Discharge CO Laser Research Articles

Dynamics of magnetically stabilized laser discharge

The dynamics of a rotating plasma in a transverse electrode geometry appropriate to high power CO2 laser discharge pumping is examined. The magneto-gas-dynamic fluid model used reveals that the spatially nonuniform electric and magnetic fields within the discharge cause the plasma and neutral gas rotational velocities to be strongly sheared within the cathode fall and the positive column regions. This sheared rotation serves to both stabilize the discharge against glow to arc transitions and to drive a centrifugal neutral gas flow within the volume. The concomitant bulk gas cooling effect so produced is particularly beneficial in a high powered CO2 electrical discharge laser as it inhibits thermal bottle necking. An order of magnitude increase in discharge power loading has been achieved utilizing a simple ’’magnetic electrode’’ discharge configuration.

Crossed-tube fluid-ballasted electrode pair for EDL applications

The design and operational features of a fluid-ballasted electrode structure for a high powered EDL CO(2) laser are described. This new device incorporates a crossed-tube geometry and as such is much simpler and inexpensive to fabricate than previous multielement or pin electrodes. Hot boundary layer formation at the electrode surfaces is minimized by a ducted flow-through feature also incorporated into the structure. This further assists in suppression of glow-to-arc transitions. The modular design used allows easy scaling to very large discharge volumes. A 40-liter cw CO(2) laser discharge system has recently been developed using this simplified crossed-tube fluid-ballasted electrode pair concept.

Numerical predictions of phase distortions due to a heating differential in a CO2 laser discharge tube

The quality of the output beam from a CO2 electron beam laser may be seriously affected by density perturbations within the laser cavity. Results are given from numerical modelling which indicate that the heating differential between lasing and non-lasing regimes is sufficient to account for these density perturbations. The authors' numerical results are in good agreement with results from experiment for the case of the stable resonator. They have extended their model to predict the degradation of the output beam from an unstable resonator.

Stability of Pin electrode CO2-lasers

The stability of parallel pin electrode CO2-laser discharges, coupled by carrier diffusion only, against concentration of current in one column and subsequent transition into an arc, is investigated theoretically. A small signal description of the time dependent behaviour of a column with falling characteristic caused by a wall sheath of considerable width is used. It is shown that an inductance in series to each discharge is not able to avoid glow to arc transition, though enlarges the duration of the glow.

Optical inhomogeneities in a Lamberton-and-Pearson-type CO2laser

A simple optical technique for the detection of shock waves in a pulsed discharge is described. The technique uses an apertured photomultiplier and He-Ne probe laser to obtain data on the passage of a shock front across the pulsed discharge. This method has been applied to a study of shock wave propagation in a TEA CO2 laser discharge.

Spectrophonic humidity-trace detection in hydrogen and air

A spectrophone with low resonant frequency compatible with the repetition frequency of a pulsed discharge CO laser is described. The spectrophone is used for humidity detection in hydrogen and air. The simple and inexpensive system provides a detection capacity of 10–100ppb/\(\sqrt {\operatorname{Hz} } \). The responsivity of the resonant spectrophone is interpreted taking the dependence of the acoustical quality factor on the geometry into account.

Determination of the recombination coefficient in CO2 laser discharges

The effective recombination coefficient γ* in a self-sustained CO2 laser discharge at a pressure of 1.3 bar is determined from a rate equation for the electron density using experimental values of the discharge current and voltage. It is found that γ* depends strongly on the electric field strength as well as on the electron density.

The influence of additives and contaminants in TEA CO2 laser discharges evaluated by electrical measurements

Observation of the electrical characteristic of the glow discharge allows easy study of the influence of additives and contaminants. Moreover, when arcing occurs, interesting information is obtained by observing the damping of the arc current. The efficacy of the low-ionization organic additive to prevent arcing is confirmed, and the deleterious influence of oxygen contamination is demonstrated. This result accounts for the technical rule of preventing air from entering the discharge vessel. Increased output peak power is observed, resulting from the use of a suitable additive.

