Discharge CO Laser Research Articles

Transitions between Glow and Arc Modes and Its Influences to the Performance of a Hollow-Cathode Discharge CO2 Laser

Hollow cathode discharge is used for excitation of a CO2 laser. With the aluminum alloy cathode a maximum laser output power of 4.1 W is obtained. This laser output power is related to the formation of aluminum oxide at the cathode surface. These oxide layers induce the transitions between the glow and arc modes of the discharge and formation of microarcs at the cathode surface. This was found beneficial for the laser operation. The results of the time resolved study of the laser parameters are also reported.

Parameter limits of thermal instabilities in high-frequency CO2 laser discharges

A simple formula giving parameter limits for stable operation with respect to thermal instabilities of high-frequency excitation of CO2 laser discharges is derived from a discharge model including the continuity equation for the electron density and the neutral gas density, the energy equation for the neutral gas and Ampere's law for the electric field. For stable discharge operation, a limiting value of the product of the neutral gas density and the discharge width must not be exceeded. This limit is also determined by the excitation frequency. The discharge is stabilized mainly by the frequency-dependent reduction in the electric field accompanying a corresponding increase in the electrical conductivity and by those processes that lead to a smoothing of particle densities and the temperature. These processes are diffusion and heat conduction and especially turbulence-induced transport processes. A quantitative description of the turbulent transport processes is beyond the scope of the present paper. The effect of the turbulent gas flow is roughly approximated by an effective heat conductivity.

CO2 Laser Discharge Influenced by a Rotating Magnetic Field

This paper deals with experimental investigations of the effect of a rotating magnetic field on a diffusion cooled CO2 laser discharge. Some properties relevant to laser activity are discussed, e.g. neutral gas temperature, magnetically driven gas rotation, pumping rate, and distribution of small signal gain within the discharge tube. By means of an intracavity method the radial distribution and the time behavior of the laser output power were measured.

Influence of gas dissociation and xenon addition on steady-state microwave-excited CO2 laser discharges

The electron energy distribution in a microwave-excited mixture of He, N2, CO2, Xe, CO and O2 was calculated by numerically solving the Boltzmann equation, using collision cross section data for 69 elastic and inelastic collision processes. At an excitation frequency of 2.45 GHz the energy distribution is quasi-stationary for gas pressures up to about 100 mbar. By balancing the most important processes of electron generation and loss, the reduced field E/N and the electron density ne in a self-sustained steady-state discharge were calculated for various gas mixtures and excitation parameters. Curves are presented showing that with increasing CO2 dissociation, the stationary value of E/N within the plasma rises, combined with drastically reduced rates for vibrational excitation of CO2 (00 degrees 1) and N2. In contrast the admixture of xenon reduces the steady-state E/N value and thus the mean electron energy, leading to an increased electron density and excitation efficiency. These effects are in good agreement with experimental observations.

Diagnostics of a microwave CO2 laser discharge by means of narrow-band BOXCARS

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.

Gain and saturation parameters of a multichannel large-area discharge CO2 laser

Details of an experimental investigation of the gain and saturation intensity of a multichannel extended-area discharge CO2 laser are presented. Measurements were made as a function of several discharge parameters. The gain coefficient was found to be similar in each of the individual discharge regions, having a maximum value of 0.70%/cm. Effects of gas pressure, gas mixture, and input power on gain and saturation parameters were studied. The saturation intensity varied as the square of the total gas pressure. The central interelectrode region of the device, which contained no visible discharge, exhibited a significant level of gain. The investigation reported herein provided important data for the design and optimization of optical extraction configurations.

Investigations of the transition from glow to arc in a transverse-flow CO2 laser discharge

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.

