Transversely Excited Atmospheric Research Articles

简单谐振腔的高能量光泵NH3太赫兹激光器

A high energy pulsed terahertz (THz) laser is studied experimentally. The laser cavity simply consists of a quartz glass waveguide, a coated GaAs input window, and a crystal quartz output window. NH3 is filled in the cavity as gain medium, and pumped by an 8-J line-tunable transversely excited atmospheric (TEA) CO2 laser. When 9R(16) transition acts as the pump line, 55.6-mJ THz radiation (90 μm) is obtained at 730-Pa NH3 pressure. The corresponding conversion efficiency is 13.54%. Energy and optimal pressure of amplified spontaneous emission and laser oscillation are compared.

Analysis of Organic Powder Samples by Using the Metal-Assisted Subtarget Effect in a Transversely-Excited Atmospheric (TEA) CO$_2$ Laser-Induced He Gas Plasma at 1 atm

Analysis of Organic Powder Samples by Using the Metal-Assisted Subtarget Effect in a Transversely-Excited Atmospheric (TEA) CO$_2$ Laser-Induced He Gas Plasma at 1 atm

Micro- and nanosecond laser TiN coating/steel modification: Morphology studies

Morphology effects induced during interaction of μs- (Transversely Excited Atmospheric (TEA) CO2 laser) or ns- (HF laser) pulses with titanium nitride (TiN) coating, deposited on austenitic stainless steel AISI 316, were studied. Experiments were carried out in regime of focused laser beam in air at atmospheric pressure. The used laser fluences were found to be sufficient for inducing intensive surface modifications of the target. The energy absorbed from the CO2 as well as HF laser beam is mainly converted into thermal energy, causing different effects like ablation, appearance of hydrodynamic features, etc. Morphology characteristics obtained during ns-pulses irradiation (HF laser) were different to those initiated by μs-pulses (TEA CO2 laser). The changes on the target surface in form of massive resolidifed droplets and crown-like structures were observed only for ns- (HF laser) pulses. It was found that these effects are a consequence of higher temperature and better coupling of the HF laser radiation with the target. Recent investigations of ps-Nd:YAG laser interaction with the same TiN coating showed that morphology picture is quite different including the reduction of thermal effect.

Investigation of the convex mirror cooling of a transversely excited atmospheric (TEA) CO2coaxial output unstable resonator laser

A coaxial unstable resonator using a vortex tube to cool a convex mirror has been developed for a transversely excited atmospheric (TEA) CO2 laser. The time dependent thermal deformation of the convex mirror was analyzed by using the finite element method. The simulated results indicate that the vortex-tube-cooling system can remarkably reduce the thermal deformation of the convex mirror, especially at long working times. Also, an experimental verification was carried out, and the results show that the developed vortex-tube-cooling coaxial unstable resonator system is an effective and applicable cooling method for a convex mirror. The divergence angle drops from 3.19 to 1.78 mrad, and the times-diffraction-limit factor β decreases from 6.94 to 1.78 if using the vortex-tube-cooling system when the repetition rate is 3 Hz and the continuous working time is 300 s.

New Technique for the Direct Analysis of Food Powders Confined in a Small Hole Using Transversely Excited Atmospheric CO2 Laser-Induced Gas Plasma

Taking advantage of the differences between the interactions of transversely excited atmospheric (TEA) CO(2) lasers with metal and with organic powder, a new technique for the direct analysis of food powder samples has been developed. In this technique, the powder samples were placed into a small hole with a diameter of 2 mm and a depth of 3 mm and covered by a metal mesh. The TEA CO(2) laser (1500 mJ, 200 ns) was focused on the powder sample surfaces, passing through the metal mesh, at atmospheric pressure in nitrogen gas. It is hypothesized that the small hole functions to confine the powder particles and suppresses the blowing-off of sample, while the metal mesh works as the source of electrons to initiate the strong gas breakdown plasma. The confined powder particles are then ablated by laser irradiation and the ablated particles move into the strong gas breakdown plasma region to be atomized and excited; this method cannot be applied for the case of Nd:YAG lasers because in such case the metal mesh itself was ablated by the laser irradiation. A quantitative analysis of a milk powder sample containing different concentrations of Ca was successfully demonstrated, resulting in a good linear calibration curve with high precision.

