Two-photon Absorption Laser-induced Fluorescence Research Articles

Temporally resolved relative krypton neutral density during breathing mode of a hall effect thruster recorded by TALIF

A key issue in the development of theory and models for plasma propulsion devices is to describe the instabilities and fluctuations of the devices. It has been widely recognized that many Hall effect thrusters (HETs) exhibit oscillations at frequencies in the range of ∼ 20 kHz. These ionization-related oscillations are generally referred to as Breathing Mode oscillations and have been the subject of considerable research. Here, for the first time, we report direct temporally resolved measurements of the ground state neutral density variation during the period of the oscillation. We used the laser-based Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique to measure neutrals within the plume of a 1.5 kW HET operating on krypton (Kr). Our TALIF scheme employs a frequency-doubled, pulsed dye laser operating at ∼ 212 nm to probe ground state Kr atoms. A novel phase-binning approach is used to recover the time-dependent signal by assigning the timing of each collected TALIF signal (laser shot) relative to the phase of the discharge current. We find that the neutral density fluctuates quite strongly over the period of the oscillation, and that this fluctuation leads the current fluctuation as expected.

Enhancing selective production of atomic oxygen through nanosecond pulse-burst driven mode in a He/O2 dielectric barrier discharge

In this study, the temporal evolution of O atoms in a nanosecond burst-pulsed dielectric barrier discharge (DBD) is measured by two-photon absorption laser-induced fluorescence spectroscopy. The experiment is conducted at burst conditions of 50, 100, and 200 kHz pulse frequency, 10 Hz burst frequency, and 20–400 pulses in 0.1%–2% O2 + He mixtures. The accumulation effect of O atoms in the burst mode is observed and the density gradually saturates at around 100 pulses. Increasing the pulse frequency effectively enhances the O saturation density. The 0-dimensional kinetic model reveals that the saturation effect is primarily balanced by the formation and loss characteristics of O atoms. Similar saturation effect is also observed in the typical continuous periodic pulse mode (one pulse each cycle), but with a saturation density about one order of magnitude lower than that in the burst case, highlighting the burst excitation mode as an effective method for enhancing the instantaneous peak production of O atoms. Further investigations into the influence of O2 proportion on the selective production of O atoms are also performed. The results suggest that a low O2 proportion (<2%) and pulse-burst driven mode for the He/O2 DBD facilitates the selective production of O atoms while competing with O3 formation.

Spatially resolved TALIF investigation of atomic oxygen in the effluent of a CO2 microwave discharge

A diagnostic setup for one-dimensionally spatially resolved two-photon absorption laser-induced fluorescence (TALIF) detection of ground state oxygen atoms ( 2p43P2,1,0 ) is developed. The goal of this study is to investigate the evolution of temperatures and absolute number densities of oxygen atoms along the effluent of a low-pressure CO2 microwave discharge in order to gain insights into some of the mechanisms governing the post-discharge regime. The plasma source is operated at conditions of 600 W– 1200 W of absorbed power with flow rates of 74 sccm and 370 sccm pure CO2 at pressures between 1.2 mbar and 5 mbar with specific energy inputs up to 111.9 eV/molecule. These operating conditions exhibit high CO2 conversions (up to 90%) at low energy efficiencies (2%–7.4%), due to direct electron impact dissociation driving the conversion process resulting in splitting of CO2 into CO and metastable oxygen atoms. The TALIF measurements yield spatially resolved translational temperatures between 1000 K– 1600 K for most operating conditions and axial positions along the effluent. Reference measurements with xenon 6p′[3/2]2 are used for absolute number density calibration. The resulting axially resolved number density profiles of ground state atomic oxygen increase along the effluent, even at considerable distances of several centimeters from the active discharge, before they reach a maximum between 5×1020 m−3 and 2.2×1021 m−3 depending on the condition, and decrease after that. This behavior indicates the potential significance of quenching of metastable oxygen atoms within the post-discharge regime of the investigated CO2 discharges. The measured spatially resolved number density evolutions are qualitatively consistent with quenching via wall collisions being the dominant deactivation mechanism, underlining the importance of particle-wall interactions.