Electrode surface field and preionization effects on the spatial distribution of arcs in CO2 laser discharges

We have studied the spatial distribution of arcing as a function of electrode spacing and preionization pattern in CO2 laser discharge mixtures, using circular electrodes which allowed a gap of up to 10 cm and a discharge volume of ∼3 liters. We utilized both cathode-field enhancement with uniform preionization and apertured preionization with uniform field to define the discharge boundaries. In both cases, we found a threshold value of cathode-field enhancement below which arcing was uniform and confined to the discharge region and above which arcing became highly nonuniform, with arcs localized to the high-field region.

Pulsed Cold Cathode Electron Beam Gun for Laser Application

A cold cathode gun of high current pulsed electron beam was investigated for an eventualuse as a CO2 laser discharge sustainer.Measured performance characteristics, suchas gun impedance, gun current behaviors as a function of applied voltage and cathodematerlal Ta and Ti were consistent with the pnasma cathode model using the evapoiationof cathode metal. The electron beam current was determined by the space chargelimited flow from the expanding plasma cathode surface.The maximum current densitywas determined by the electron satumtion current on the cathode plasma surface. By the fandamental investigations, optimum design for a large apature and uniform electronbeam source are presented.

Characteristics of a 600-W convective flow CO laser

Operating characteristics of a moderate power, convective flow, longitudinal discharge CO laser are presented. Incorporation of a novel electrode geometry has resulted in a highly stable and reliable device, useful as a laboratory tool.

Gas temperature and cooling in waveguide CO2 lasers

Gas temperatures in waveguide laser discharges have been measured. The temperature obeys the scaling laws for similarity with conventional positive column CO2 laser discharges. Beryllia and alumina are equally efficient in cooling the CO2-N2-He gas, but Pyrex is less effective.

High-pressure pulsed electrical CO laser

The design and performance of a 16-l high-pressure pulsed electric discharge CO laser is reported. The gas was excited using the electron-beam sustained discharge technique. Cryogenically cooled gas flowed subsonically through a room-temperature cavity. Thermal isolation was provided by the formation of free convection boundary layers on the warm flow walls which preserved the cryogenic gas temperature and maintained good medium uniformity. Beam-quality measurements indicated better than 1.5 times diffraction-limited peformance with single pulse energies of 1600 J and an electrical efficiency of approximately 40%. Comparisons are made between the actual laser performance and predictions based on the CO laser kinetics code.

Acoustic waves and heating due to molecular energy transfer in an electric discharge CO laser

This paper summarizes analytical studies and the interpretation of experimental results for the compression and rarefaction waves generated in the cavity of a pulsed CO electric discharge laser. A one-dimensional analysis of acoustic waves is applied to a transversely excited laser. The influences of heating in the cathode fall, heat transfer to the cathode, flow through both the anode and cathode, and bulk heating of the plasma are included. The analysis is used to relate the bulk heating rate to observable features of the pressure and density waves. Data obtained from interferograms and reported elsewhere are used to infer the bulk heating rates in a pulsed CO laser. Results are presented for CO/Ar, CO/N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , and N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> plasmas. Comparison of the data with recent theoretical results for the heating due to electron/ neutral collisions and the anharmonic defect associated with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-V</tex> energy transfer shows substantial differences at lower values of total energy deposition. The change of heating With <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E/N</tex> is in fairly good agreement with predicted values.

Picosecond resolution, real-time linear detection system for 10-μm radiation

A 10-μm detection system based on up-conversion in proustite has been developed for the measurement of subnanosecond pulses of CO2 laser radiation. This has involved the development of a ’’stretched-pulse’’ Q-switched Nd:YAG oscillator which can be synchronized with the output of a transverse discharge CO2 laser. The 1.06-μm radiation generated by this oscillator and the incoming 10-μm radiation pass collinearly through a 0.6-cm-long proustite crystal which is oriented for type II phase matching. An investigation of the bandwidth of the up-conversion process has shown that it should be possible to resolve 10-μm pulses with durations of ∼5 psec. Using an image converter streak camera to record the 0.96-μm sum frequency, the overall performance of the system has been investigated with the aid of a mode-locked CO2 laser operating at a pressure of 12 atm. The results have demonstrated that mode-locked CO2 pulses as short as ∼75 psec can be generated and that the temporal resolution of the complete detection system corresponds to that of the streak camera.

