Investigation of E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma

In this paper, E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction(0%–100%), operating pressure(1 Pa, 5 Pa, 10 Pa and 50 Pa), and radio frequency(RF) power(5–100 W). An RF compensated Langmuir probe and optical emission spectroscopy are used for the diagnostics of the plasma under study. Owing to the lower ionization potential and higher collision cross-section of argon, when its fraction in the discharge is increased, the mode transition occurs at lower RF power; i.e. for 0% argon and1 Pa pressure, the threshold power of the E–H mode transition is 65 W, which reduces to 20 W when the argon fraction is increased. The electron density increases with the argon fraction at afixed pressure, whereas the temperature decreases with the argon fraction. The relaxation length of the low-energy electrons increases, and decreases for high-energy electrons with argon fraction, due to the Ramseur effect. However, the relaxation length of both groups of electrons decreases with pressure due to reduction in the mean free path. The electron energy probability function(EEPF) profiles are non-Maxwellian in E-mode, attributable to the nonlocal electron kinetics in this mode; however, they evolve to Maxwellian distribution when the discharge transforms to H-mode due to lower electron temperature and higher electron density in H-mode. The tail of the measured EEPFs is found to deplete in both E-and H-modes when the argon fraction in the discharge is increased, because argon has a much lower excitation potential(11.5 eV) than neon(16.6 eV).     

Experimental and simulation investigation of electrical and plasma parameters in a low pressure inductively coupled argon plasma

The electrical and plasma parameters of a low pressure inductively coupled argon plasma are investigated over a wide range of parameters(RF power, flow rate and pressure) by diverse characterizations. The external antenna voltage and current increase with the augment of RF power, whereas decline with the enhancement of gas pressure and flow rate conversely.Compared with gas flow rate and pressure, the power transfer efficiency is significantly improved by RF power, and achieved its maximum value of 0.85 after RF power injected excess125 W. Optical emission spectroscopy(OES) provides the local mean values of electron excited temperature and electron density in inductively coupled plasma(ICP) post regime, which vary in a range of 0.81 eV to 1.15 eV and 3.7×10~(16)m~(-3)to 8.7×10~(17)m~(-3)respectively. Numerical results of the average magnitudes of electron temperature and electron density in twodimensional distribution exhibit similar variation trend with the experimental results under different operating condition by using COMSOL Multiphysics. By comprehensively understanding the characteristics in a low pressure ICP, optimized operating conditions could be anticipated aiming at different academic and industrial applications.     

Characteristics of Single Cathode Cascaded Bias Voltage Arc Plasma

A single cathode with a cascaded bias voltage arc plasma source has been developed with a new quartz cathode chamber,instead of the previous copper chambers,to provide better diagnostic observation and access to the plasma optical emission.The cathode chamber cooling scheme is also modified to be naturally cooled only by light emission without cooling water to improve the optical thin performance in the optical path.A single-parameter physical model has been developed to describe the power dissipated in the cascaded bias voltage arc discharge argon plasmas,which have been investigated by utilizing optical emission spectroscopy(OES) and Langmuir probe.In the experiments,discharge currents from 50 A to 100 A,argon flow rates from 800 sccm to 2000 sccm and magnetic fields of 0.1 T and 0.2 T were chosen.The results show:(a) the relationship between the averaged resistivity and the averaged current density exhibits an empirical scaling law as η∝ j~(-0.63369) and the power dissipated in the arc has a strong relation with the filling factor;(b) through the quartz,the argon ions optical emission lines have been easily observed and are dominating with wavelengths between 340 nm and 520 nm,which are the emissions of Ar~+-434.81 nm and Ar~+-442.60 nm line,and theintensities are increasing with the arc current and decreasing with the inlet argon flow rate;and(c) the electron density and temperature can reach 2.0 × 10~(19) m~(-3) and 0.48 eV,respectively,under the conditions of an arc current of 90 A and a magnetic field of 0.2 T.The half-width of the n_e radial profile is approximatively equal to a few Larmor radii of electrons and can be regarded as the diameter of the plasma jet in the experiments.     