Absolute atomic nitrogen density spatial mapping in three MHCD configurations

In this work, nanosecond two-photon absorption laser induced fluorescence (TALIF) is used to perform spatial mappings of the absolute density of nitrogen atoms generated in a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, with a DC discharge current of 1.6 mA and the plasma is ignited in a 20% Ar/ 80% N2 gas mixture. A 1-inch diameter aluminum substrate, acting as a third electrode (second anode), is placed further away from the MHCD to emulate a deposition substrate. The spatial profile of the N atoms is measured in three MHCD configurations. First, we study a MHCD having the same pressure (50 mbar) on both sides of the anode/cathode electrodes and the N atoms simply diffuse in three dimensions from the MHCD. The recorded N atoms density profile in this case satisfies our expectations, i.e. the maximal density is found at the axis of the hole, close to the MHCD. However, when we introduce a pressure differential, thus creating a plasma jet, an unexpected N atoms distribution is measured with maximum densities away from the jet axis. This behavior cannot be simply explained by the TALIF measurements. Then, as a first simplified approach in this work, we turn our attention to the role of the gas flow pattern. Compressible gas flow simulations show a correlation between the jet width and the radial distribution of the N atoms at different axial distances from the gap. Finally, a DC positive voltage is applied to the third electrode (second anode), which ignites a micro cathode sustained discharge (MCSD). The presence of the pressure differential unveils two stable working regimes depending on the current repartition between the two anodes. The MCSD enables an homogenization of the density profile along the surface of the substrate, which is suitable for nitride deposition applications.

Absolute calibration of the ratio of Xe/O two-photon absorption cross-sections for O-TALIF applications

The paper presents a calibration of the ratio of two-photon absorption cross-sections, σXe(2)/σO(2) , necessary for the absolute O-atom density measurements by two-photon absorption laser-induced fluorescence (TALIF) technique. To calibrate the ratio of the cross-sections, a special discharge with 100% dissociation of molecular oxygen, and so with a known ‘reference’ density of O-atom [O] ref=2⋅ [O2] was suggested. This is a nanosecond capillary discharge in N2:O2 mixtures with a few percent of oxygen at a reduced electric field of a few hundred of Townsend and specific deposited energy of about 1 eV mol−1. Voltage at the electrodes, electrical current in the plasma, longitudinal electric field and energy delivered to the gas were measured with 0.2 ns synchronisation. Additionally, radial distribution of emission of excited nitrogen molecules and gas temperature in the discharge and afterglow were obtained experimentally. Detailed 1D kinetic modeling was suggested to confirm complete O2 dissociation and to analyse the main reactions. By comparing the data measured by TALIF technique with the ‘reference’ density of oxygen atoms [O] ref , the ratio of the two-photon absorption cross-sections σXe(2)/σO(2) was determined.

Concentration measurements of atomic nitrogen in an atmospheric-pressure RF plasma jet using a picosecond TALIF

The absolute concentration and spatial distribution of ground-state atomic nitrogen (N) in an atmospheric pressure plasma jet were measured using the two-photon absorption laser-induced fluorescence. The jet was ignited by radio frequency voltage in argon (or argon with nitrogen admixture) flowing through a silica tube. The spatially resolved measurements of atomic nitrogen concentration were realized in the effluent of the jet. In a pure argon plasma, the N concentration was increased with the distance from the silica tube and reached the maximum value ( 3.5⋅1014 cm−3) at the distance of 15 mm, and then sharply decreased at the end of the plume. On the contrary, plasma ignited in Ar with nitrogen admixture, the maximum N concentration was located directly at the end of the silica tube, where plasma starts to blow out into the ambient air. The highest N concentrations for 0.5% and 2% of N2 in the feed gas were 1.3⋅1015 cm−3 and 4⋅1015 cm−3, respectively.

‘Self-calibration’ method for TALIF measurement of O atoms by full photofragmentation of O3 using a 226 nm UV laser

Quantification of atomic oxygen through the method of two-photon absorption laser-induced fluorescence (TALIF) is common in the fields of plasma fundamental research and application treatments. Fluorescence signal calibration is required to absolutely quantify the O amount and normally achieved with the help of TALIF measurement of a known-density Xe gas. In this study, an alternative calibration method is proposed based on the full photofragmentation (FPF) of O3 with a known density in a known gas composition by the same UV laser beam as that of the TALIF detection of atomic O. This is achieved by an equivalent amount of O fragment contributing the same fluorescence intensity as that of the O3 FPF-TALIF process under the same experimental conditions. The validity of this calibration method is proved by comparing it to the TALIF measurement of the Xe gas. It provides a ‘self-calibration’ method for the TALIF detection of O atoms without any need to change the laser optical arrangements including the laser wavelength. In addition, it only requires a gas flow with known O3 density through the studied medium reactor or chamber (such as plasma discharges). Detailed theoretical and practical principles of this self-calibration approach are presented and discussed in this study.

Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities

Terahertz (THz) absorption spectroscopy has recently been developed as a diagnostic technique for measuring absolute ground-state atomic oxygen densities in plasmas. To demonstrate the validity of this approach, we present in this Letter a benchmark against a more established method. Atomic oxygen densities were measured with THz absorption spectroscopy and compared to those obtained from picosecond (ps) two-photon absorption laser induced fluorescence (TALIF) measurements on the same capacitively coupled radio frequency oxygen discharge. Similar changes in the atomic oxygen density were observed with both diagnostics when varying the applied power (20–100 W) and the gas pressure (0.7–1.3 mbar). Quantitatively, the results are in good agreement as well, especially when considering the total margin of error of the two diagnostics. For example, for a gas pressure of 1.3 mbar and an applied power of 30 W, atomic oxygen densities measured with THz absorption spectroscopy and TALIF were (7.0 ± 1.7)×1014 cm−3 and (5.3 ± 3.2)×1014 cm−3, respectively. This shows that THz absorption spectroscopy is an accurate technique that can be reliably used for real-world applications to determine atomic oxygen densities in plasmas.

Nitriding Effects of Ammonia Flames on Iron-based Metal Walls

Effects of NH3/O2/N2 premixed flame/mixture on iron-based metal walls have been investigated experimentally. A premixed NH3/O2/N2 flame or mixture formed by a quartz tube burner impinged on the disc-shaped metal test plate, of which the temperature was kept at 550 oC by an infrared lamp heater. As materials for the test plate, SACM645, SUS304 and SUS310S, which are widely used as industrial steels, were examined. After being exposed to either the NH3/O2/N2 flame or mixture for 5 hours, the surface hardness was measured by using a nano indenter. Distributions of nitrogen atom diffused into the metal wall were measured by means of the Electron Probe Micro Analyzer (EPMA) and Secondary Ion Mass Spectrometry (SIMS). In addition, spatial distributions of the NH3 concentration in the NH3/O2/N2 flame and mixture were measured by employing a two-photon absorption laser-induced fluorescence (TALIF) technique. It was found that the surface hardness of the test plates increased significantly after being exposed to the NH3/O2/N2 flame/mixture. The distributions of surface hardness and nitrogen atom concentration show that nitriding occurred by the present NH3/O2/N2 flame/mixture with the equivalent level of the conventional gas nitriding method. Moreover, it is also found that NH3 concentration in the vicinity of the wall plays an important role in the surface hardness increase, and a wider distribution of NH3 can result in a broader surface hardness distribution.

Ammonia thermal decomposition on quartz and stainless steel walls

Ammonia has been proposed as a promising solution for hydrogen carriers as well as clean fuels. However, the reaction kinetics is still not robust in many engineering applications, where the deviation is mainly caused by the surface reactions of ammonia on the wall. To examine the ammonia surface reaction on engineering materials, in the present study, the thermal decomposition of ammonia is systematically investigated in uniformly heated quartz/SUS304 tubular flow reactors with the prescribed heating length. A decrease in the inner diameter of the quartz tubular reactor leads to an increase in the ammonia thermal decomposition rate, showing that the thermal decomposition reaction occurs even on quartz surfaces, which is usually believed to be inert. The thermal decomposition on the SUS304 reactor surface starts from as low as 700 K, indicating it is much more reactive than the quartz surface. One-step surface reaction models of ammonia for quartz/SUS304 surfaces are proposed, with which the reaction rates are estimated based on the experimental data. The inner surface of the SUS304 tubular reactor after the thermal decomposition experiments is examined by X-ray photoelectron spectroscopy. The formation of iron nitride on SUS304, which in turn facilitates ammonia thermal decomposition, reveals the positive feedback between ammonia decomposition and nitriding. Moreover, the present one-step surface reaction on stainless steel has been validated by measuring ammonia distributions in the preheat zone of NH3/O2/N2 premixed flame impinging on a stainless steel plate through a two-photon absorption laser-induced fluorescence (TALIF) technique.

