Metastable Argon Research Articles

Demonstration of a metastable argon laser of 10 W by transverse pumping.

Optically pumped metastable rare gas lasers have been extensively investigated as promising high-energy lasers. These systems employ discharge-excited metastable inert gases as the lasing medium. Following optical pumping, a buffer gas, typically helium, is introduced to facilitate a collisional population transfer to the p2[1/2]1 level, thereby establishing population inversion. To date, laser outputs in the watt level have been demonstrated. However, further power scaling crucially depends on the ability to stably generate high metastable densities at elevated pressures approaching atmospheric conditions. In this Letter, we report a pulsed discharge technique based on a peaking capacitor rapid discharge circuit, which is capable of producing metastable particle densities exceeding 1014 cm-3 at pressures up to 900 mbar. By employing this discharge approach in conjunction with transverse optical pumping, we have realized a maximum output power of 12.5 W from a semiconductor-pumped metastable argon laser system.

Time-resolved analysis of Ar metastable and electron populations in low-pressure misty plasma processes using optical emission spectroscopy

Misty plasmas have recently emerged as a promising tool for nanocomposite thin films deposition. However, aerosol-plasma interactions remain poorly documented, especially at low working pressure. In this work, optical emission spectroscopy is used to probe the temporal evolution of three fundamental plasma parameters during pulsed liquid injection in an inductively coupled argon plasma at low-pressure. Time-resolved values of metastable argon density, electron temperature, and electron density are determined from radiation trapping analysis and particle balance equations of selected argon 1s and 2p levels. Pulsed liquid injection is found to induce a sudden drop in metastable density and electron temperature, and an increase in electron density. These results are attributed to the lower ionization thresholds of the injected molecular species compared to the one of argon. In addition, upstream liquid temperature is found to affect the transitory kinetics for non-volatile solvents more than volatile ones, in accordance with a previously reported flash boiling atomization mechanism.

Study of Mid-Pressure Ar Radiofrequency Plasma Used in Plasma-Enhanced Atomic Layer Deposition of α-Al2O3

This study investigated the characteristics of radiofrequency, middle-pressure argon plasma used in the atomic layer deposition (ALD) of Al2O3 films. Based on the electrical characteristics—the current, voltage, and phase shift between them—and the stability of the plasma plume, the optimum plasma power, allowing reliable switching on of the plasma for any step of an ALD cycle, was determined. Spectral measurements were performed to determine the gas temperature and reactive species that could be important in the ALD process. The density of metastable argon atoms was estimated using tunable laser absorption spectroscopy. It was concluded that plasma heating of substrates did not affect film growth. The crystallization-enhancing effect of plasma observed in these experiments was due to the action of OH radicals produced in the plasma.

Modeling of the influence of field electron emission from a cathode with a thin insulating film on its sputtering in a gas discharge in a mixture of argon and mercury vapor

A model of the low-current gas discharge in a mixture of argon and mercury vapor in the presence of a thin insulating film on the cathode surface is proposed. The model takes into account that in such a mixture a substantial contribution to the ionization of the working gas can come from the ionization of mercury atoms during their collisions with metastable excited argon atoms. In the discharge, positive charges accumulate on the film surface, creating an electric field in the film sufficient to cause field emission of electrons from the cathode metal substrate into the insulator. Such electrons are accelerated in the film by the field and can escape from it into the discharge volume. As a result, the effective yield of ion-electron emission from the cathode increases. The temperature dependences of discharge characteristics are calculated and it is shown that, due to a rapid decrease in the concentration of mercury vapor in the mixture with decreasing temperature, the electric field strength in the discharge gap and the discharge voltage increase. The presence of a thin insulating film on the cathode can result in an improvement in its emission characteristics and a significant reduction in the discharge voltage. This causes a decrease in the energies of the ions and atoms bombarding the cathode surface, and, consequently, in the intensity of cathode sputtering in the discharge.

Investigation on the kinetics of a diode-pumped plasma-jet type metastable argon laser

Diode-pumped rare gas lasers (DPRGLs) have the potential for high-energy laser output with high beam quality. The metastable concentration >1014 cm−3 and pump intensity >10 kW/cm2 are required in developing high-power and efficient DPRGLs. However, the kinetics of 1s and 2p levels (Paschen notation) under these conditions is unclear yet, particularly regarding the adverse effects of interference energy levels (e.g., 1s4 and 2p1–2p8). This study addresses this gap by measuring the number densities of 1s and 2p levels in a diode-pumped plasma jet metastable argon laser. The experiment was conducted with a much higher number density of 1014 cm−3 and a higher pump intensity of 12 kW/cm2 than our previous works. The results indicate that the adverse effect of 1s4 accumulation could be partially compensated, and the relative population distribution of 2p energy levels remained unchanged. However, the high gas temperature (>1000 K) caused a significant negative impact of high 2p levels (2p1–2p8), and the air environment weakened the effect of He collision relaxation.