Nonequilibrium Atmospheric Pressure Ar/O2 Plasma Jet: Properties and Application to Surface Cleaning

In this study an atmospheric pressure Ar/O_2 plasma jet is generated to study the effects of applied voltage and gas flux rate to the behavior of discharge and the metal surface cleaning.The increase in applied voltage leads to increases of the root mean square(rms) current,the input power and the gas temperature.Furthermore,the optical emission spectra show that the emission intensities of metastable argon and atomic oxygen increase with increasing applied voltage.However,the increase in gas flux rate leads to a reduction of the rms current,the input power and the gas temperature.Furthermore,the emission intensities of metastable argon and atomic oxygen decrease when gas flux rate increases.Contact angles are measured to estimate the cleaning performance,and the results show that the increase of applied voltage can improve the cleaning performance.Nevertheless,the increase of gas flux rate cannot improve the cleaning performance.Contact angles are compared for different input powers and gas flux rates to search for a better understanding of the major mechanism for surface cleaning by plasma jets.     

Preliminary results of optical emission spectroscopy by line ratio method in the RF negative ion source at ASIPP

Optical emission spectroscopy (OES) using the trace rare gases of Ar and Xe have been carried out in a radio frequency (RF) driven negative ion source at Institute of Plasma Physics, Chinese Academy of Science (ASIPP), in order to determine the electron temperature and density of the hydrogen plasma. The line-ratio methods based on population models are applied to describe the radiation process of the excited state particles and establish their relations with the plasma parameters. The spectral lines from the argon and xenon excited state atoms with the wavelength of 750.4 and 828.0 nm are used to calculate the electron temperature based on the corona model. The argon ions emission lines with the wavelength of 480 and 488 nm are selected to calculate the electron density based on the collisional radiative model. OES has given the preliminary results of the electron temperature and density by varying the discharge gas pressure and RF power. According to the experimental results, the typical plasma parameters is T_e ≈ 2–4 eV and n_e ≈ 1×10 ¹⁷– 8×10¹⁷ m⁻³ in front of plasma grid.     

Volume Diffuse Dielectric Barrier Discharge Plasma Produced by Nanosecond High Voltage Pulse in Airflow

Volume diffuse dielectric barrier discharge (DBD) plasma is produced in subsonic airflow by nanosecond high-voltage pulse power supply with a plate-to-plate discharge cell at 6 mm air gap length.The discharge images,optical emission spectra (OES),the applied voltage and current waveforms of the discharge at the changed airflow rates are obtained.When airflow rate is increased,the transition of the discharge mode and the variations of discharge intensity,breakdown characteristics and the temperature of the discharge plasma are investigated.The results show that the discharge becomes more diffuse,discharge intensity is decreased accompanied by the increased breakdown voltage and time lag,and the temperature of the discharge plasma reduces when airflow of small vclocity is introduced into the discharge gap.These phenomena are because that the airflow changes the spatial distribution of the heat and the space charge in the discharge gap.     

Comparative Study of Plasma Parameters in Magnetic Pole Enhanced Inductively Coupled Argon Plasmas

Langmuir probe measurements of radio frequency (RF) magnetic pole enhanced inductively coupled (MaPE-ICP) argon plasma were accomplished to obtain the electron number densities and electron temperatures. The measurements were carried out with a fixed RF frequency of 13.56 MHz in a pressure range of 7.5 mTorr to 75 mTorr at an applied RF power of 10 W and 100 W. These results are compared with a global (volume average) model. The results show good agreement between theoretical and experimental measurements. The electron number density shows an increasing trend with both RF power and pressure while the electron temperature shows decreasing trend as the pressure increases. The difference in the plasma potential and floating potential as a function of electron temperature measured from the electrical probe and that obtained theoretically shows a linear relation with a small difference in the coefficient of proportionality. The intensity of the emission line at 750.4 nm due to 2p 1 → 1s 2 (Paschen’s notation) transition closely follows the variation of n e with RF power and filling gas pressure. Measured electron energy probability function (EEPF) shows that electron occupation changes mostly in the high-energy tail, which highlights close similarity of 750.4 nm argon line to n e .     