A narrow line high average power CO2 laser for pumping far-infrared laser media

A 10 MW peak power, 1000 pps transverse discharge CO2 laser is under construction. This device will be employed to optically pump a far-infrared medium, NH3 for example, and comprise the submillimeter master oscillator of an alpha Thomson scattering diagnostic for fusion plasmas. Transverse laser discharge stability will be maintained by a compact flow recirculation flow loop which is contained within a compact (1 m diam ×1.5 m long) cylindrical pressure vessel. Magnetic pulse compression techniques will also be employed to efficiently generate the laser discharge. System details and current fabrication status will be presented.

Narrow-band BoxCARS applied to CO2 laser discharges

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.

Mass Spectrometric Measurements of CO2 Laser Discharges with Time Resolution

In mass spectrometric measurements, knowledge of the gas dwell time in the glow discharge region has been utilized to time-resolve the variations of CO2 partial pressure in CO2 laser discharges. The measured data are interpreted by a dissociation-reformation scheme of CO2 molecules in the discharge. Rate coefficients that characterize the CO2 equilibrium process are evaluated, and their technical implications are discussed.

Theoretical and experimental investigations of the filamentation of high-frequency excited CO2 laser discharges

The filamentation of high-frequency excited CO2 laser discharges with dielectric electrodes is investigated theoretically and experimentally. A self-consistent two-dimensional discharge model including equations for the electron density, the electric field and the neutral gas density is used to calculate the spatio-temporal development of filaments in a high-frequency discharge. The space charge field is described implicitly by the complex dielectric constant of the discharge plasma. The side-on light emission of high-frequency excited discharges is recorded, time-integrated by a camera and time-resolved by using a framing camera. Filaments develop above a critical value of the power density which depends on the pressure. The development of the filaments is partly suppressed by the dielectric electrodes.

Generation of, and tolerance to, dissociation products in an electron-beam sustained carbon dioxide laser

The extent of carbon dioxide dissociation in an electron-beam sustained CO2 laser discharge was found to be proportional only to the total charge passed and the CO2 partial pressure; the rate of oxygen production was found to be 150 mu mol C-1 referred to pure carbon dioxide. The dissociation occurs in the cathode sheath. The effects of dissociation products on small-signal gain, discharge impedance, and on the glow-arc transition were studied. It was found that only oxygen had any influence and that this was confined mostly to the increase of impedance caused by attachment. An analysis of the implications of impedance matching to discharge stability is presented.

Numerical modelling of RF excited CO2 laser discharges

Current density-voltage (j-U) characteristics and vibrational excitation efficiencies of RF excited CO2 laser discharges have been calculated in the frame of a self-consistent numerical discharge model. The model includes the continuity equations for electrons and positive ions Poisson's equation and the energy balance equation for the neutral gas. The results are compared with experiments. The slope of the j-U characteristic decreases with increasing frequency and with increasing pressure. The vibrational excitation efficiency increases with increasing frequency and decreases with increasing RF power and decreasing pressure.

Investigation of microwave excited CO2 laser discharges

Microwave discharges for the excitation of CO2 lasers have been investigated. A waveguide structure with closed front and end, and with the discharge tube located parallel to the waveguide axis is used. The field distribution within the waveguide but outside the discharge tube is measured before and after ignition of the discharge. The wavelength of the microwave is reduced by the discharge plasma. The side-on light emission is detected. Light emission is detected over a distance within which the measured field strength in the waveguide varies fivefold. The experimental results are compared to the results of a self-consistent one-dimensional microwave discharge model. The model includes an equation for the electron density, the neutral gas density and a simplified wave equation for the electric field. The results show, in accordance with experimental results, a reduction of the wavelength due to the discharge plasma and the discharge excited over a range within which the electric field outside the plasma varies appreciably. In contrast to the electric field outside the plasma, the electric field inside is almost constant within the excited region. This is due to the high plasma conductivity which is a result of the high electron density which reaches values of up to 1012 cm-3. This leads to a power density up to several tens of W cm-3. In order not to overheat the gas the discharge has to be operated in a pulsed mode. Another method of limiting the gas temperature is to use a waveguide structure with a small electrode spacing or with a fast gas flow.