Possible evidence for the production of Ar2∗-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2^{*-}$$\\end{document} metastable negative molecular ions in gaseous argon of two-phase detectors for dark matter searches

Our recent studies of electroluminescence (EL) properties in two-phase argon detectors for dark matter searches have revealed the presence of unusual delayed pulses in the EL signal in the form of two slow components with time constants of about 5 and 50 μs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \ ext {s}$$\\end{document}. These components were shown to be present in the charge signal itself, which clearly indicates that drifting electrons are temporarily trapped on two states of metastable negative argon ions which have never been observed before. In this work, using the pressure dependence of the ratio of slow component contributions measured in experiment, it is deduced that these states are those of two types of metastable negative molecular ions, Ar2∗-(b4Σu-)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{Ar}}}_2^{*-}(b \\ ^4\\varSigma _u^-)$$\\end{document} and Ar2∗-(a4Σg+)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{Ar}}}_2^{*-}(a \\ ^4\\varSigma _g^+)$$\\end{document} for the higher and lower energy level respectively.

Production of high-density metastable argon atoms with periodic fast pulsed DC discharge.

Optically pumped metastable rare gas laser (OPRGL), as a potential high-energy laser has attracted much attention. During extensive research on OPRGL, the realization of volume discharge with high-metastable density has always been a priority and is essential to the efficient lasing of OPRGL. A large-volume plasma of He/Ar with high metastable density at atmospheric pressure can be generated by applying peaking capacitors near the electrodes. In this article, 0.8 cm3 of He/Ar plasma at a pressure of 900 mBar, with a peak value of metastable argon density higher than 1014 cm-3 was realized. The metastable density was measured by absorption spectroscopy based on the absorption bandwidths acquired at a pressure range of 400-900 mbar where a probe beam at a weak line with wavelength 772.38 nm was applied. Furthermore, the high metastable density was verified by the laterally-observed laser-induced fluorescence, as well as the laser oscillation of 100 mW under a longitudinal pumping with a short gain length of 8 mm.

Experimental and numerical investigation on the uniformity of nanosecond pulsed dielectric barrier discharge influenced by pulse parameters

Nanosecond (ns) pulsed dielectric barrier discharge (DBD) is considered as a promising method to produce controllable large-volume and high activity low-temperature plasma at atmospheric pressure, which makes it suitable for wide applications. In this work, the ns pulse power supply is used to excite Ar DBD and the influences of the pulse parameters (voltage amplitude, pulse width, pulse rise and fall times) on the DBD uniformity are investigated. The gas gap voltage (U g) and conduct current (I g) are separated from the measured voltage and current waveforms to analyze the influence of electrical parameters. The spectral line intensity ratio of two Ar excited species is used as an indicator of the electron temperature (T e). The time resolved discharge processes are recorded by an intensified charge-coupled device camera and a one-dimensional fluid model is employed to simulate the spatial and temporal distributions of electrons, ions, metastable argon atoms and T e. Combining the experimental and numerical results, the mechanism of the pulse parameters influencing on the discharge uniformity is discussed. It is shown that the space electric field intensity and the space particles’ densities are mainly responsible for the variation of discharge uniformity. With the increase of voltage and pulse width, the electric field intensity and the density of space particles increased, which results in the discharge mode transition from non-uniform to uniform, and then non-uniform. Furthermore, the extension of pulse rise and fall times leads to the discharge transition from uniform to non-uniform. The results are helpful to reveal the mechanism of ns pulsed DBD mode transition and to realize controllable and uniform plasma sources at atmospheric pressure.

Metastable argon atom kinetics in a low-pressure capacitively coupled radio frequency discharge

The kinetics of excited atoms in a low-pressure argon capacitively coupled plasma source are investigated by an extended particle-in-cell/Monte Carlo Collisions simulation code coupled with a diffusion-reaction-radiation code which considers a large number of excited states of Ar atoms. The spatial density distribution of Ar atoms in the 1s5 state within the electrode gap and the gas temperature are also determined experimentally using tunable diode laser absorption spectroscopy. Processes involving the excited states, especially the four lower-lying 1s states are found to have significant effects on the ionization balance of the discharge. The level of agreement achieved between the computational and experimental results indicates that the discharge model is reasonably accurate and the computations based on this model allow the identification of the populating and de-populating processes of the excited states.

