Metastable Density Research Articles

Modeling study on the enhancement of atmospheric pulse-modulated radio-frequency discharge assisted by pulsed voltage

In this paper, we performed a one-dimensional fluid model to study the mechanism and optimization of pulse-modulated Radio-Frequency (RF) discharges at atmospheric pressure assisted by short pulse voltages. The evolution of discharge current density, helium metastable (He*) density, and total electron density from the simulation demonstrates that the ignition of RF discharge could be effectively enhanced by the short pulsed discharge, and a large Peak Current in the First Period (PCFP) can be produced, which agrees well with the experimental measurements. Due to the assistance of pulsed voltage, a strong electric field could be formed near the anode with the same polarity of that near the cathode, which can reaccelerate the electrons near the anode to generate a large PCFP. Based on the simulation results, reducing the time interval and increasing the pulse rise rate are very helpful to enhance the ignition of subsequent RF discharge by strengthening the electric field near the anode. It is shown that by choosing the appropriate time interval and pulse rise rate, the pulse-modulated RF discharge assisted by the pulsed discharge can be effectively modulated and optimized for applications.

Energy efficiency of voltage waveform tailoring for the generation of excited species in RF plasma jets operated in He/N2 mixtures

Based on tunable diode laser absorption spectroscopy (TDLAS) measurements of the spatially averaged and peak helium metastable atom densities in a capacitively coupled micro atmospheric pressure plasma jet operated in He/N2 mixtures, the energy efficiency of metastable species (He-I 23S1) generation is compared for three different scenarios: single frequency operation at (i) 13.56 MHz and (ii) 54.12 MHz, and voltage waveform tailoring (VWT) at (iii) ‘valleys’-waveforms synthesized from four consecutive harmonics of 13.56 MHz. For each case, the dissipated power is measured based on a careful calibration procedure of voltage and current measurements. It is shown that the range of powers, at which the jet can be stably operated, is noticeably expanded by VWT. The results are compared to particle-in-cell/Monte Carlo collisions simulation results and very good agreement is found. The computational results show that the choice of the surface coefficients in the simulation is important to reproduce the experimental data correctly. Due to the enhanced control of the spatio-temporal electron power absorption dynamics and, thus, of the electron energy distribution function by VWT, this approach does not only provide better control of the generation of excited and reactive species compared to single frequency excitation, but in case of helium metastables the energy efficiency is also shown to be significantly higher in case of VWT.

Dynamics of Ar metastable and resonance states in pulsed capacitively coupled plasmas

The experimental and theoretical study of radiofrequency capacitively coupled plasma discharge afterglow has been carried out. Temporal dynamics of Ar metastable and resonance states densities are measured by the absorption method with separate capacitive discharge as a radiation source. The collision radiative model is applied to calculate the corresponding dynamics of Ar excited states in the afterglow with electron density and temperature obtained from self-consistent particle in cell simulation. Different temporal dynamics for metastable and for resonance states densities are revealed and explained. The numerical results and experimental data on Ar excited state dynamics are in good agreement with each other. The decrease in vacuum ultraviolet photon fluxes from the discharge to the substrate is shown. The possible damage free time for plasma treatment of specific dielectric film in a pulsed discharge regime is estimated.

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.

Study Of The Optimum Conditions And Parameters To Perform Simulation Of Low Temperature Capacitive Radio Frequency Argon Discharge

Capacitively coupled plasma (CCP) is used extensively in plasma processing, such as, plasma etching, deposition, and sputtering. Particle-in-Cell (PIC) is used to explore the discharge dynamics microscopically. Considering all species in PIC simulation is challenging. Also, considering various atomic transitions, e.g., ionization, recombination, excitation, and deexcitation is not doable. The state of the art is to carry out fluid simulation. We compare between PIC and fluid simulation for radio frequency CCP to reveal the effect of various approximations as assuming constant temperature, simplifying Navier-Stokes equations in terms of particles mobility and diffusivity, and considering artificial boundary conditions at the electrodes. The fluid model predicts qualitatively PIC results in few minutes. For Argon discharge in geometrically symmetric CCP, the plasma bulk is quasineutral. Over the electrodes, two sheaths are built up due to the difference between ion and electron fluxes. The dynamics of two sheaths are out of phase. In the fluid model, chemical reactions and atomic processes can be considered. Argon metastable states density is maximum in the plasma bulk. In semi dark sheaths, metastable states concentration is small. The proposed fluid model could be used as a simulation platform to find the optimum conditions and to interpret experimental results.