Electron Energy Distribution Functions in CO2 Laser Mixture: The Effects of Second Kind Collisions from Metastable Electronic States

AbstractElectron energy distribution functions (eedf) in CO2 laser discharge (He—CO2—N2—CO mixture) have been calculated by solving the Boltzmann equation in the presence of given concentrations of excited (vibrational and electronic) states. The results show a well structured eedf as a result of second kind collisions coming from metastable electronic states of N2 and He as well as a strong dependence of rate coefficients for CO2 dissociation and for the ionization of the different species.

Electron dynamics in magnetized CO2 laser and He discharges

The energy density of the electron flux in magnetized 30-Torr He and He:N2:CO2 dc glow discharges was obtained by Monte Carlo simulation of the cathode region. The application of transverse magnetic fields to the cathode region caused an increase in collisions. This effect became increasingly important in the second half of the cathode region, where magnetic trapping of high-energy electrons occurred. Ionization was enhanced in the cathode region as well as the negative glow, with maximal electron multiplication observed for magnetic field strengths of approximately 0.1 T. This suggests both that a lower cathode fall may be possible in a magnetized discharge and that the electric field reversal in the negative glow will be accentuated. Thus, magnetization of the cathode region may improve the electrical efficiency of CO2 lasers.

Reduction of the CO2 decomposition in the transversely excited atmospheric CO2 laser discharge plasma by a very small amount of the water vapor

Using a comprehensive theoretical model, we have theoretically clarified for the first time that the homogeneous catalytic reaction caused by a very small amount of water vapor (<100 ppm) in the closed-cycle, transversely excited atmospheric (TEA) CO2 laser discharge plasma could considerably reduce the saturation value of the CO2 molecules decomposition. Effects of the water vapor at this concentration level have been overlooked in the previous investigations. A small concentration variation of the residual water vapor in the experimental laser chambers is thought to be one of the reasons for the considerable disagreements among the published data about the CO2 equilibrium decomposition level.

Effects of H2 and CO Additives on the O2 Accumulation in the TEA CO2 Laser

A change in gas mixture in a sealed-off TEA CO2 laser discharge was analyzed gas-chromatographically. It has been found that the addition of a small amount of H2 and CO will reduce the O2 formation in the mixture. In particular, when a mixture of N2:CO2:CO:H2:He=12:12:6:1.6:68.4% was used, a sealed-operation lifetime exceeding 5 × 105 shots without any oxygen formation was obtained. However, it has also been found that CO2 was decomposed by discharge and formed O2 was missing. No nitrogen oxide formation was recognized, implying that the missing oxygen was dissipated to some other reactions. Furthermore, notable differences in the change in the electrode surface, with addition of CO or H2 to the mixture, were recognized by visual observation. Hence, we proposed electrode surface reactions for the reduction in the O2 formation in the mixture.

Laser diagnostics of the working medium in an electrical discharge CO2-laser with a closed pumping loop

Results are presented of an experimental investigation of the amplifier characteristics and temperature measurements of the upper laser level and the progressive tempeature of the working medium of a pumping electrical discharge CO2 laser in the range of pumping power variation 18–65 kW. It is shown that growth of the progressive temperature of the active medium is the main reason for degradiation of laser operation efficiency at large (above 50 kW) values of the pumping power.

Volumetric stability of two-step ionization dominated discharges

The stability of self-sustained discharges is investigated both analytically and numerically. The dominant ionization mechanism in the discharge is assumed to be two-step ionization while both electron loss mechanisms attachment and recombination are considered. It is shown that the stiff voltage source is marginally stable when the electron loss rate is due solely to electron-molecular ion recombination. In the presence of attachment the stiff voltage source is unstable and the growth rate of the instability is approximately the attachment rate. A stiff current is shown to be more stable and the discharge is stable irrespective of the loss rate. The theory is verified numerically for at atmospheric Co2 laser discharge.