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Similarity laws (SLs) for low-pressure capacitive radio-frequency plasmas are generalized from one- to two-dimensional (2D) frameworks based on kinetic particle-in-cell simulations. Fundamental discharge parameters, such as the 2D distributions of electron densities and electric fields, are examined to assess the applicability of SLs to such discharges. The discharge characteristics are found to remain invariant when external control parameters are changed according to SLs. Even under conditions where nonlinear electron resonance heating caused by the self-excitation of the plasma series resonance due to the geometric reactor asymmetry plays an important role, the electron kinetics are shown to be invariant. Moreover, the validity of SLs for the ion dynamics is demonstrated. The results advance the applicability of SLs to a 2D cylindrical reactor geometry with azimuthal symmetry, indicating broad application prospects in practice. [ABSTRACT FROM AUTHOR]

The combined influence of airflows and a parallel magnetic field on an AC-driven dielectric barrier discharge plasma is experimentally investigated through image analyses, electrical measurements, and optical diagnoses. After applying a parallel magnetic field, more discharge filaments are generated during one discharge cycle. Besides, the electrical and optical diagnoses show that the magnetic field can increase the plasma parameters, such as the electron temperature and electron density. When airflows and a parallel magnetic field are applied in combination, the discharge uniformity presented in the long-exposure images is significantly enhanced by the airflows and slightly improved by the magnetic field. With increasing airflow velocity, the distribution of discharge filaments goes through four phases, namely, spot-like distribution, line-like distribution, cotton-like distribution, and stripe-like distribution, among which the stripe-like distribution exhibits the highest discharge uniformity. High-speed video analyses reveal that the improved discharge uniformity can be attributed to the changed breakdown positions and the increased number of filaments. Although airflow can significantly improve the macroscopic uniformity of the discharge, it leads to a decrease in the maximum current pulse amplitude, electron temperature, electron density, and gas temperature. Applying a magnetic field in flowing air can not only improve the discharge uniformity but also ensure that the discharge has high maximum current pulse amplitude intensity, electron temperature, and electron density. Based on the analyses of the electron trajectory and the estimation of the force condition of the micro-discharge remnants, the modulated charged particles, reduced electric field, and pre-ionization degree are responsible for the changed discharge uniformity and plasma parameters in the parallel magnetic field and flowing air. [ABSTRACT FROM AUTHOR]

It is recognized that in very-high-frequency capacitively coupled plasma (VHF CCP) reactors, the standing wave effects can be enhanced by the presence of nonlinear higher harmonics self-excited by the plasma series resonance (PSR). In this work, we investigated the influence of the dc bias on the excitation of the nonlinear standing waves in a parallel-plate capacitively coupled dc/VHF (60 MHz) hybrid discharge sustained in argon, with both the dc source and the VHF source applied to the top electrode. A hairpin probe was employed to determine the axial/radial distribution of the electron density, while a high-frequency B-dot probe was used to measure the radial distributions of the harmonic magnetic field and the harmonic current density. With no dc bias applied, the nonlinear standing wave excitation was observed to be predominant; the high-order harmonic current density exhibited a main peak at the electrode center and one or more minor peaks between the electrode center and the edge, leading to a center-high electron density profile. With the addition of the dc source, the harmonic excitations and the corresponding nonlinear standing wave effect tend to be suppressed with the increase of dc bias, resulting in improved plasma uniformities. Underlying mechanisms of the suppression effect of the dc bias on the nonlinear standing wave excitation were analyzed. [ABSTRACT FROM AUTHOR]

A three-dimensional fluid model is developed to investigate the radio-frequency inductively coupled H2 plasma in a reactor with a rectangular expansion chamber and a cylindrical driver chamber, for neutral beam injection system in CFETR. In this model, the electron effective collision frequency and the ion mobility at high E-fields are employed, for accurate simulation of discharges at low pressures (0.3 Pa–2 Pa) and high powers (40 kW–100 kW). The results indicate that when the high E-field ion mobility is taken into account, the electron density is about four times higher than the value in the low E-field case. In addition, the influences of the magnetic field, pressure and power on the electron density and electron temperature are demonstrated. It is found that the electron density and electron temperature in the xz-plane along permanent magnet side become much more asymmetric when magnetic field enhances. However, the plasma parameters in the yz-plane without permanent magnet side are symmetric no matter the magnetic field is applied or not. Besides, the maximum of the electron density first increases and then decreases with magnetic field, while the electron temperature at the bottom of the expansion region first decreases and then almost keeps constant. As the pressure increases from 0.3 Pa to 2 Pa, the electron density becomes higher, with the maximum moving upwards to the driver region, and the symmetry of the electron temperature in the xz-plane becomes much better. As power increases, the electron density rises, whereas the spatial distribution is similar. It can be summarized that the magnetic field and gas pressure have great influence on the symmetry of the plasma parameters, while the power only has little effect. [ABSTRACT FROM AUTHOR]