O3 full photo-fragmentation TALIF spectroscopy for quantitative diagnostics of non-thermal O2-mixed plasmas

In this study, we explore the potential of using laser-induced photo-fragmentation of O3 by UV radiation as a quantitative diagnostic tool in non-thermal O2-mixed plasmas. We analyze the optical processes of O3 using a comprehensive kinetic model with a 226 nm laser, which is typically used in the two-photon absorption laser-induced fluorescence (TALIF) measurement of O atoms. Our model demonstrates that the fluorescence intensity from atomic O fragments produced by the same laser is directly proportional to the population of precursor O3. This makes various diagnostic purposes achievable through the proposed O3 full photo-fragmentation (FPF) TALIF spectroscopy, including calibration of TALIF signals of O atoms and quantification of both O and O3 in O2-mixed plasmas. We present detailed theoretical principles, technical requirements, and successful examples of implementation for different diagnostic aims using the proposed O3 FPF-TALIF spectroscopy. However, we also specify the limitations of the developed diagnostic methods, particularly under low E/N conditions (&amp;lt;30 Td), where other interferential species such as the vibrationally excited ground-state O 2 ( X 3 Σ g − , v ≠ 0 ) are abundantly produced.

Two-dimensional laser-induced fluorescence thermometry in a medium- to high-enthalpy CO2 flow

Ground-based experimental facilities are needed to qualify materials subjected to aerothermodynamic loads during an entry into the Martian atmosphere. In the present work, the focus is on quantifying the temperature field in a medium- to high-enthalpy CO_{2} flow (from 4.5 to {10.5}, hbox {MJ}/hbox {kg}) of a plasma wind tunnel using one- and two-dimensional two-photon absorption laser-induced fluorescence (CO-TALIF) thermometry at a CO number density in the range of 1–2 times 10^{17} cm^{-3}. However, the measurement method developed here can also be used to study satellite propulsion systems. Due to absorption by air, a two-photon excitation scheme of the CO molecule at 115 nm (VUV) was used. Experimental and simulated excitation spectra are compared for temperature determination. For spectrum simulation in the B^1varSigma ^+leftarrow X^1varSigma ^+ system, an existing program was extended and a polynomial regression correlation was developed for signals with low signal-to-noise ratios. After the first one-dimensional temperature measurements using CO-TALIF in the free-stream to quantify the radial temperature profile for three conditions in the medium- to high- enthalpy range, respectively, two-dimensional measurements in a steady state test chamber at room temperature were carried out. For the two-dimensional investigations, a light sheet optic was designed to generate a light sheet free of divergence in width and thickness. The results of the one- and two-dimensional temperature measurements performed at medium- to high-enthalpy conditions provided plausible results concerning both the values and the course in radial and axial direction, the latter being, to the authors’ knowledge, the first two-dimensional temperature measurements under such conditions using CO-TALIF.

TALIF at H− ion sources for the determination of the density and EDF of atomic hydrogen

The production of H− ions in negative ion sources relevant for particle accelerator facilities and neutral beam injection systems is based predominantly on the surface conversion of H atoms at a low work function surface covered with caesium (the plasma grid (PG)). Therefore, the H atom density n H and energy distribution function (EDF) close to the PG determine the amount of surface produced H− ions. As a direct method for the density and EDF determination, two-photon absorption laser induced fluorescence (TALIF) on H atoms was implemented at the ion source of the teststand BATMAN Upgrade (BUG) being the first time that this was accomplished at an H− ion source. Several challenges had to be overcome concerning the application of the diagnostic at the complex facility and the evaluation of the fluorescence signals against a bright H background. The observed line profiles suggest a Maxwellian EDF with an H atom temperature of K. The presence of highly energetic H atoms (measured by optical emission spectroscopy, (OES)) could not be resolved by the TALIF system due to the insufficient signal-to-noise ratio. Atomic densities were measured for H2 and D2 plasmas for varying ion source parameters at BUG resulting in values between m−3 and m−3 for hydrogen. For the operation with deuterium, higher atomic densities are observed for similar ion source parameters which agree well with the previous results obtained with OES.

