High-power Pulsed CO2 Laser Research Articles

TRANSPARENT MATERIALS FOR HIGH-POWER PULSED CO2-LASERS

In one experiment, having the aperture of the beam, comparable with dimensions of the sample, we compared the real optical damage of the main transparent materials for CO2-lasers: BaF2, NaCl, KCl, KBr, RbI, AgCl, CsI, KРС-5, KPC-6, ZnSe, ZnS, GaAs и Ge. Experimentally and theoretically we compared the nonlinear losses in 13 crystals, when the radiation in the range of 107—4 × 108 W/сm2 passed through them. The «tail» of the impulse was practically completely absorbed by plasma of optical breakdown of air in TEA CO2-laser, but the «peak» — only partially — by «hot» disbalanced charge carriers in the crystal. The losses of laser radiation exponentially depend on band gap of material. he peculiarities of damage Ge single crystals by high power pulsed CO2-laser were investigated. For the first time windows from Ge (diameter — 420 mm) were elaborated, produced and tested. We looked into, how parameters of crystal lattice, admixtures and structure of point defects, which can be changed by influence X -ray irradiation and annealing, can influence on the process of laser volume pores generation in alkali halides. We revealed dependence of pore dimensions from influence conditions, parameters of high-power laser beam, energy of crystal lattice. We studied kinetics of thermal annealing for to diminish dimensions of this pores.

Influence of molecular absorption of a ground atmospheric path on characteristics of filamentation region of high-power CO2 laser radiation

Theoretical aspects of self-focusing and filamentation in air of high-power pulsed CO2 laser radiation with a carrier wavelength of 10.6 μm are considered. The frequency-differential molecular absorption of atmospheric air is included in a theoretical model. The super-broad pulse spectrum is shown to be strongly affected by atmospheric absorption that destabilizes the filamentation process and considerably reduces the length of a plasma channel formed in a beam channel under the conditions of pulse self-phase modulation.

Initiation of ignition of a combustible gas mixture in a closed volume by the radiation of a high-power pulsed CO2 laser

The results of experiments on initiating the ignition of a CH4 — O2 — SF6 triple gas mixture in a closed volume by the radiation of a high-power CO2 laser are presented. It is shown that spatially nonuniform (in the direction of the laser beam) gas heating by the laser radiation leads to formation of a fast combustion wave, propagating along the chamber axis and giving rise to 'instantaneous' ignition. At the threshold value 16.5 J of the laser radiation energy the fast combustion wave is transformed into a detonation wave, which causes an explosion and destruction of the reaction chamber

Influence of the salinity on the characteristics of acoustic signal in salt water generated by pulsed CO2 laser

The experimental study on the laser acoustic signal in salt water with different salinities using high power pulsed CO2 laser has been carried out. By using a hydrophone to receive the sound signal and further analyzing in time domain, the effects of the salinity and pulse energy on the characteristics of the sound signals were disclosed. The results show that the salinity has significant effect on the characteristics of the laser acoustic signal. As the salinity increases, the amplitude of the sound signal decreases. The profile of the sound signal also changes with the salinity. As the salinity decreases, the changes in the signal waveform show the loss of its originally bimodal shape. This phenomenon is also found in the laser energy which decreases when the salinity is kept constant.

High-power, high-pressure pulsed CO2 lasers and their applications

The paper is devoted to problems associated with the construction of high-power pulsed CO2 lasers and high-pressure amplifiers and to an analysis of the possible ways of their solution. Prospects of the development of such lasers and their applications in technological processes are considered. Original designs of a laser complex for obtaining the carbon-13 isotope and a superatmospheric-pressure CO2 amplifier are presented.

Collective Thomson Scattering Diagnostic for Fast Ion and Deuterium to Tritium Ratio Measurement in Open Magnetic Systems

An advanced diagnostic technique to measure the ion temperature and fast ions in open magnetic field systems is described. The method for the determination of deuterium to tritium ratio and the helium ash density in DT burning plasmas is also proposed. The measurement is made by small angle collective Thomson scattering (CTS) using a high power pulsed CO2 laser and heterodyne receiver system. The axial profiles can be measured by an axially injected beamline. Attenuation and refraction in the plasma are unimportant even for large devices. Scattered spectrum of the expected DT fusion plasma with currently developed laser and receiver system is presented. The component of scattered laser power nearly perpendicular to the magnetic field giving rise to ion cyclotron modulation of the scattered spectrum. Spectrum from pure deuterium and from D-T mixed plasma show the possibility of fuel ratio measurement. The scattered spectrum of D-T plasmas with thermalized helium ash is also calculated. Experimental arrangement to improve the S/N ratio in low density open magnetic systems are also discussed. The possibility of ion temperature and modulation measurement using a homodyne receiver system in GAMMA 10 are also evaluated.