Micro atmospheric pressure plasma jets excited in He/O2 by voltage waveform tailoring: a study based on a numerical hybrid model and experiments

A hybrid simulation code is developed to treat electrons fully kinetically by the particle-in-cell/Monte Carlo collision (PIC/MCC) algorithm, while ions and neutral species are handled by a fluid model, including a time slicing technique to reduce the computational expenses caused by the responses of various species on different time scales. The code is used to investigate a capacitively coupled COST reference micro atmospheric pressure helium plasma jet with 0.1% oxygen admixture excited by a valley-type tailored voltage waveform with a fixed peak-to-peak voltage of 400 V, and a fundamental frequency of 13.56 MHz. The computational results are compared to experiments based on several sophisticated diagnostics, showing good agreement in the electron impact helium excitation rate, the helium metastable density, and the atomic oxygen density. The spatio-temporal electron heating dynamics, are found to be asymmetrical due to the specific shape of the driving voltage waveform. Tailoring the voltage waveform is shown to enable to control the electron energy probability function (EEPF) in distinct spatio-temporal regions of interest. As a consequence, the generation of reactive neutral species can be enhanced by increasing the number of consecutive harmonics. Based on a simplified two dimensional neutral transport model in the hybrid code, it is demonstrated that the transport between the electrodes, as well as the gas flow have different effects on various neutral species distributions due to the relevant chemical reaction rates for the generation and destruction of species.

Systematic investigation of the pulsed barrier discharges in flowing and stationary gas: From differences to similarities

Pulsed dielectric barrier discharges in flowing and stationary helium at atmospheric pressure are comparatively studied. The discharges are generated in plane-parallel configuration and driven by positive voltage pulses. The intensified charge coupled device imaging and electrical measurements are used for temporally, spatially, and spectrally resolved discharge emission. The populations of excited states are investigated by optical emission and absorption spectroscopy. The discharge operating in flowing helium is characterized by a single current pulse per each voltage flank and a “residual current peak.” In stationary helium or in flowing helium with low velocity (<10 cm/s), multiple current pulses are observed. The appearance of the subsequent current pulse(s) is related to the disappearance of the “residual current peak.” Besides these differences between the two discharge regimes, some similarities are also registered. The maximum emission intensity is reached at the cathode, even for the subsequent current pulses formed in stationary helium, confirming the generation of the glow and multi-glow discharges. Also, the most important similarities between the discharges working in stationary helium and in flowing gas mixture, in relation to flowing helium mode, are discussed. Finally, the subsequent current pulses formed in multi-glow discharges are due to re-ignition of the discharge dominantly by direct electronic excitation and ionization processes, not by Penning ionization, because of the decrease in metastable atoms density and their lifetime. In addition, the metastable lifetime reaches a critical value, for which the discharge transits from glow to multi-glow mode, when the gas velocity or its composition changes.

A study on the E-H mode transition with the line integrated He [formula omitted] metastable atom density in an inductively coupled plasma

Helium (He) inductively coupled plasma (ICP) was generated by a right-helical-type RF antenna. At a fixed He pressure of 20 mTorr, the optical emission spectrum from the He plasma was measured with RF power. The E-H mode transition and the characteristic hysteresis of the mode transition were confirmed by a change in the spectral line intensity of the 33D−23P transition at 587.6 nm. By laser absorption spectroscopy using a laser light with a wavelength of 1083 nm resonant to the 23S→23P absorption transition, the line-integrated He 23S metastable density was measured and found to show consistent behavior with the spectral line intensity of the 33D−23P transition during the E-H mode transition. The line-integrated He 23S metastable atom density exhibited distinctly different longitudinal distributions in the E and H modes.