Two-photon absorption laser induced fluorescence (TALIF) detection of atomic iodine in low-temperature plasmas and a revision of the energy levels of I I

In order to perfect the use of iodine, instead of xenon, in plasma thrusters, an experiment has been set up to investigate the feasibility of two-photon absorption laser induced fluorescence (TALIF) diagnostics in iodine plasmas. Levels and of atomic iodine were found qualified for that purpose, with a radiative lifetime of the former found equal to , which appears consistent with the predicted by the most recent calculations. Doppler-free two-photon spectroscopy has confirmed that the relative intensities of the hyperfine components follow well-known general formulas, which makes it possible to disentangle the hyperfine structure of from Doppler broadening. Accurate temperature measurements are shown to be possible, even though the relatively heavy mass of the iodine atom does not make it the most favorable gauge for measurements based on Doppler effect. Using an injection seeded pulsed laser as the primary laser source, the frequency-tripling of which has provided the ca 300 nm radiation used for excitation, absolute energy measurements of the two-photon excited levels revealed that all upper energy levels of I I have to be revised down, by , which pulls every energy level off its presently tabulated value by more than thirty times the currently admitted uncertainty bar.

Species structures in preheated ammonia micro flames

Species structure of ammonia (NH3) micro flames stabilized on a preheated 2-mm diameter tube is investigated using laser-induced fluorescence (LIF) measurements and numerical simulations with detailed reaction mechanisms. Nitric oxide (NO) and hydroxyl (OH) radicals are measured using single-photon absorption laser-induced fluorescence (LIF), while hydrogen atoms (H) are measured using two-photon absorption LIF (TPLIF). NO and OH are measured using nanosecond excitation pulses while both nanosecond and femtosecond methods are employed for TPLIF of H. 1-D and 2-D flame simulations are performed using Cantera and ANSYS FLUENT, separately, and the results are compared with the experimental data. It was observed that experimental H-atom signals increased with increasing equivalence ratio (ϕ), while the model predicted an opposite trend. Compared to the model predicted a rapid drop of OH as a function of ϕ, experimentally measured OH profiles exhibit opposite trends in different height locations of the flame. It is conjectured that the thermal dissociation of NH3 to generate reactive radicals such as H and OH is enhanced with preheating rich mixtures. Measured NO-LIF signal decreases with increasing ϕ, which agrees with the predicted trend by the simulation. NO reduction in fuel-rich flames is expected to be due to NHi radicals generated from excess NH3. A negative correlation is observed between NO and H/OH as a function of ϕ, which contradicts with the simulation that shows a positive relationship. This discrepancy implies that the effect of NH3 thermal dissociation has to be considered in the simulation of rich NH3 flames. This study helps to fulfill the fundamental knowledge gap of NH3 dissociation kinetics and the role of reactive intermediates in oxidation and NO formation pathways, hence an important step towards realizing practical combustion devices operating of NH3 as an energy carrier.

Absolute N-atom density measurement in an Ar/N2 micro-hollow cathode discharge jet by means of ns-two-photon absorption laser-induced fluorescence

In this work, nanosecond two-photon absorption laser-induced fluorescence (TALIF) is used to probe the absolute density of nitrogen atoms in a plasma generated using a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, and the plasma is ignited in an Ar/N2 gas mixture. First, we study a MHCD configuration having the same pressure (50 mbar) on both sides of the electrodes. A good agreement is found between the density of N atoms measured using TALIF in this work and previous measurements using vacuum ultraviolet Fourier transform absorption spectroscopy. Then, we introduce a pressure differential between the two electrodes of the MHCD, creating a plasma jet. The influence of the discharge current, the percentage of N2 in the gas mixture, and pressures on both sides of the MHCD is studied. The current has a small impact on the N-atom density. Furthermore, an optimal N-atom density is found at around 95% of N2 in the discharge. Finally, we demonstrate that the pressure has a different impact depending on the side of the MHCD: the density of N atoms is much more sensitive to the change of the pressure in the low-pressure side when compared to the pressure change in the high-pressure side. This could be due to several competing phenomena: gas residence time in the cathodic region, recirculation, or recombination of the N atoms at the wall. This study contributes to the optimization of MHCD as an efficient N-atom source for material deposition applications.

