharge and flow conditions on the instability threshold was investigated. The favourable effect of preionization and flow turbulence on the discharge stability was confirmed. Also the possibility of raising the instability threshold by using an uncooled cathode was ascertained. An adjustment of the cathode profile is proposed as a simple and effective means of ensuring a uniform current distribution over the discharge channel width, which is a prerequisite of a stable discharge operation and laser action at increased electric power densities.

CARS (Coherent Anti-Stokes Raman Scattering) has developed into a powerful tool for studying molecular systems. One of its possibilities is to derive vibrational and rotational temperatures as well as concentrations of molecules from measurements of the energy level population differences. A very good spatial resolution of CARS technique is one of its important advantages. This feature has been utilized for making spatially resolved measurements of the vibrational and rotational temperatures of N2 in a d.c.-excited transverse-flow CO2 laser discharge. Apart from that also spectra of CO2, CO and O2 in the discharge have been taken, which allowed us to evaluate the spatial distributions of those components in the discharge. Additionally first investigations of a microwave-excited CO2 laser module have been performed for comparison.

An electric probe was used for measurements of the normal cathode fall in a glow discharge in flowing CO2 laser mixtures (CO2:N2:He = 4.5:13.5:82 and 5:20:75) for uncooled copper, stainless-steel and tungsten cathodes. The results imply that the cathode fall value in the mixture is governed mainly by the presence of carbon dioxide.

Investigations of the laboratory set-up of a high-power CO2 laser have been carried out to determine the best conditions of performance. The measurement results and sample characteristics are presented and discussed in this paper. Also a brief description of the facility is given. The laser, developed for investigation of beam generation processes and of laser beam interaction with matter is characterized by relative good efficiency of generation of cw radiation power in the range up to 8 kW as well as by its experimental potential that has been confirmed by a number of tests.

The paper contains the review of experiments with a high-power CO2 faow laser. The experiments were carried out for selecting the most effective electric discharge and optical resonator geometries.

Investigations of a high-power transverse-flow transverse-discharge closed-cycle cw CO2 laser, designed as a laboratory facility primarily for materials processing research, are reported. A self-sustained dc electric discharge between a multipin cathode (tungsten) and a planar anode (polished copper) has been used forexcitation of the working medium, a mixture of CO2, N2, and He. The discharge volume is 5 dm3. A controllable-speed centrifugal compressor circulated the gas at a pressure of ~ 80 kPa, with the mass flow rate reaching 0.5 kg s-1. The specific power per unit mass flow amounts to 200 kW/kg s-1. The discharge characteristics have been measured in various flow conditions to evaluate the effect of flow conditioning devices on the discharge stability. Various multipass unstable optical resonator configurations have been adopted based on previous numerical analyses. Measurements of small-signal gain distribution along the discharge channel (with a maximum of 1 m–1) allowed optimization of the resonator position relative to the discharge. The dependence of the output power on the electric power dissipated in the discharge was measured. For a two-pass resonator (M = 1.8 kanigen mirrors) with one amplifying pass, the maximum output beam power was 4.4 kW at an electro-optical efficiency of 10%. The beam divergence was ~2 times larger than the diffraction-limited value. This allowed satisfactory tests of laser welding and cutting. (Poster paper)

A high-power CO2 laser for investigations on materials processing is described. Measurements of the output characteristics of the laser are discussed on the basis of an adequate model.

Laser action at 18.4 μm has been obtained between the (0310) and (1000) levels in a shock-tube-driven CO2/Ar gas-dynamic laser. The pulse duration was equal to about 4 ms, giving the output power up to 2 W extracted from a 10-cm3 volume cavity mounted within a nozzle of 60° divergence angle and 0.5-mm throat height 67-mm downstream of the nozzle throat.

The small-signal gain of cw CO2 and/or N2O mixing gasdynamic lasers has been measured by varying the cavity losses with the aid of two rotating coupled NaCl plates placed within the cavity and looking for the generation cutoff point. The saturation parameter has been determined on the basis of measured values of respective small-signal gains by experimentally recovering the output power-cavity losses relation. The measurements yielded α0≃1.1 m−1 and Is≃3.2 kW/cm2 for CO2, and α0≃1.0 m−1 and Is≃3.0 kW/cm2 for a cw N2O mixing gasdynamic laser.