Electron heating mode transitions in radio-frequency driven micro atmospheric pressure plasma jets in He/O2: a fluid dynamics approach

A two-dimensional fluid model is used to investigate the electron heating dynamics and the production of neutral species in a capacitively coupled radio-frequency micro atmospheric pressure helium plasma jet—specifically the COST jet—with a small oxygen admixture. Electron heating mode transitions are found to be induced by varying the driving voltage amplitude and the O2 concentration numerically and experimentally. The helium metastable density, and the charged species densities are highly relevant to the electron heating dynamics. By analyzing the creation and destruction mechanisms of the negative ions, we find that the generation of negative ions strongly depends on the O2 concentration. The increase of the electronegativity with the increasing O2 concentration leads to an enhancement of the bulk drift electric field. The distributions of the different neutral species densities along the direction of the gas flow inside the jet, as well as in the effluent differ a lot due to the relevant chemical reaction rates and the effect of the gas flow. The simulated results show that a fluid model can be an effective tool for qualitative investigations of micro atmospheric pressure plasma jets.

Optically pumped argon metastable laser with repetitively pulsed discharge in a closed chamber

The optically pumped rare gas laser (OPRGL) is an innovative type of gas laser, which has the potential to be a candidate for high-energy lasers (HELs), in which metastable rare gas atoms generated by gas discharge are used as active particles. To realize these kinds of lasers as HELs in field application, high efficiency and low cost are necessary. For this purpose, we demonstrated a closed-chamber operation with repetitively pulsed gas discharge for an OPRGL of argon metastables. The pulsed discharge was powered by a home-made LC generator, which provided a high efficiency for the deposition of electric energy. The metastable density produced was higher than 1012 cm−3, quite enough for efficient laser operation. The dependencies of laser radiation on the parameters of discharge voltage, gas pressure, and argon concentration were studied, which showed an optimized argon concentration of 1%. Long-time operations were also performed, which demonstrated the practicability of the operation of OPRGL in a closed chamber, with the avoiding of the running away of rare gases in the open cycle of flowing operation.

Ar metastable densities (3P2) in the effluent of a filamentary atmospheric pressure plasma jet with humidified feed gas

The Ar(3P2) metastable density in the effluent of the cold atmospheric pressure plasma jet kINPen was investigated as a function of the feed gas humidity, the gas curtain composition, and the distance from the nozzle by means of laser atomic absorption spectroscopy. The filamentary character of the plasma jet was considered as the absorption signals are from single individual events. From the effective lifetime, the quenching coefficient for Ar(3P2) by water was determined. A maximum Ar(3P2) density of (6.0±0.7)×1013 cm−3 was obtained close to the nozzle for a feed gas humidity of 20 ppm. The densities near the nozzle decreased inversely proportional to the water content in the feed gas leveling off at approximately 1×1013 cm−3, independently from the gas curtain composition. With the addition of water to the feed gas, the excitation dynamics changed, and the production and lifetime of Ar(3P2) was reduced. The impact of Ar(3P2) on the reactive species composition in the effluent was found to be smaller than within the plasma zone inside the plasma jet device.

Metastable helium Faraday filter for helium lidar to measure the density of the thermosphere.

We demonstrate a metastable helium Faraday optical filter operating on the 23S1 - 23P1 and 23S1 - 23P2 transition at 1083 nm by using a 3 cm long helium cell. The influence of the magnetic field and gas pressure of the helium cell on the filter characteristics is experimental studied. When the magnetic field is 230 Gs and the gas pressure of helium cell is about 110 Pa, the peak transmission corresponding to the two energy level transitions is about 32% and 57%, respectively. The equivalent noise bandwidth (ENBW) under this working condition is about 1.9 GHz. The metastable helium Faraday filter can be used to improve the optical inefficiency of a helium resonance fluorescence lidar to achieve the metastable helium density detection at 200-1000 km thermosphere.

Compact, portable, laser induced fluorescence diagnostic for laboratory plasma sources.