Small-Angle Pulse CO2 Laser Scattering with a Heterodyne Detection for Investigation of Ion-Acoustic Waves in Plasma with Strong Langmuir Turbulence

A diagnostic system to measure the spatial and frequency spectra of ion-acoustic waves in plasma with strong Langmuir turbulence is being designed. High power pulsed CO2 laser system is used as primary source of radiation. A heterodyne light mixing scheme is used to detect the scattered power. Diagnostics is tested in the experiment on the interaction of high power relativistic electron beam with magnetized plasma.

A filter bank system for scattered spectrum analysis in collective Thomson scattering diagnostic on JT-60

In JT-60, a collective Thomson scattering (CTS) diagnostic system based on a pulsed CO2 laser (wavelength 10.6 μm, beam energy 15 J, pulse width 1 μs) has been developed to measure ion temperature and velocity distribution of fast ions to demonstrate the feasibility of measurements of confined alpha particles. A high power pulsed CO2 laser and heterodyne receiver system using a quantum-well infrared photodetector (QWIP) has been developed and installed in the diagnostic room. In order to resolve frequency distribution of pulsed scattered light (1 μs), we developed a wideband filter bank system (0.4–4.5 GHz) for scattered spectrum analysis in JT-60 CTS diagnostic. The detector bandwidth and frequency resolution are both determined by the calculation results of expected scattered spectrum. In order to realize wideband multichannel detection, cascade connected seventh order simultaneous-Chebyshev (elliptic) bandpass filters were adopted. Signal to noise ratio based on the noise equivalent power of the QWIP receiver and laser pulse length are also evaluated.

Advanced impurity measurement for deuterium–tritium-burning plasmas using pulsed CO2 laser collective Thomson scattering

Knowledge of the dynamics and concentration of impurities in the plasma core is crucial to the development of a deuterium–tritium (DT) tokamak fusion reactor. In this article, we first describe an advanced diagnostic technique to measure the impurities, thermalized helium density, and the deuterium-to-tritium density ratio in DT-burning plasmas. The measurement is made by small-angle collective Thomson scattering using a high-power pulsed CO2 laser and heterodyne receiver system. The component of scattered laser power nearly perpendicular to the magnetic field gives rise to ion cyclotron modulation of the scattered spectrum. Scattered spectrum of expected impurities (Ar, Be, and He ash) are presented for the International Thermonuclear Experimental Reactor plasmas. The method for the determination of the helium ash density and deuterium-to-tritium ratio are also proposed. Modulation spectra as a function of frequency for different magnetic-field angles are confirmed. Experimental requirements to measure a scattered spectrum with fine details resulting from ion cyclotron oscillations are also discussed.

Saturation of the photon-drag voltage at high CO2 laser intensities

p-Type Ge is investigated as a photon-drag detector for high-power pulsed CO2 laser, using 3-samples of different dimensions and resistivities. Nonlinear behaviour of the photon-drag effect is observed. This phenomena could be accounted for by taking into consideration the decrease of absorption and the increase of resistivity instantaneously at the moment of interaction, rather than the effect of multireflection. The observed saturation at intensities higher than 30 MW/cm2 could be due to multireflection since its role increases at high intensities. Using the quantum mechanical model with the above considerations it was possible to obtain good agreement to the measured results.

Feasibility of dc to visible high-power conversion employing a stimulated Compton free electron laser with a waveguided CO2 laser pump wave and an axial electric field

We examine the feasibility of high power generation of visible radiation by a process of applying an axial accelerating electric field on electrons trapped in the ponderomotive potential of a Compton scattering free electron laser. We consider a scheme where the pump (wiggler) field is produced by a high-power pulsed CO2 laser and the signal wave is the radiation of a high-power pulsed dye laser. We propose to use a hollow dielectric waveguide in order to overcome the pump wave diffraction and obtain a long interaction length.