Preliminary Study of a Hybrid Helicon-ECR Plasma Source

A new type of hybrid discharge is experimentally investigated in this work. A helicon source and an electron cyclotron resonance(ECR) source were combined to produce plasma. As a preliminary study of this type of plasma, the optical emission spectroscopy(OES) method was used to obtain values of electron temperature and density under a series of typical conditions. Generally,it was observed that the electron temperature decreases and the electron density increases as the pressure increased. When increasing the applied power at a certain pressure, the average electron density at certain positions in the discharge does not increase significantly possibly due to the high degree of neutral depletion. Electron temperature increased with power in the hybrid mode.Possible mechanisms of these preliminary observations are discussed.     

Trace-Rare-Gas Optical Emission Spectroscopy of Nitrogen Plasma Generated at a Frequency of 13.56 MHz

Optical emission spectroscopic measurement of trace rare gas is carried out to determine the density of nitrogen (N) atom, in a nitrogen plasma, as a function of filling pressure and RF power applied. 2% of argon, used as an actinometer, is mixed with nitrogen. In order to normalize the changes in the excitation cross section and electron energy distribution function at different operational conditions, the Ar-I emission line at 419.83 nm is used, which is of nearly the same excitation efficiency coefficients as that of the nitrogen emission line at 493.51 nm. It is observed that the emission intensity of the selected argon and atomic nitrogen lines increases with both pressure and RF power, as does the nitrogen atomic density.     

Effects of O2 addition on the plasma uniformity and reactivity of Ar DBD excited by ns pulsed and AC power supplies

Dielectric barrier discharges (DBDs) have been widely used in ozone synthesis, materials surface treatment, and plasma medicine for their advantages of uniform discharge and high plasma-chemical reactivity. To improve the reactivity of DBDs, in this work, the O₂ is added into Ar nanosecond (ns) pulsed and AC DBDs. The uniformity and discharge characteristics of Ar ns pulsed and AC DBDs with different O₂ contents are investigated with optical and electrical diagnosis methods. The DBD uniformity is quantitatively analyzed by gray value standard deviation method. The electrical parameters are extracted from voltage and current waveforms separation to characterize the discharge processes and calculate electron density n_e. The optical emission spectroscopy is measured to show the plasma reactivity and calculate the trend of electron temperature T_e with the ratio of two emission lines. It is found that the ns pulsed DBD has a much better uniformity than AC DBD for the fast rising and falling time. With the addition of O₂, the uniformity of ns pulsed DBD gets worse for the space electric field distortion by O₂⁻, which promotes the filamentary formation. While, in AC DBD, the added O₂ can reduce the intensity of filaments, which enhances the discharge uniformity. The ns pulsed DBD has a much higher instantaneous power and energy efficiency than AC DBD. The ratio of Ar emission intensities indicates that the T_e drops quickly with the addition of O₂ both ns pulsed and AC DBDs and the ns pulsed DBD has an obvious higher T_e and n_e than AC DBD. The results are helpful for the realization of the reactive and uniform low temperature plasma sources.     

Axial profiles of argon helicon plasma by optical emission spectroscope and Langmuir probe

We present the axial profiles of argon helicon plasma measured by a local optical emission spectroscope (OES) and Langmuir RF-compensated probe. The results show that the emission intensity of the argon atom lines (750 nm, 811 nm) is proportional to the plasma density determined by the Langmuir probe. The axial profile of helicon plasma depends on the discharge mode which changes with the RF power. Excited by helical antenna, the axial distribution of plasma density is similar to that of the external magnetic field in the capacitive coupled mode (E-mode). As the discharge mode changes into the inductively coupled mode (H-mode), the axial distribution of plasma density in the downstream can still be similar to that of the external magnetic field, but becomes more uniform in the upstream. When the discharge entered wave coupled mode (W-mode), the plasma becomes nearly uniform along the axis, showing a completely different profile from the magnetic field. The W-mode is expected to be a mixed pattern of helicon (H) and Trivelpiece-Gould (TG) waves.     