As diagnostic groups are increasingly called upon to participate in experimental campaigns at remote facilities, there is a need to develop portable versions of plasma diagnostic systems. One such diagnostic is laser induced fluorescence (LIF). Here, we describe a portable LIF apparatus that eliminates the need for an optical table, beam splitters, and an optical chopper. All of the light exiting the laser system is coupled through optical fibers to the experiment and housekeeping diagnostics. The collected light is coupled through an optical fiber as well. A key feature is modulation of the tapered amplifier current instead of physical modulation of the laser output. Using this portable LIF system, measurements of ion temperature, ion flow, and relative metastable ion density are reported for two different remote experiments.

Argon/dust and pure argon pulsed plasmas explored using a spatially-averaged model

The properties (densities of electrons and metastable argon atoms, effective electron temperature and dust charge) of argon/dust and pure argon pulsed plasmas are studied using a spatially-averaged model. The calculated time-dependencies for the densities of electrons and metastable atoms are compared with the experimental measurements and are found to be in a good qualitative agreement. It is analyzed how the plasma properties depend on the shape of the electron energy probability function (EEPF), the pulsing frequency and the duty cycle for both dust-free and dusty plasma. The analysis reveals that the agreement between theory and experiment is better with Druyvesteyn EEPF than the Maxwellian EEPF. Further, the variation in the pulsing frequency ν p differently affects the metastable density n m in a dust-free and in a dusty plasma. For large ν p, the metastable density in the dust-free pulsed plasma is larger than in the continuous-wave (CW) discharge, while the opposite is obtained in the presence of dust particles. This difference probably arises because of faster variation in the effective electron temperature in the dusty plasma due to collection of electrons by dust particles. Our calculations also show that dust particles may affect the behavior of electron density in the beginning of the on-period due to an enhancement in electron collection by dust particles.

Admittance spectroscopy on single-crystal Cu2ZnSnSe4 solar cells: Back-contact effects and metastabilities

Admittance spectroscopy has become a commonly used device-level technique to probe the defect structure of kesterite materials. While this technique holds promise, phenomena such as current barriers and metastabilities cause difficulty in the interpretation of results. In this work, devices fabricated on single-crystalline CZTSe absorber layers are used to explore these effects in a more idealized system than thin films. Differences in behavior between admittance signatures of absorbers with Cu/Zn + Sn ratios of 0.86 and 0.77 are observed and mainly attributed to the formation of a low-temperature current barrier at the CZTSe/carbon-based back-contact interface in the more Cu-depleted devices. The effect of this barrier is significantly reduced by the implementation of higher-work function Au or MoO3−x back-contact material. Furthermore, distinct metastable doping densities are observed in each device type with the implementation of 3-h external bias conditioning of white, blue, or red light as well as 1 V applied forward bias. The origin of this behavior is attributed to a VSe–VCu defect complex that is predicted to exist in related chalcogenide materials.

Spectroscopic investigation of a neon-operated DBD at atmospheric and intermediate pressure

A sinusoidally-excited Venturi-DBD operating in neon has been investigated. The Ne(1s5) metastable density has been quantified spatially resolved using laser atomic absorption spectroscopy for different pressure levels. Density values of up to 7 · 1016 m−3 could be determined at atmospheric pressure and up to 3 · 1016 m−3 at 100 mbar. For all investigated parameters, the Ne(1s5) density was found to be distinctly higher in the proximity of the cathode than in the anode region. Complementary investigations of the discharge development using phase-resolved optical emission spectroscopy complete the characterization of the device. The discharge was found to show typical properties of a glow-like discharge regarding current waveform and luminosity distribution. In addition, the influence of nitrogen and oxygen impurities and admixtures in the process gas has been determined. A substantial impact was found on both the Ne(1s5) concentration and the current waveform.