Kinetics of collisionless dissociation of N2F4in a highpower infrared radiation field

An on-line diagnostic system was used to investigate the kinetics of N2F4→2 NF2 dissociation in a highpower pulsed CO2 laser field. It was found that, at N2F4 pressures of 3–15 Torr and incident energy densities of 0.2–3 J/cm2, collisions did not have a significant influence on the excitation and dissociation process and its rate was governed by the rate of radiative activation of the molecules. The intramolecular relaxation time of N2F4(τννi < 15) nsec was estimated from the experimental results. The experiment was interpreted satisfactorily assuming that the behavior of N2F4 molecules in the field is characterized only by the dependence of the stimulated transition cross section on the molecular energy σ(E) and by the dissociation energy which corresponds to the breaking energy of the weakest N–N bond. The experimental data were used to construct the time dependence of the molecular energy distribution function and to determine the character of the dependence σ(E): σE –k(1<k< 2).

Investigation of the reflection of CO2laser radiation from targets in air

An experimental study was made of the reflection of high-power pulsed CO2 laser radiation from targets of graphite and polished aluminum in air. It was found that for a flux density in the range 2×106 – 4×108W/cm2 the radiation was reflected both from the target surface and from a plasma formed close to it. The conditions and the formation time of a plasma layer screening the target were determined. The directional polar diagram of the radiation reflected by the plasma was studied and the reflection was found to be strongly nonspecular. The total laser energy losses on reflection were measured; they reached 43% for the graphite and more than 50% for the aluminum.

Impulse Transfer from Pulsed CO, Laser Irradiation at Reduced Ambient Pressures

EXPERIMENTAL and theoretical studies have been made of the impulse received by a target surface when irradiated by a high-power pulsed CO2 laser at reduced ambient pressures. Correlation of experimental and theoretical results for aluminum targets indicates a maximum impulse-energy coupling coefficient ap~2 dyn s/J and an effective thermal coupling coefficient a,—0.1. Both theory and experiment show an increase in ap with increasing ambient pressure. Structural deformation was experimentally demonstrated for an 1100 aluminum target. Contents Most of the previous work on impulse transfer from highpower laser pulses to surfaces, both experimental and theoretical, has been carried out for ambient air at or near 1 atm pressure, the measured impulse being dominated by the air environment.1 The air above the target surface can become ionized, forming an absorptive plasma. At laser intensities above about 107 W/cm2, this air plasma moves supersonically up the laser beam and away from the target, shielding the target surface from further direct radiation. This phenomenon is the well-known laser-supported detonation (LSD).2'3 At low ambient pressures (densities), on the other hand, impulse data are more limited in quantity, and theoretical predictions and correlations are comparatively less well established. As the ambient pressure is lowered, the air plasma absorption length for laser energy increases, becoming comparable to or exceeding the width of the laser beam; the LSD wave in air becomes harder to maintain and eventually disappears.3 In such cases, vaporization of the target material and the subsequent behavior of the vapor become the dominant factor in determining the impulse transfer. In the present work, both experimental measurements and theoretical calculations have been made of the impulse received by a target surface when irradiated by a high-power pulsed CO2 laser at reduced ambient pressures. The impulse experiments were carried out inside a 120-cmdiamx 160-cm-long vacuum tank. The pressure was varied from 10 to 0.01 Torr. The laser beam was admitted through a scintered NaCl window whose outside face was tilted to avoid reflecting energy back into the laser cavity. The 7.5-cm-diam beam was concentrated on the target by means of a copper mirror having a focal length of 114 cm located in the vacuum tank. By adjusting the distance between the mirror and the target, the laser spot diameter was varied from 0.38 to 2.3 cm.

Stimulated Brillouin backscatter detection from a ϑ-pinch plasma

Detection of prompt stimulated Brillouin backscatter is reported from an experiment in which a high-power pulsed CO2 laser irradiates an independently produced ϑ-pinch plasma. Analysis of the backscatter signal indicates that significant SBS gain is limited to a time period near peak incident power. The restriction on SBS gain is attributed to a varying temperature gradient.

Observation of wedge fringes on CO2laser optical components

During the operation of a high power pulsed CO2 laser system, damage was observed to a GaAs modulator crystal. The cause of the damage was possibly the creation of wedge fringes within the crystal. Wedge fringes were also formed in the system by a Ge polarizing plate. The authors describe how the formation of such fringes can be avoided and also how they can be used to measure small wedge angles over long path differences.