The effect of the pulse-off duration on the time evolution of the plasma and electrical parameters during the ignition phase in a pulsed capacitively coupled radio frequency argon discharge operated at 450 mTorr and 12.5 MHz is investigated synergistically by multifold experimental diagnostics, particle-in-cell/Monte Carlo collision simulations and an analytical model. In the experiment, the electron density is measured time-resolved by a hairpin probe, the spatio-temporal distribution of the electron impact excitation dynamics is studied by phase resolved optical emission spectroscopy, and the amplitudes and the relative phase, φvi, of the discharge voltage and current are determined based on the waveforms measured by a voltage and a current probe. The experimental results show that the plasma and electrical parameters during the ignition process depend strongly on the duration of the afterglow period, Toff, primarily because of the dependence of the remaining charge density on this parameter. Computed values of φvi show a similar time-dependence compared to the experiment, if the simulations are initialized with specific initial charged particle densities, nini. This allows us to further understand the time evolution of φvi for different values of Toff based on the simulation results together with an analytical model. In particular, the optical emission intensity is found to change with time in the same fashion as the power deposition into the system at Toff ⩾ 100 μs, suggesting that the power is primarily absorbed by the electrons, which dissipate their energy via inelastic collisions. The system goes through different mode transitions of electron power absorption during the ignition phase depending on Toff. Specifically, for short Toff (high nini), the α mode dominates during the entire ignition process, as the electric field is largely shielded by the abundant charge located in the interelectrode space. For intermediate values of Toff (moderate nini), another excitation pattern caused by an enhanced drift electric field at the center of the gap is observed, since a large fraction of the externally applied potential can penetrate into the central region in the absence of high charged particle densities. For longer Toff (very low nini), the ignition of the pulsed plasma behaves like a gas breakdown. [ABSTRACT FROM AUTHOR]

The time evolution of dielectric barrier discharge driven by nanosecond pulse high-voltage power is investigated by high-speed video analysis, electrical measurements and spectral diagnostics. It is found that the discharge mode generally goes through the evolution process of filamentary discharge → diffuse discharge → filamentary discharge with the increase in discharge cycle. The time-dependent changes in the standard deviation of image gray levels indicate that the discharge uniformity first improves and then deteriorates in this evolution process. The different pre-ionization density and modulated distribution of space charges and surface charges are considered to be the main reasons for the time evolution of discharge uniformity. In addition, the experiments under different frequencies and voltages show that the transition of the discharge mode is more likely to occur at higher frequency and higher voltage. Further measurement and calculation reveal that the discharge at high frequency and high voltage has the same characteristics, that is, high pre-ionization degree, thick filament diameter and short time lag. These characteristics usually lead to higher seed electron density, larger critical avalanche size and weaker lateral inhibition effect, which make the discharge mode transition more likely to occur. [ABSTRACT FROM AUTHOR]

The effects of a negative direct-current (dc) voltage on the striated structures and the electrical parameters have been studied by multi-fold experimental diagnostics and particle-in-cell/Monte Carlo collision (PIC/MCC) simulations in a dc superposed rf (radio-frequency, 8 MHz) capacitive discharge in CF4. The electron density, the spatio-temporal distribution of electron-impact excitation rate and the electrical parameters measured by a hairpin probe, phase resolved optical emission spectroscopy and a voltage and a current probe are compared with the corresponding simulation results. With the increase of the magnitude of the dc voltage, , all the experimental observations are well reproduced and analyzed by the simulations. It was found that the plasma bulk is compressed and the electron density decreases slightly at low , while the plasma bulk broadens at high when the ionization at the edge of the completely expanded sheath adjacent to the powered electrode is significantly enhanced due to the secondary electron emission (SEE). By increasing , the region of the strong ionization/excitation shrinks towards the edge of the collapsing sheath adjacent to the grounded electrode. And the ionization inside the bulk region is significantly suppressed during the collapsing phase of the sheath adjacent to the powered electrode. This is mainly attributed to the fact that the dc component of the bulk electric field and the spatial gradient of the electron density are simultaneously enhanced during the transition from the 'striation' mode to 'striation-γ' hybrid mode when increasing. Besides, we found that increasing can somewhat suppress the rf power deposition at low , and enhance the rf power deposition at high. The magnitude of the dc current exhibits a complex dependence on , due to nonlinear change of the dc resistance of the plasma. [ABSTRACT FROM AUTHOR]

Pulse inductively coupled plasma has been widely used in the microelectronics industry, but the existence of overshoot phenomenon may affect the uniformity of plasma and generate high-energy ions, which could damage the chip. The overshoot phenomenon at various spatial locations in pulsed inductively coupled Ar and Ar/CF4 discharges is studied in this work. The electron density, effective electron temperature, relative light intensity, and electron energy probability function (EEPF) are measured by using a time-resolved Langmuir probe and an optical probe, as a function of axial and radial locations. At the initial stage of pulse, both electron density and relative light intensity exhibit overshoot phenomenon, i.e., they first increase to a peak value and then decrease to a convergent value. The overshoot phenomenon gradually decays, when the probe moves away from the coils. Meanwhile, a delay appears in the variation of the electron densities, and the effective electron temperature decreases, which may be related to the reduced strength of electric field at a distance, and the consequent fewer high-energy electrons, inducing limited ionization and excitation rate. The overshoot phenomenon gradually disappears and the electron density decreases, when the probe moves away from reactor centre. In Ar/CF4 discharge, the overshoot phenomenon of electron density is weaker than that in the Ar discharge, and the plasma reaches a steady density within a much shorter time, which is probably due to the more ionization channels and lower ionization thresholds in the Ar/CF4 plasma. [ABSTRACT FROM AUTHOR]