Spatio-temporal profile of atomic oxygen in a 1 kHz repetition atmospheric-pressure plasma jet in He–O2–H2O mixture

Atomic oxygen (O) is one of the essential reactive species in plasma oxidation processes. We investigated the behavior of atomic oxygen in a 1 kHz-repetition pulsed plasma jet in atmospheric-pressure He/O2/H2O mixture. By two-photon absorption laser-induced fluorescence, the spatio-temporal profiles of O density were measured under various conditions. In the dry ([H2O] 100 ppm) condition, the rate of O production did not depend on the [O2] fraction in the range of [O2] = 275–8600 ppm. The analysis of the O-production rate indicates that the atomic oxygen in this plasma jet arises from electron-impact dissociation and quenching of O(1 D), similar to the O-production mechanism in radio-frequency plasma jet. The dependence of O-production in each discharge pulse (Δ[O]) on the discharge energy and [O2] in the plasma region at dry condition is formulated as . The decay rate of atomic oxygen was not explained by self-recombination or ozone-generation reactions; it was consistent with the reaction rate of O + OH O2 + H at [OH] = cm−3. This result suggests that the small amount of [OH] with 1013 cm−3 density is more responsible for O behavior than [O2] with large fraction of 1015 cm−3. We conducted a chemical reaction simulation considering the measured results of [O] and [OH] production, resulting in good agreement with the spatial distribution of [O]. Chemical reaction analysis revealed that the cyclic reproduction of OH via chain reaction with O and O2 is important, therefore a small amount of OH catalytically consumes atomic oxygen with two-order higher density.

A diagnostic to measure neutral-atom density in fusion-research plasmas.

A femtosecond two-photon-absorption laser-induced-fluorescence (TALIF) diagnostic was designed, installed, and operated on the Princeton-Field-Reversed Configuration-2 device to provide non-invasive measurements of the time and spatially resolved neutral-atom densities in its plasmas. Calibration of the Ho density was accomplished by comparison with Kr TALIF. Measurements on plasmas formed of either H2 or Kr fill gases allowed examination of nominally long and short ionization mean-free-path regimes. With multi-kW plasma heating and H2 fill gas, a spatially uniform Ho density of order 1017m-3 was measured with better than ±2mm and 10µs resolution. Under similar plasma conditions but with Kr fill gas, a 3-fold decrease in the in-plasma Kr density was observed.

Absolute OH density measurements in a CO2–H2O glow discharge by laser-induced fluorescence spectroscopy

Time-resolved absolute OH density measurements in a millisecond-pulsed DC glow discharge from carbon dioxide with water admixture, by laser-induced fluorescence (LIF) spectroscopy, are presented. For this purpose, a novel technique is suggested to calibrate the OH LIF setup by means of two-photon absorption LIF spectroscopy measurements on a CO-filled gas cell. OH densities in the order of 1 × 1018 m−3 are reported at a pressure of 6.67 mbar with a water admixture of 20% and a discharge current of 50 mA. Furthermore, the time evolution of the CO2 conversion is determined from collision energy transfer LIF and validated against the literature. The time-dependent rotational temperature of OH is obtained with LIF thermometry.

Importance of stepwise ionization from the metastable state in electron cyclotron resonance ion thrusters

In electron cyclotron resonance (ECR) thrusters, the plasma mode transition is a critical phenomenon because it determines the maximum thrust performance. In ECR ion thrusters, ionization generally occurs in the magnetic confinement region, where electrons are continuously heated by ECR and confined by magnetic mirrors. However, as the flow rate increases, ionization is also observed outside the magnetic confinement region, and this induces the plasma mode transition. In our previous work, two-photon absorption laser-induced fluorescence (TALIF) analysis revealed that the stepwise ionization from the metastable state plays an important role in the ionization process. However, the distribution of the stepwise ionization has not yet been revealed because of the long lifetime of the metastable state. In this study, this distribution was investigated using one experimental and two numerical approaches. First, TALIF was applied to two types of gas injection with clear differences in thrust performance and ground-state neutral density distribution. In the first simulation, the metastable state particle simulation was used to estimate the excitation rate distribution. In the second study, simulations of the electric field of microwaves were used to estimate the contribution of the stepwise ionization to the plasma density. The experimental and numerical results revealed that the stepwise ionization spreads outside the magnetic confinement region because of the diffusion of metastable particles, and this spread induces the plasma mode transition, explaining the difference between the two types of gas injection.