Design and operation characteristics of a high power transverse flow pulser sustained cw CO2 laser

A transverse flow, transverse discharge cw CO₂ laser in which de discharge is sustained by employing high repetition rate high voltage pulses has been developed. Pulser sustained discharge through electrodes of innovative design provided uniform excitation at electrical input power densities more than 10 W/cc. Laser output power more than 2.5 kW was obtained in a laser gas mixture consisting of 0.5 mbar of CO₂, 16 mbar of N₂ and 38.5 mbar of He. Design details and operational characteristics of this laser are presented.     

Dissociation of molecular iodine in RF discharge for oxygen-iodine lasers

The dissociation of molecular iodine in 40 MHz-RF discharge was studied experimentally. This generation of atomic iodine is aimed at use in oxygen-iodine lasers. The discharge was ignited in a mixture of I₂ + buffer gas fast-flowing through the cylindrical chamber and the discharge products were injected into a supersonic flow of nitrogen. The atomic iodine number density was measured in a low-pressure cavity after mixing with nitrogen and the dissociation fraction was calculated related to the input I₂ flow rate. The dissociation fraction of 46.2% was achieved at 0.22 mmol/s of I₂ and 7 mmol/s of Ar and RF power of 500 W. Argon and helium were used as a buffer gas; discharge stability and dissociation efficiency were better with argon. At the I₂ flow rate corresponding to the operation of a 1 kW chemical oxygen-iodine laser, the dissociation fraction was about 20%. The dissociation efficiency (the fraction of absorbed energy used for the dissociation) significantly decreased with increasing in the specific energy. At a reasonable I₂ flow rate (0.32 mmol/s), the maximum achieved efficiency was 8.5% and the corresponding energy cost was 8.9 eV per dissociating of one I₂ molecule. The input energy of more than 3 kJ per 1 mmol of I₂ is needed for dissociating at least 50% of I₂. The obtained dependencies on the gas flow rates infer a good chance for scaling-up of the tested RF discharge generator for the intended application.     

Design considerations and scaling laws for high power convective cooled CW CO2 lasers

Various criteria for designing high power convective cooled CO₂ lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO₂ lasers have been established. In transverse flow CO₂ lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 10⁴ p ^1/2 V ^-1 cms andW = 0.04P ^1/2 cms. The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO₂ lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.     

Operational characteristics and power scaling of a transverse flow transversely excited CW CO<Subscript>2</Subscript> laser

Transverse flow transversely excited (TFTE) CO₂ lasers are easily scalable to multikilowatt level. The laser power can be scaled up by increasing the volumetric gas flow and discharge volume. It was observed in a TFTE CW CO₂ laser having single row of pins as an anode and tubular cathode that the laser power was not increasing when the discharge volume and the gas volumetric flow were increased by increasing the electrode separation keeping the gas flow velocity constant. The discharge voltage too remained almost constant with the change of electrode separation at the same gas flow velocity. This necessitated revision of the scaling laws for designing this type of high power CO₂ laser. Experimental results of laser performance for different electrode separations are discussed and the modifications in the scaling laws are presented.     

Pulsed inductive discharge CO2 laser

A pulsed inductive discharge CO₂ laser with a wavelength of 10.6 μm has been created for the first time. The excitation system of a cylindrical pulsed inductive discharge (pulsed inductively coupled plasma) in the gas mixture of CO₂:N₂:He was developed. The temporal and energy parameters of the laser radiation were investigated. The maximum inductive discharge CO₂ laser radiation energy of 104 mJ was achieved. An average power of 3.2 W was obtained at laser generation energy of 65 mJ and pulse repetition rate of 50 Hz. In the cross-section, the laser radiation had the ring shape with an external diameter of 34 mm and thickness of 4-5 mm. The measured divergence of laser radiation was 12 mrad.     

Nonequilibrium Ionization by Vibrationally Excited Molecules in Supersonic Flows

The results of experimental investigations on thermal nonequilibrium ionization in CO₂: N₂: He mixtures are presented. Measurements of electron density, n_e, in vibrationally excited nitrogen were made in a supersonic flow with different CO₂ contents as well as in a CO₂: N₂: He = 1 : 5 : 4 mixture laser gas. The mixtures were heated in a shock tube and expanded through a supersonic nozzle. Furthermore, supersonic mixing of N₂ and CO₂ + He was used in some experiments. The measured values of n_e in the plenum chamber and in the supersonic nozzle are reported, and the processes responsible for nonequilibrium ionization in a laser-active medium are discussed.     