Moderate-Acceleration Launch Using Repetitive-Pulse Laser Ablation in a Tube

Projectile launch experiments were conducted using a new laser-driven, in-tube accelerator (LITA) in which the propellant was supplied not as pre-filled gas but from laser ablation. The launch tube had an inner diameter of 25 mm and was 1 m in length, made of acrylic. The 7.1-gram projectile had a parabolic nozzle and an ablator rod made of polyacetal that was located at the focus of the parabola. Laser pulses of 3.1 J to 3.8 were repetitively sent at a frequency of 50 Hz from a Transversely-Excited Atmospheric (TEA) CO2 laser (wavelength; 10.6 μm). Time-modulating the laser-ablative pressure, a large impulse performance was obtained with a moderate acceleration level. A momentum coupling coefficient of 1.5 mNs/J and a gross acceleration of 2.5g were obtained.

Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses

Interaction of a nanosecond transversely excited atmospheric (TEA) CO 2 laser, operating at 10.6 μm, with tungsten–titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24–49 J/cm 2. The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses.

Analysis of Heavy Metal Pollution in Soil Using Transversely Excited Atmospheric CO2 Laser-Induced Plasma by Trapping the Soil in Microstructured Holes on Metal Subtargets

A unique technique for direct analysis of soil samples utilizing a special advantage of a transversely excited atmospheric (TEA) CO(2) laser-induced plasma generated at atmospheric pressure on a metal target has been developed. In this technique, a metal subtarget, such as nickel plate, structured with intentional microholes on its surface, each with dimensions of around 100 microm in diameter and depth, was used to selectively trap small sized soil particles by immersing the metal plate subtarget into the polluted soil sample. The trapped small soil particles on the metal subtarget were irradiated by a TEA CO(2) laser (10.6 microm, 1.5 J, 200 ns) at atmospheric pressure under defocused condition with a spot size of 3 mm x 3 mm. This trapping and confining scheme substantially suppresses the blowing off effect; thus, the trapped soil particles can effectively be dissociated and atomized in the microstructured holes. Using this method of a microstructured metal plate subtarget, quantitative analysis was carried out on loam soil samples polluted by Pb. A linear calibration curve was obtained with a detection limit of approximately 50 mg/kg. Preliminary quantitative studies were carried out for a quartz sand sample containing Cr and Hg, resulting in linear calibration curves with detection limits of approximately 25 mg/kg and 10 mg/kg, respectively, at this stage. This technique is promising as a potential field screening tool for soil analysis.

Surface modifications of TiN coatings by a pulsed TEA CO2 laser: Coating thickness effects

Interactions of a transversely excited atmospheric (TEA) CO2 laser, pulse duration ∼2 μs (initial spike FWHM ∼120 ns), with polycrystalline titanium nitride (TiN) coatings deposited on high-quality steel (AISI 316 or M2) were studied. The experiments were carried out in a regime of high laser energy densities: 25, 48, and 50 J/cm2. The energy absorbed from the laser beam was partially converted to thermal energy and the effects of the TiN coating thickness on the morphological changes were considered. The morphological features and processes that accompany the interaction can be summarized as follows: (i) exfoliation of the TiN coating in the central zone of the irradiated area (for coating thickness of 1 μm) or appearance of grainy structure (for coating thicknesses 3 and 10 μm); (ii) appearance of hydrodynamic changes in the surrounding peripheral zone; and (iii) appearance of plasma in front of the target during sample irradiation.

Observation of Discharge Instability Induced by Shockwave in Self-Sustained High-Pressure Pulsed Glow Discharge

Self-sustained high-pressure pulsed glow discharge is applied to excitation discharge on transversely excited atmospheric (TEA) gas lasers. Influence of shockwaves with Mach number M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> of 1.1 and 1.35 on the discharge has been investigated. The normal Shockwave, which is normal to gas flow direction in TEA gas lasers, is produced by a shock tube with gas mixture of helium and argon to simulate reflected shockwave in discharge cavity. Discharge instability depends on both the shock Mach number and the position of shockwave in the cavity. The discharge does not collapse by gas density disturbance caused by the shockwave with M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> = 1.1, where the gas densities in front of and behind the shockwave are rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> = 0.52 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> = 0.60 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively. In the two shockwaves with identical M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> = 1.35 having rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> = 0.83/0.55 or 0.67 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /0.45 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , the recovery of discharge depends on rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and the gas density disturbance caused by oblique shockwave destroys the discharge even if the cavity is occupied by the gas having suitable density in still gas. The discharge may not be disturbed with spatial distribution of preionization electron density due to the shockwave.