Electron population properties with different energies in a helicon plasma source

The characteristics of electrons play a dominant role in determining the ionization and acceleration processes of plasmas. Compared with electrostatic diagnostics, the optical method is independent of the radio frequency(RF) noise, magnetic field, and electric field. In this paper, an optical emission spectroscope was used to determine the plasma emission spectra, electron excitation energy population distributions(EEEPDs), growth rates of low-energy and highenergy electrons, and their intensity jumps with input powers. The 56 emission lines with the highest signal-to-noise ratio and their corresponding electron excitation energy were used for the translation of the spectrum into EEEPD. One discrete EEEPD has two clear different regions,namely the low-energy electron excitation region(neutral lines with threshold energy of13–15 eV) and the high-energy electron excitation region(ionic lines with threshold energy?19 e V). The EEEPD variations with different diameters of discharge tubes(20 mm, 40 mm,and 60 mm) and different input RF powers(200–1800 W) were investigated. By normalized intensity comparison of the ionic and neutral lines, the growth rate of the ionic population was higher than the neutral one, especially when the tube diameter was less than 40 mm and the input power was higher than 1000 W. Moreover, we found that the intensities of low-energy electrons and high-energy electrons jump at different input powers from inductively coupled(H) mode to helicon(W) mode; therefore, the determination of W mode needs to be carefully considered.     

Plasma characteristics of direct current enhanced cylindrical inductively coupled plasma source

Experimental results of a direct current enhanced inductively coupled plasma (DCE-ICP) source which consists of a typical cylindrical ICP source and a plate-to-grid DC electrode are reported.With the use of this new source,the plasma characteristic parameters,namely,electron density,electron temperature and plasma uniformity,are measured by Langmuir floating double probe.It is found that DC discharge enhances the electron density and decreases the electron temperature,dramatically.Moreover,the plasma uniformity is obviously improved with the operation of DC and radio frequency (RF) hybrid discharge.Furthermore,the nonlinear enhancement effect of electron density with DC + RF hybrid discharge is confirmed.The presented observation indicates that the DCE-ICP source provides an effective method to obtain high-density uniform plasma,which is desirable for practical industrial applications.     

Comparison of heating mechanisms of argon helicon plasma in different wave modes with and without blue core

In this work,we investigated the discharge characteristics and heating mechanisms of argon helicon plasma in different wave coupled modes with and without blue core.Spatially resolved spectroscopy and emission intensity of argon atom and ion lines were measured via local optical emission spectroscopy,and electron density was measured experimentally by an RFcompensated Langmuir probe.The relation between the emission intensity and the electron density was obtained and the wavenumbers of helicon a...     

Comparison of double layer in argon helicon plasma and magnetized DC discharge plasma

We present in this paper the comparison of an electric double layer (DL) in argon helicon plasma and magnetized direct current (DC) discharge plasma. DL in high-density argon helicon plasma of 13.56 MHz RF discharge was investigated experimentally by a floating electrostatic probe and local optical emission spectroscopy (LOES). The DL characteristics at different operating parameters, including RF power (300–1500 W), tube diameter (8–60 mm), and external magnetic field (0–300 G), were measured. For comparison, DL in magnetized plasma channel of a DC discharge under different conditions was also measured experimentally. The results show that in both cases, DL appears in a divergent magnetic field where the magnetic field gradient is the largest and when the plasma density is sufficiently high. DL strength (or potential drop of DL) increases with the magnetic field in two different structures. It is suggested that the electric DL should be a common phenomenon in dense plasma under a gradient external magnetic field. DL in magnetized plasmas can be controlled properly by magnetic field structure and discharge mode (hence the plasma density).     

Numerical simulation and experimental research on an inductively coupled RF plasma cathode

In this study, numerical simulation and discharge current tests were conducted on an inductively coupled radio frequency (RF) plasma cathode. Numerical simulations and experimental measurements were performed to study the factors influencing the electron extraction characteristics, including the gas type, gas flow, input power and extracting voltage. The simulation results were approximately consistent with the experimental results. We experimentally found that the RF input power mainly determines the extracted electron current. An electron current greater than 1 A was acquired at 270 W (RF input power), 2.766 sccm (xenon gas). Our results prove that an inductively coupled RF plasma cathode can be reasonable and feasible, particularly for low power electric propulsion devices.     