Deep penetration welding using a CO laser with an unstable resonator

An unstable resonator (M = 2) has been applied to reduce the beam divergence of a gas-dynamically cooled supersonic CO laser operating at 105 K in a semiclosed gas cycle. A 4.7 kW laser beam with a total divergence of 2.5 mrad is obtained with an efficiency of 9.4%. The results of welding experiments are compared with those using a CO₂ laser. The weld depths obtained with the CO₂ laser are drastically reduced using argon as the assist gas, whereas the results with the CO laser are independent of the assist gas because of the lower plasma absorption coefficient for the shorter CO laser wavelength.     

Numerical investigation of a fast-axial-flow CW CO2 laser

This paper presents the results of the calculation of the parameters of the active medium of a fast-axial-flow CO₂ laser using numerical methods in the framework of a one-dimensional approximation of the set of continuity equations, Bernoulli equation, equation of gas state, energy equation and multi-temperature rate equations with regard to diffusion for the gas flow in the cylindrical discharge tube. The spatial distribution of the small-signal gain and gas temperature along the gas flow direction have been calculated for a given set of initial conditions, namely, gas flow velocity, gas pressure and the tube diameter. In addition, the dependence of small-signal gain, the asymmetric stretch vibrational temperature of CO₂ (T₃) and the gas temperature on the discharge current were studied.     

A computer model of a transverse discharge cw CO laser

A kinetic model has been developed for the investigation of the novel performance of a CO laser, on which efficient extraction of laser power was obtained by exciting a subsonic gas mixture of CO/N₂/He/O₂ through transverse dc discharge. Kinetic equations for direct excitation by electron impact, V-V and V-R/T energy transfer, and stimulated emission are coupled with a semi-one-dimensional flow model. Careful consideration is devoted especially to the V-V transfer process of CO–N₂ and N₂–N₂. The laser power was calculated by a constant gain method. The laser output performance, examined as a function of gas mixture ratio, temperature, flow velocity, and discharge current, was in good agreement with the experiment.     

Laser plasmatron for diamond coating deposition

An experimental installation with a laser plasmatron based on a continuous wave CO₂ laser with a radiation power of up to 3.5 kW has been created. The plasmatron design makes it possible to bring out the plasma jet into atmospheric air both along and across the laser beam direction. The spatial temperature distributions on the metal substrate surface heated by the plasma jet are measured. The threshold power for optical discharge maintenance as a function of the gas flow rate and the focal length of the focusing lens are obtained for an Ar and Ar/CH₄/H₂ gas mixture under atmospheric pressure; the radiation spectrum of the discharge plasma is measured. A one-dimensional model of the discharge for estimation of its geometrical parameters in a convergent laser beam with consideration of radiation refraction on the discharge is given.     

Numerical modeling of a fast-axial-flow CW–CO2 laser

A four-temperature model has been applied on a fast axial flow, longitudinal discharge CO₂ laser. Using Runge–Kutta method, a set of differential equations of the model is numericaly solved. These equations describe the operation of the laser with certain ratio 1:3:6 of the mixture CO₂:N₂: He and average output power of 550 W.The temporal behaviour of the output power and photon density was obtained. The effects of kinetic temperature, coupled mirror reflectivity, gas flow speed, and cavity loss on the output power were studied.Calculated output power was compared with its measured value taken from experiment and a good agreement was observed.     

Thermal instability of glow discharge in the gas loop of CO2 laser with extended disk fans-heat exchangers

CO₂ lasers with transverse discharge and convective gas cooling find ever-increasing application. On strategy in making such lasers more efficient radiators is increasing the rate of the gas flow through the discharge zone with the help of diametral disk fans-heat exchangers. The application of such fans-heat exchangers, however, entails serious difficulties related to the glow discharge-gas flow interaction. In the present study, we investigate the stability problem for volume discharge in the gas loop of a CO₂ laser with diametral dis fans-heat exchangers.     

Determination of the fractal dimension of activated carbons: Two alternative methods

The fractal dimension of three samples of activated carbon has been determined according to two different experimental methods. The first method is based on the vaporization of gas species from the sample into a CO₂ flow up to a constant temperature of 573 K, and the adsorption of CO₂ from the CO₂ flow onto the surface of the sample under cooling. The second method is based on the application of the equation proposed by Frankel, Halsey and Hill. The degree of concordance between both methods is excellent.     