Formation mechanisms of striations in a filamentary dielectric barrier discharge in atmospheric-pressure argon

Formation mechanisms of striations along the discharge channel of a single-filament dielectric barrier discharge (DBD) in argon at atmospheric pressure are investigated by means of a time-dependent, spatially two-dimensional fluid-Poisson model. The model is applied to a one-sided DBD arrangement with a 1.5 mm gap using a sinusoidal high voltage at the powered metal electrode. The discharge conditions are chosen to mimic experimental conditions for which striations have been observed. It is found that the striations form in both half-periods during the transient glow phase, which follows the streamer breakdown phase. The modelling results show that the distinct striated structures feature local spatial maxima and minima in charged and excited particle densities, which are more pronounced during the positive polarity. Their formation is explained by a repetitive stepwise ionisation of metastable argon atoms and ionisation of excimers, causing a disturbance of the spatial distribution of charge carriers along the discharge channel. The results emphasise the importance of excited states and stepwise ionisation processes on the formation of repetitive ionisation waves, eventually leading to striations along the discharge channel.

Metastable argon dynamics in a pulsed microplasma at 43 GHz

Short pulses of millimeter wave (MMW) radiation at 43 GHz create microplasma within a photonic crystal for pressures from 40 to 600 Torr (1.3 × 103–8.0 × 104 Pa). Gas breakdown occurs within a photonic crystal, which acts as an electromagnetic resonator to create a strong initial electric field. The time response of the argon metastable density is experimentally determined during the pulse and in the afterglow using laser absorption. The metastable density overshoots the steady-state condition at the beginning of the pulse and during the afterglow. Modeling is presented to understand these observations. The overproduction of argon 1s5 at the beginning of each pulse is due to a concurrent overshoot in the MMW electric field within the photonic crystal. This field overheats the plasma electrons and enhances the production of excited states. The burst of argon metastables observed in the afterglow is due to the pooled energy of the plasma stored in electrons, ions, and excited states of argon. Understanding metastable production is an important intermediate step to ionization and is also critical in the study of diode-pumped rare gas lasers.

Puzzling time properties of proportional electroluminescence in two-phase argon detectors for dark matter searches

Proportional electroluminescence (EL) in noble gases is a physical process routinely used in two-phase (liquid–gas) detectors for low-energy astroparticle-physics experiments. In this work, the time properties of visible-light EL in two-phase argon detectors have been systematically studied for the first time. In particular, two unusual slow components in the EL signal, with their contributions and time constants increasing with electric field, were observed. This puzzling property is not expected in any of the known mechanisms of photon and electron emission in two-phase media. Time constants of these components is about 4–5 upmu s and 50 upmu s. In addition, a specific threshold behavior of the slow components was revealed: they emerged at a threshold in reduced electric field of 4.8 ± 0.2 Td regardless of the gas phase density, which is about 1 Td above the onset of standard (excimer) EL. There is a conspicuous similarity between this threshold and reduced field threshold of EL in NIR occurring via higher atomic excited states Ar^{*}(3p^{5}4p). An unexpected temperature dependence of slow components was also observed: their contribution decreased with temperature, practically disappearing at room temperature. We show that the puzzling properties of slow components can be explained in the framework of hypothesis that these are produced in the charge signal itself due to trapping of drifting electrons on metastable negative argon ions.

Demonstration of transversely pumped Ar[formula omitted] laser with continuous-wave diode stack and repetitively pulsed discharge

As a potential high-energy laser, optically pumped metastable rare gas laser (OPRGL) has drawn attention worldwide. In this article, a stack-pumped metastable argon laser was demonstrated at 800 mbar of Ar/He mixture. The metastable particles were produced by repetitively pulsed discharge in the gas mixture, and laser oscillation was realized in a planar-concave cavity with the gain medium longitudinally or transversely pumped by the pumping source of a diode-stack with an external cavity of gratings. Maximum averaged power of 750 mW was obtained with transverse pumping in a repetition rate of 30 kHz of pulsed discharge. Compared to longitudinal pumped OPRGLs, extraction ability and pumping uniformity would be improved at transverse pumping configuration. And it is found that the average output power similar to that of longitudinal pumping can be achieved at a shorter gain length with transverse pumping, which is linearly proportional to the discharge repetition rate. This demonstration implies that power scaling of OPRGL is feasible by combining diode-stacks and multi-pass transverse pumping.

Influence of Metastable Argon Atoms and Dust Particles on Gas Discharge Plasma

The model of a DC glow discharge with metastable argon atoms and dust particles based on the Boltzmann equation for the electron energy distribution function (EEDF), dust particle charging, and balance equation for metastable argon atoms is presented. The processes of direct and stepwise electron impact ionization, metastable-metastable collisions, and recombination of electrons and ions on the dust particle surface and discharge tube wall are taken into account. The results show that the densities of metastable argon atoms and dust particles are in strong correlation, and both sufficiently influence gas discharge parameters.