Characterization of a microsecond pulsed non-equilibrium atmospheric pressure Ar plasma using laser scattering and optical emission spectroscopy

A non-equilibrium atmospheric pressure argon(Ar) plasma excited by microsecond pulse is studied experimentally by laser scattering and optical emission spectroscopy(OES), and theoretically by collisional-radiative(CR) model. More specifically, the electron temperature and electron density of plasma are obtained directly by the laser Thomson scattering, the gas temperature is measured by laser Raman scattering, the optical emissions of excited Ar states of plasma are measured by OES. The laser scattering results show that the electron temperature is about 1 eV which is similar to that excited by 60 Hz AC power, but the gas temperature is as low as 300 K compared to about 700 K excited by 60 Hz AC power. It is shown that the microsecond pulsed power supply, rather than nanosecond ones, is short enough to reduce the gas temperature of atmospheric pressure plasma to near room temperature. The electron temperature and electron density are also obtained by CR model based on OES, and find that the intensities of the optical emission intensity lines of 727.41, 811.73, 841.08, 842.83, 852.44 and 912.86 nm of Ar can be used to characterize the behavior of electron density and electron temperature, it is very useful to quickly estimate the activity of the atmospheric pressure Ar plasma in many applications.     

Asphaltene Erosion Process in Air Plasma: Emission Spectroscopy and Surface Analysis for Air-Plasma Reactions

Optical emission spectroscopy (OES) was applied for plasma characterization during the erosion of asphaltene substrates. An amount of 100 mg of asphaltene was carefully applied to an electrode and exposed to air-plasma glow discharge at a pressure of 1.0 Torr. The plasma was generated in a stainless steel discharge chamber by an ac generator at a frequency of 60 Hz, output power of 50 W and a gas flow rate of 1.8 L/min. The electron temperature and ion density were estimated to be 2.15±0.11 eV and (1.24±0.05)×1016 m-3 , respectively, using a double Langmuir probe. OES was employed to observe the emission from the asphaltene exposed to air plasma. Both molecular band emission from N2 , N+2 , OH, CH, NH, O2 as well as CN, and atomic light emission from V and Hγ were observed and used to monitor the evolution of asphaltene erosion. The asphaltene erosion was analyzed with the aid of a scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) detector. The EDX analysis showed that the time evolution of elements C, O, S and V were similar; and the chemical composition of the exposed asphaltenes remained constant. Particle size evolution was measured, showing a maximum size of 2307 μm after 60 min. This behavior is most likely related to particle agglomeration as a function of time.     

Diagnostic of capacitively coupled radio frequency plasma from electrical discharge characteristics: comparison with optical emission spectroscopy and fluid model simulation

The capacitively coupled radio frequency(CCRF)plasma has been widely used in various fields.In some cases,it requires us to estimate the range of key plasma parameters simpler and quicker in order to understand the behavior in plasma.In this paper,a glass vacuum chamber and a pair of plate electrodes were designed and fabricated,using 13.56 MHz radio frequency(RF)discharge technology to ionize the working gas of Ar.This discharge was mathematically described with equivalent circuit model.The discharge voltage and current of the plasma were measured atdifferent pressures and different powers.Based on the capacitively coupled homogeneous discharge model,the equivalent circuit and the analytical formula were established.The plasma density and temperature were calculated by using the equivalent impedance principle and energy balance equation.The experimental results show that when RF discharge power is 50–300 W and pressure is 25–250 Pa,the average electron temperature is about 1.7–2.1 e V and the average electron density is about 0.5?×?10~(17)–3.6?×?10~(17)m~(-3).Agreement was found when the results were compared to those given by optical emission spectroscopy and COMSOL simulation.     

Study of the Characteristics of DC and ICP Hybrid Discharge Plasmas

In this paper,the double-discharge plasma generated by radio frequency(RF) and direct current(DC) has been investigated.In comparison with their single-frequency counterpart,the interaction between the two excitations is significant and beneficial.The results show that the RF discharge can effectively increase the DC discharge current and decrease the DC voltage;meanwhile the DC discharge is favorable to feed abundant high energy seed electrons to the ICP discharge sustaining at 13.56 MHz for the latter to acquire higher plasma density and lower plasma potential by increasing the ionization rate.The innovative design has been demonstrated to facilitate more homogeneous performance with higher plasma density.