Gasdynamic CO laser with closed-cycle gas flow

The operation of an electrically excited gasdynamic CO laser with closed-cycle gas flow is described. Details of the gasdynamics, the recompression, and the cooling of the flowing gas are given. The electrical discharge produces contaminating gas compounds—in particular CO₂—which accumulate in the recirculating gas. The resulting transient behavior of the laser gas composition, the discharge characteristics, and the laser performance are studied and compared with the results obtained with the same CO laser in an open-flow system. It is found that the closed-cycle system may be operated for considerable time if the CO₂ contamination is removed from the laser gas.     

High-power transverse flow CW CO2 laser for material processing applications

A transverse flow transversely excited (TFTE) CW CO₂ with a maximum output power about 15 kW has been developed. This is excited by pulser sustained DC discharge applied between a pair of multi-pins anodes and a common tubular cathode. Though the laser power in convective cooled CO₂ laser scales proportionally with the volumetric gas flow, it did not increase in this laser when the volumetric gas flow was increased by increasing the electrode separation keeping the flow velocity constant. The discharge voltage too remained almost unchanged with increase of the electrode separation. These observations are explained considering the electrical discharge being controlled by ionization instability. Laser materials processing applications often demand programming facilities for laser power modulation. A four-stage cascaded multilevel DC–DC converter-based high-frequency switch mode power supply has been developed to modulate the output power of the laser. Laser was operated up to 15 kW output power in four different modes viz. continuous wave mode, pulse periodic mode, single shot mode and processing velocity-dependent power mode with 1.2 kHz modulation bandwidth. We describe briefly the laser system, the SMPS, and the temporal behavior of laser beam.     

Influence of the gas mixture temperature on the efficiency of synthesis gas production from ethanol in a nonequilibrium plasma

The mechanism behind the plasma conversion of a mixture of ethanol vapor, water vapor, air, and carbon dioxide CO₂ in the nonequilibrium plasma of a tornado discharge is studied. The influence of the CO₂ flow rate, the current through the discharge, and the gas temperature in the discharge on the concentrations of molecular hydrogen and carbon monoxide CO is studied. Comparison between the concentrations of the gaseous mixture’s main components at the output from the reactor obtained experimentally and by numerical simulation shows that the adopted kinetic mechanism adequately describes the plasma kinetics in the mixture.     

Quasi-simultaneous ultraviolet and infrared emission from a plasma cathode TEA laser

Quasi-simultaneous laser action in the UV (0.337 μm) and the IR (10.6 μm) was observed from a pulsed laser with a sliding discharge plasma cathode. The laser operates at atmospheric pressure, with a gas mixture of CO₂/N₂/He, at a 0.26/0.50/4.0 lmin⁻¹ flow rate. Output energies of 30 mJ in the IR and 0.35 mJ in the UV were obtained, from a laser discharge volume of 38.0×1.0×2.8 cm³. The optimum gas mixtures have been determined and the temporal behavior of the discharge parameters, the performance characteristics of the laser and the beam spatial distributions were investigated.     

Novel considerations of functions of helium and nitrogen in a fast flow CO2 laser

In a research of fast axial flow CO₂ laser sustained by 150 kHz silent discharge, we found the optimized gas mixing ratio was CO₂:N₂:He=1:22:5 or the content of helium was only about 18%. This result upset the situation of common CO₂ lasers in which the most important laser gas is helium. An explanation of our particular results and supporting experimental evidence are given.     

CO2 laser-induced plasma CVD synthesis of diamond

2 laser maintenance of a stationary optical discharge in a gas stream, exhausting over a substrate into the air (laser plasmatron). Nano- and polycrystalline-diamond films were deposited on tungsten substrates from atmospheric-pressure Xe(Ar):H₂:CH₄ gas mixtures at flow rates of 2 ?/min. A 2.5-kW CO₂ laser focused beam produced plasma. The deposition area was about 1 cm² and growth rates were up to 30–50 μm/h. Peculiarities and advantages of laser plasmatrons are discussed. Received: 15 January 1998/Accepted: 16 January 1998     

An efficient,RF excited,waveguide CO2 laser

A CW waveguide CO₂ laser excited by a transverse radiofrequency discharge is described. An efficiency of 8.5% (laser power/RF power from supply) has been achieved and an output power of up to 4.6 W.