Experimental investigation of a ns-pulsed argon plasma jet for the fast desorption of weakly volatile organic compounds deposited on glass substrates at variable electric potential

A ns-pulsed argon atmospheric pressure plasma jet (APPJ) was studied for the fast desorption of weakly volatile organic molecules (bibenzyl) deposited on glass substrates at variable electric potential (floating-potential or grounded). The experiments focused particularly on thin resistant bibenzyl films (a large thick bibenzyl deposit was also studied), which are more difficult to be desorbed when using a substrate at a floating-potential. The APPJ was probed by means of high-resolution laser absorption spectroscopy to map the Ar(1s5) metastable absolute density (spatially and temporally resolved) at the close vicinity of the glass plate where bibenzyl was deposited. Furthermore, the electrical, optical and thermal features of the APPJ were investigated systematically. In this way, the plasma desorption efficacy on thin resistant bibenzyl deposits was evaluated for the envisaged application, by varying the exposure time of the molecules to the APPJ (texp, from 10 s up to 180 s). The obtained results confirm the relatively low desorption efficacy in the case of a floating-potential substrate, which improves to some extent with increasing texp. However, when the substrate is grounded, the effect of the plasma becomes much more significant (i.e. much higher desorption efficacy). Besides, contrary to the case of a floating-potential substrate, an almost complete desorption of bibenzyl is achieved for texp = 180 s. Similar effects of the APPJ were recorded on a thick bibenzyl deposit, validating the previous results. For both operating conditions (floating-potential and grounded substrate), the plasma action should be due to the production close to the glass surface of relatively high densities of Ar(1s5) (up to 2 1013 cm−3) and of oxidative species, such as atomic oxygen, hydroxyl radical and ozone. Thermal effects might play a synergistic role only when the substrate is grounded, since relatively high gas and glass-surface temperatures (>60 °C) are reached only in this case. The present results are of interest for public-security applications related to the fast detection of resistant prohibited substances, such as narcotics and explosives.

Steady-State Model of an Argon–Helium High-Pressure Radio Frequency Dielectric Barrier Discharge

A steady-state kinetic model of argon–helium high-pressure radio frequency dielectric barrier discharge is developed using a simplified reaction rate package appropriate for pressures ranging from 200 to 500 torr. Electron production and loss rates are analyzed as a function of pressure and Ar–He mixture, highlighting the importance of the Ar2* dimer for higher pressures and Ar rich mixtures. As pressure increases, the primary ionization mechanism shifts between ionization of ground state Ar to dimer ionization, which affects the steady-state reduced electric field due to the electron energy required for the two dominant ionization mechanisms. The resulting reduced electric field profile controls the metastable Ar(1s5) production, which, combined with the Ar(1s5) loss rates, reveals the metastable density as a function of pressure and Ar-fraction. This simplified 0-D steady-state model shows close agreement with previous time-dependent simulations using a robust reaction rate package.

Maximizing optical production of metastable xenon.

The wide range of applications using metastable noble gas atoms has led to a number of different approaches for producing large metastable state densities. Here we investigate a recently proposed hybrid approach that combines RF discharge techniques with optical pumping from an auxiliary state in xenon. We study the effect of xenon pressure on establishing initial population in both the auxiliary state and metastable state via the RF discharge, and the role of the optical pumping beam power in transferring population between the states. We find experimental conditions that maximize the effects, and provide a robust platform for producing relatively large long-term metastable state densities.

Influence of metastable atoms in low pressure magnetized radio-frequency argon discharges

One-dimensional particle-in-cell simulations with Monte Carlo collisions are used to investigate the influence of metastable atoms in low pressure radio-frequency argon discharges with magnetic fields ranging from 0 G to 60 G. Two metastable levels of argon species are included and tracked as particles, enabling multistep ionization and metastable pooling. At low magnetic fields, the metastable argon atoms have little influence on the discharge. At higher magnetic fields, the electron density increases and the electron temperature decreases at the center of the discharge with the inclusion of metastable atoms. The reduction in electron temperature is attributed to the depletion of energetic electrons due to low-energy-threshold reactions related to the metastable atoms. The reduced electron temperature leads to a reduction in the total ionization rate, albeit the contribution of multistep ionization increases with the magnetic field. The suppression of plasma diffusion at low electron temperatures plays a greater role than the reduction in ionization rate, resulting in a higher electron density. A transformation in metastable density profiles from parabolic to saddle type is observed with the increase in the magnetic field. Metastable atoms may play an important role in modulating the electron temperature in low pressure magnetized discharges.