Segmented pulsed discharge for metastable argon lasing medium

Segmented pulsed discharge for metastable argon lasing medium

Formation and evolution of striation plasma in high-pressure argon glow discharge

The self-organized striation phenomenon in the positive column region of glow discharge plasma is a basic problem in gas discharge physics, which involves rich physics such as electron dynamics, transport process, discharge instability and nonlinear phenomenon. It is an important topic in basic physics and practical application. In this work an argon glow discharge striation plasma at high pressure is reported. The electrical, optical and ionization wave propagation characteristics of the striation plasma, and the evolution of the striation plasma with pressure and impurity gas are investigated experimentally. The generation and quenching mechanism of argon striation plasma are analyzed. The results show that the striation length is about 1.5 mm, and decreases with pressure increasing, and the velocity and frequency of the ionization wave are estimated at 1.87 m/s and 1.25 kHz, respectively. The measurement of optical emission spectrum shows that the generation of striation plasma is probably related to the argon metastable atoms. The stepwise ionization process caused by metastable atoms triggers off an ionization instability. The instability propagates in the form of ionization wave, which leads the plasma parameters to be modulated longitudinally, thus, forming an alternating bright and dark striation plasma. The adding of nitrogen can effectively quench metastable argon atoms and change the electron energy distribution function, which destroys the instability conditions of the plasma, therefore, the striation plasma disappears. This work provides a new insight into the understanding of the formation and annihilation mechanism of glow discharge striation plasma at high pressure.

On the delayed emission from a laser-produced aluminum plasma under an argon environment

We report long time (250 μs) emission from Al I in laser produced plasma in argon ambient and its dependence on laser power density, background pressure and distance from the target surface. Role of argon metastables as energy reservoirs is discussed.

A linewidth narrowed diode laser for metastable Argon atom pumping

A volume Bragg grating-based external cavity diode laser array produced more than 100 W output power for metastable argon atoms pumping centered at 811.53 nm with a 0.15 nm spectral linewidth (FWHM). The device has only 4.6 % loss compared to the free-running case operating at full power and a spectral contrast of 40 dB. The emission spectrum can be precisely tuned to 811.53 nm and has an over 200 pm tuning range. Pump absorption was demonstrated by using a repetitively pulsed glow discharge operating with 620 Torr Ar/He mixture, and an average absorption of 24 % of the pump radiation was realized.

Characterising a tunable, pulsed atomic beam using matter-wave interferometry

We describe the creation and characterisation of a velocity tunable, spin-polarized beam of slow metastable argon atoms. We show that the beam velocity can be determined with a precision below 1% using matter-wave interferometry. The profile of the interference pattern was also used to determine the velocity spread of the beam, as well as the Van der Waals (VdW) co-efficient for the interaction between the metastable atoms and the multi-slit silicon nitride grating. The VdW co-efficient was determined to be C 3 = 1.84 ± 0.17 a.u., in good agreement with values derived from spectroscopic data. Finally, the spin polarization of the beam produced during acceleration of the beam was also measured, demonstrating a spatially uniform spin polarization of 96% in the m = +2 state.

Zeeman-resolved TDLAS using metastable levels of Ar in the weakly magnetized plasma of the linear plasma device PSI-2

Tunable diode laser absorption spectroscopy was applied at the linear plasma device PSI-2 to measure the magnetic field, temperature of argon and density of metastable species in a low density gas discharge. The measurements on the two metastable levels of Ar were performed by scanning the plasma column of PSI-2 at different radii. The obtained magnetic field using the lines at 763 and 772 nm (Ar) was found to be systematically lower (by 5% to 17%) than the calculated vacuum field. Part of the deviation arises from the line integration of the absorption signal. The radial gradient of the magnetic field strength combined with the radial metastable density determines the magnitude of this contribution (2%–3%). The temperature of the neutral gas was found to be essentially constant within the discharge chamber. The gas temperature rises with increasing cathode current and magnetic field due to an increase in the plasma density and, consequently, an increase in the energy transferred to the neutral gas by collisions with the charged particles. The density of the 4 s metastable level with J = 2 was found to be 8–9 times higher than that of the level with J = 0 similarly to observations by others in non-magnetized plasmas. To understand this trend a simple collisional-radiative model for the metastable argon 4s J = 2 level was developed. Depending on the treatment of the 4p levels it predicts a lower and an upper limit of the metastable density. The experimental values are within the limits predicted by the model indicating that the complex kinetics of the excitation and deexcitation collisional-radiative processes lead to this deviation from the statistical equilibrium.