CO2 Laser Collective Thomson Scattering Diagnostic of α-Particles in Burning Plasmas

Collective Thomson scattering (CTS) diagnostic based on a pulsed CO2 laser (wavelength 10.6 μm) has been developed to establish a diagnostic method of confined α-particles in burning plasmas. A high-repetition and high-energy transversely excited atmospheric (TEA) laser has been developed as a source of the CTS diagnostic. In order to obtain single-mode output, which is needed for CTS diagnostic, seed laser is injected into the cavity with unstable resonator. Pulse energy of 17 J with a repetition rate of 15 Hz has been achieved in a single-mode operation. This result gives a prospect for the CTS diagnostic on International Thermonuclear Experimental Reactor (ITER), which requires energy of 20 J with repetition rate of 40 Hz. Proof-of-principle test will be carried out in the JT-60U tokamak by using the newly developed laser. Preliminary consideration of the CTS diagnostic in the tandem mirror GAMMA 10 shows that axial profiles of ion temperature will be obtained using a circumferential collection mirror of scattered power.

Ambient Pressure Dependence of Laser-Induced Impulse onto Polyacetal

L ASER ablation can be usefully employed to generate a propulsive impulse on an object not only in the atmosphere but also in vacuum [1–3]. Larson et al. [4] proposed a launch vehicle propelled by repetitively pulsed laser ablation. Several authors [5–9] show that favorable propulsion performance in the atmosphere can be achieved with a polymer material, polyacetal, which is commercially named “Delrin” or is abbreviated as “POM.” The ablated gas from this material does not contain much air pollutant. Targeting space applications, the laser-ablative-propulsion performance of metals and polymers at low ambient pressures has been intensively investigated [10–17]. For many kinds of metals and polymers, Cm ranges from 10 to 100 N-s=J, and depends strongly on the characteristics of the laser pulse. According to measurements byGregg andThomas [10], metallic materials have an optimum laser intensity that maximizes Cm. Phipps et al. [12] formulated experimental Cm characteristics of aluminum alloys and several polymers in terms of the intensity, width, andwavelength of the laser pulse.Cm can be further increased by utilizing the so-called “volume absorber” [13] or layered target [14]. Recent measurements by D’Souza and Ketsdever [17] of Cm for polyacetal at low ambient pressure using a frequency-doubled Nd:YAG pulsed laser showed peak of 110 N-s=J at a laser intensity of the order of 10 W=cm. Several authors investigated the influence of ambient pressure on Cm [18,19]. Pakhomov et al. [18]measured the impulse generated on an aluminum surface irradiated by a CO2 laser pulse whose pulse width was 200 ns; Cm decreased monotonically with decreasing ambient pressure. In Dufresne et al.’s surface-pressure measurement [19], the ambient-pressure dependence of Cm for aluminum was quite sensitive to the laser pulse width. For polymer materials, Beverly and Walters [20] measured the CO2-laser-induced shock pressure in cellulose acetate and polymethylmethacrylate. The peak shock pressures exhibited the complex dependence on the ambient pressure. The ablative impulse dependence of materials other than aluminum warrants further investigations. To quantitatively evaluate the feasibility of laser-ablativepropulsion systems, the propulsive performance of polyacetal over a wide range of ambient pressure needs to be known. In the present paper, the influence of the ambient pressure on the impulse characteristics of polyacetal is investigated experimentally using a transversely excited atmospheric (TEA) CO2 pulse laser.

Dissolution studies of bovine dental enamel surfaces modified by high-speed scanning ablation with aλ = 9.3-µm TEA CO2 laser