Self-organized striated structure has been observed experimentally and numerically in CF4 plasmas in radio-frequency capacitively coupled plasmas recently (Liu et al 2016 Phys. Rev. Lett. 116 255002). In this work, the striated structure is investigated in a capacitively coupled oxygen discharge with the introduction of the effect from the secondary electron emission, based on a particle-in-cell/Monte Carlo collision model. As we know, the transport of positive and negative ions plays a key role in the formation of striations in electronegative gases, for which, the electronegativity needs to be large enough. As the secondary electron emission increases, electrons in the sheaths gradually contribute more ionization to the discharge. Meanwhile, the increase of the electron density, especially in the plasma bulk, leads to an increased electrical conductivity and a reduced bulk electric field, which would shield the ions' mobility. These changes result in enlarged striation gaps. And then, with more emitted electrons, obvious disruption of the striations is observed accompanied with a transition of electron heating mode. Due to the weakened field, the impact ionization in the plasma bulk is attenuated, compared with the enhanced ionization caused by secondary electrons. This would lead to the electron heating mode transition from striated (STR) mode to γ-mode. Besides, our investigation further reveals that γ-mode is more likely to dominate the discharge under high gas pressures or driving voltages. [ABSTRACT FROM AUTHOR]

In this work particle-in-cell/Monte Carlo collision simulations are performed to study the heating mechanism and plasma characteristics in direct current (dc) superposed radio-frequency (RF) capacitively coupled plasmas, operated both in single-frequency (SF) and dual-frequency (DF) regimes. An RF (60/2 MHz) source is applied on the bottom electrode to sustain the discharge, and a dc source is fixed on the top electrode. The heating mechanism appears to be very different in dc superposed SF and DF discharges. When only a single source of 60MHz is applied, the plasma bulk region is reduced by the dc source, thus the ionization rate and hence the electron density decrease with rising dc voltage. However, when a DF source of 60 and 2MHz is applied, the electron density can increase upon addition of a dc voltage, depending on the gap length and applied dc voltage. This is explained from the spatiotemporal ionization rates in the DF discharge. In fact, a completely different behavior is observed for the ionization rate in the two half-periods of the LF source. In the first LF half-period, the situation resembles the dc superposed SF discharge, and the reduced plasma bulk region due to the negative dc bias results in a very small effective discharge area and a low ionization rate. On the other hand, in the second half-period, the negative dc bias is to some extent counteracted by the LF voltage, and the sheath close to the dc electrode becomes particularly thin. Consequently, the amplitude of the high-frequency sheath oscillations at the top electrode is largely enhanced, while the LF sheath at the bottom electrode is in its expanding phase and can thus well confine the high-energy electrons. Therefore, the ionization rate increases considerably in this second LF half-period. Furthermore, in addition to the comparison between SF and DF discharges and the effect of gap length and dc voltage, the effect of secondary electrons is examined. [ABSTRACT FROM AUTHOR]

The effects of a negative direct-current (dc) voltage on the striated structures and the electrical parameters have been studied by multi-fold experimental diagnostics and particle-in-cell/Monte Carlo collision (PIC/MCC) simulations in a dc superposed rf (radio-frequency, 8 MHz) capacitive discharge in CF4. The electron density, the spatio-temporal distribution of electron-impact excitation rate and the electrical parameters measured by a hairpin probe, phase resolved optical emission spectroscopy and a voltage and a current probe are compared with the corresponding simulation results. With the increase of the magnitude of the dc voltage, , all the experimental observations are well reproduced and analyzed by the simulations. It was found that the plasma bulk is compressed and the electron density decreases slightly at low , while the plasma bulk broadens at high when the ionization at the edge of the completely expanded sheath adjacent to the powered electrode is significantly enhanced due to the secondary electron emission (SEE). By increasing , the region of the strong ionization/excitation shrinks towards the edge of the collapsing sheath adjacent to the grounded electrode. And the ionization inside the bulk region is significantly suppressed during the collapsing phase of the sheath adjacent to the powered electrode. This is mainly attributed to the fact that the dc component of the bulk electric field and the spatial gradient of the electron density are simultaneously enhanced during the transition from the 'striation' mode to 'striation-γ' hybrid mode when increasing. Besides, we found that increasing can somewhat suppress the rf power deposition at low , and enhance the rf power deposition at high. The magnitude of the dc current exhibits a complex dependence on , due to nonlinear change of the dc resistance of the plasma. [ABSTRACT FROM AUTHOR]