Mixing Effect of Organic Gas Additives on Double Discharge CO2 Laser

Mixing Effect of Organic Gas Additives on Double Discharge CO2 Laser

Electron velocity distributions, fractional energy transfer, and collision rates for potential Na, NaHe, and NaXe electrically excited laser discharges

The Boltzmann transport equation including elastic, inelastic, ionization, electron-electron, and electron-ion interactions has been numerically solved to determine the electron velocity distribution, the fractional electron energy transfer, and the collision rates for potential Na, NaHe, and NaXe electrically excited laser discharges. Previously reported theoretical and experimental cross-section data from the literature were used in the calculations. Na, like all of the alkali atoms, has a very large cross section (∼4×10−15 cm2) for electronic excitation of the first level (an allowed optical resonant ns→np transition). This value greatly exceeds all other low-energy inelastic cross sections for both the Na and the He and/or Xe buffer gases. Such a combination of cross sections enables the input electron energy to be very efficiently transferred to the potential upper laser level, Na(3 P). The computed distribution functions are shown to be strongly non-Maxwellian and exhibit an energy variation indicative of this large resonant cross section in Na. Using the electron energy conservation equation reveals that indeed the electronic excitation of the Na first level dominates the electron energy exchange processes for the Na and the Na He/Xe mixtures. For example, in pure Na 100% of the electron energy is transferred to the Na(3 P) state in the E/N range 5×10−16 to 5×10−15 V cm2. In both the NaHe (1:99) and the NaXe (1:7600) mixtures, over 90% of the electron energy is transferred to this same Na(3 P) level in the E/N range of 5×1017 to 4×10−16 and (1.2–7) ×10−17 V cm2, respectively. At the second Na electronic level, 4P, electron energy transfer is down from that of the Na(3 P) by nearly three orders of magnitude for all of the mixtures. For the optimum values of E/N as cited above the mean electron energy for the Na, NaHe, and NaXe mixture lies in the range 0.6–1.4, 0.92–2.4, and 1.2–2.7 eV, respectively. Also, the normalized collision rates, ν/N (cm3 sec−1), for the Na(3 P) level are shown to have a value approximately 10–100 times that calculated for the normalized vibrational excitation rates of CO and CO2 laser discharges. These high pumping rates are a direct result of the large resonant transition cross section and give these systems excellent potential for high-power high-efficiency cw laser operation. Other discharge transport properties, like drift velocities and characteristics energies, are presented. Electron-electron interactions have negligible effects on the ’’tail’’ of distribution for partial ionization ratios, ne/N, values up to 10−3 due to the large Na(3 P) inelastic cross. The results and their generalizations to all the alkali systems along with recommended cross-section data needed are discussed.

Power Calculations for High-Flow CO Electric Discharge Laser Systems

A self-consistent model for high-flow electric discharge CO laser systems with coincident optical and flow axes is presented. The model employed couples the kinetics of the electrons and heavy particles with the optical and fluid dynamic processes in the laser system. The resulting integrodifferential system of equations governing this multilevel system was solved by an exact numerical scheme. The effects of composition, total mass flow rate and inlet pressure and temperature on the intensities, and total power output were investigated. The results indicate that, for a given input power and N/sub 2/ fraction, the efficiency increases by reducing the inlet temperature and increasing the velocity and/or the fraction of CO. Moreover, transitions on the lower vibrational bands can be obtained at the expense of lower efficiencies by reducing the CO fraction.

Improved convective flow longitudinal discharge CO laser

Specific extraction efficiencies of 300 W g−1 sec−1 have been obtained with flow-stabilized electrodes in a longitudinal electric discharge CO laser, representing a 50% enhancement over previous results.

Electron-beam spreading and its effect on sustainer current and field distribution in CO2 lasers

An electron-beam-initiated high-pressure CO2 laser discharge is investigated. It is assumed that initially the discharge current flows along the vacuum electric field direction. The resulting distortion of the electric field is calculated by a linearization of the continuity equation. We have computed the solution for the special case of a finite electron beam and infinite planar electrodes. The predicted variation of the sustainer current and distortions of the applied voltage are compared with experimental results.