Previous studies have demonstrated that lasers can be used to modify the chemical composition of dental enamel to render the mineral phase more resistant to acid dissolution with minimal peripheral thermal damage. Transverse excited atmospheric (TEA) CO(2) lasers tuned to the strong mineral absorption of hydroxyapatite (HAP) near lambda = 9 microm are well-suited for the efficient ablation of dental hard tissues if the laser-pulse is stretched to greater than 5-10 microseconds to avoid plasma shielding phenomena. Moreover, TEA CO(2) lasers can be operated at very high repetition rates and are inherently less expensive and more versatile than Er:YAG and Er:YSGG solid-state lasers. In this study a lambda = 9.3-microm TEA CO(2) with a pulse duration of 8 microseconds and a repetition rate of 300 Hz was used to uniformly treat bovine enamel surfaces at ablative irradiation intensities. We hypothesized that a uniform surface layer of modified enamel of improved crystallinity and CaP phase composition would be formed with an enhanced resistance to acid-dissolution in the ablated areas at higher scanning rates used with the water spray. Such a modified layer of enamel formed at the base and walls of a cavity preparation under the irradiation conditions employed in this study have the potential to inhibit secondary caries under sealants and restorations. The surfaces of bovine enamel blocks (3 x 3 mm(2)) were rapidly scanned across the laser beam at rates of 2, 3, and 6 mm/second with and without a water-spray at an incident fluence of 30 J/cm(2). The resistance to acid dissolution was evaluated using controlled surface dissolution experiments on laser-irradiated and control samples. The groups irradiated at a fluence of 30 J/cm(2) with a repetition rate of 300 Hz and a high scan rate of 6 mm/second with and without water-cooling significantly reduced the overall surface dissolution rates (P < 0.001). At low scan rates (2-3 mm/second) excessive heat deposition resulted in the formation of an outer layer of asperities containing non-apatitic CaP phases that were more susceptible to acid-dissolution. At a scanning rate of 6 mm/second even without the water spray a layer of purer phase HAP was formed without thermal damage, indicating that a high scanning rate can be used to avoid excessive thermal damage during ablation. The best results (80% inhibition) were attained for the higher scanning speed 6-mm/second combined with a water spray. This study demonstrates that an enamel surface with enhanced resistance to acid dissolution is produced after ablation with lambda = 9.3-microm TEA CO(2) laser pulses delivered at high-repetition rates if sufficiently high scanning rates are used with or without a water-spray.

Surface modifications of TiN coating by pulsed TEA CO 2 and XeCl lasers

Interactions of a transversely excited atmospheric (TEA) CO 2 laser and an excimer XeCl laser, pulse durations ∼2 μs (initial spike FWHM ∼100 ns) and ∼20 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steel AISI 316, were studied. Titanium nitride was surface modified by the laser beams, with an energy density of 20.0 J/cm 2 (TEA CO 2 laser) and 2.4 J/cm 2 (XeCl laser), respectively. The energy absorbed from the CO 2 laser beam is partially converted to thermal energy, which generates a series of effects such as melting, vaporization of the molten material, shock waves, etc. The energy from the excimer XeCl laser primarily leads to fast and intense target evaporation. The calculated maximum temperatures on the target surface were 3770 and 6300 K for the TEA CO 2 and XeCl lasers, respectively. It is assumed that the TEA CO 2 laser affects the target deeper, for a longer time than the XeCl laser. The effects of the XeCl laser are confined to a localized area, near target surface, within a short time period. Morphological modifications of the titanium nitride surface can be summarized as follows: (i) both lasers produced ablation of the TiN coating in the central zone of the irradiated area and creation of grainy structure with near homogeneous distribution; (ii) a hydrodynamic feature, like resolidified droplets of the material, appeared in the surrounding peripheral zone; (iii) the process of irradiation, in both cases, was accompanied by appearance of plasma in front of the target. Target color modifications upon laser irradiation indicate possible chemical changes, possibly oxidation.

Deuterium Emission in Laser Plasma Induced by Transversely Excited Atmospheric Pressure CO2 Laser in Low-Pressure of Helium Surrounding Gas

A transversely excited atmospheric pressure (TEA) CO2 laser was focused on samples containing hydrogen of high concentration in ambient gas of low pressure. The optimum conditions for reducing the spectral width of Hα emission have been investigated. As a result, it was found that a film sample is superior to a bulk sample, and He gas should be used as the ambient gas at approximately 1 Torr, and for spectral measurement, only the front area of the plasma should be selected. We successfully demonstrated the detection of Dα emission, of which peak shifts only 0.179 nm from the Hα emission, by employing a D2O liquid film sandwiched between a Cu plate and a thin plastic wrapping film.

Surface modification of the Cr-based coatings by the pulsed TEA CO2 laser

The interaction of a transversely excited atmospheric (TEA) CO2 laser with chromium oxynitride (CrON) coating deposited on a AISI 304 steel substrate was considered. The results have shown that CrON was surface-modified by the laser beam of 45 J/cm2 energy density. The energy absorbed from the TEA CO2 laser beam was partially converted into thermal energy, which has generated a series of effects such as melting, vaporization of the molten material, and shock waves in the vapor and in the solid. Morphological manifestations on the CrON coating surface can be summarized as follows: non-uniform features with ablation and appearance of crater-like form (central zone of interaction); appearance of three damaged areas and presence of hydrodynamic effects with resolidified droplets (periphery zone of interaction). In case of applied energy density the interaction of laser radiation with CrON has been always followed by plasma creation in front of the coating.

Surface modifications of TiN coating by the pulsed TEA CO 2 and KrCl laser

Interaction of a transversely excited atmospheric (TEA) CO 2 and excimer-KrCl laser, pulse duration ∼2 μs (FWHM of initial spike ∼100 ns) and ∼12 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steels, was studied. The results have shown that the titanium nitride was surface modified by the laser beam, with an energy density (ED) of 43.0 J/cm 2 (TEA CO 2 laser system) and 3.3 J/cm 2 (KrCl system), respectively. The energy absorbed from the CO 2 laser beam is partially converted to thermal energy, which generates a series of effects such as melting, vaporization of the molten material, shock waves, etc. The energy from the excimer-KrCl laser primarily leads to a fast and intense target evaporation. Calculated maximal temperatures, on the target surface, were 3940 and 8000 K for the TEA CO 2 and the KrCl lasers, respectively. It is assumed that TEA CO 2 laser affects the target deeper, during a longer time than the KrCl laser. The effects of the KrCl laser are confined to a localized area, near target surface, in the course of short time. The following morphological changes of titanium nitride surface were observed: (i) both types of laser caused ablation of the TiN coating in the central zone of the irradiated area, (ii) appearance of a hydrodynamic feature like resolidified droplets of material in the surrounding outer zone (cracking) in the case of the TEA CO 2 laser, and (iii) appearance of wave-like microstructures (consisting of periodic parallel fringes with a period of about 20 microns) in the case of KrCl laser. Formation of the wave-like microstructures is discussed in the context of laser-induced periodic surface structure (LIPSS) effects and diffraction phenomena. The process of the CO 2 and KrCl laser interaction with titanium nitride were typically accompanied by plasma formation. Plasma appeared above the target after about 40 and 1 laser pulse, respectively.

Pulsed TEA CO2 laser surface modifications of silicon

The interaction of a transversely excited atmospheric (TEA) CO2 laser, pulse duration of about 2μs (FWHM of initial spike=120ns), with p-type single crystalline silicon doped by boron, was studied. The results have shown that the silicon was surface modified by the laser beam of 12.0Jcm−2 energy density. The energy absorbed from the CO2 laser beam is converted partially into thermal energy, which generates a series of effects such as melting, vaporisation of molten material, shock waves, etc. Morphological manifestations on the silicon surface are: nonuniform modifications (central zone of interaction); wave-like periodical microstructure (inner periphery zone); hydrodynamical structure-like droplets (outer periphery zone). Wave-like microstructure consists of periodic parallel fringes with a period of about 0.8μm. Formation of these wave-like microstructures is very complex. Explanation includes a consideration of the laser-induced periodic surface structure (LIPSS) effect. The process of the CO2 laser interaction with silicon was not initially accompanied by plasma. Plasma, in the form of a spark, typically appeared after about 100 cumulated laser pulses.

Production of laser-heated plasma in high-. pressure Ar gas and emission characteristics. of vacuum ultraviolet radiation. from Ar 2 excimers

A new pump scheme for the realization of a practical Ar2 excimer laser operating at 126 nm has been proposed and investigated experimentally. In this scheme, pre-ionized high-pressure Ar gas was excited by an intense transversely-excited atmospheric (TEA) CO2 laser irradiation. A 100-mm-long line plasma was successfully produced at an argon gas pressure of 2 MPa. The vacuum ultraviolet emission at 126 nm from the Ar2 excimers was observed and its emission characteristics were investigated under various experimental conditions.

Surface modification of stainless steels by TEA CO 2 laser

The interaction of a transversely excited atmospheric (TEA) CO 2 laser with austenitic stainless AISI 316 and high-speed tool AISI M2 steel is considered. The results have shown that both types of steel were surface modified by the laser beam, for the laser energy densities used. Morphological features on the AISI M2 were more prominent in comparison to a AISI 316 steel. On the AISI M2 steel crater like forms, solidified boundary and hydrodynamical modifications were observed, while on the AISI 316 one, corrugation, cracking and resolidified areas were registered. The morphology changes of steel surfaces in dependence of laser pulse temporal shape were analyzed especially. The pulse with a tail for difference of a tail-free pulse, for the same incident peak power density, as a rule caused higher level of damage.