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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.

In this work, a fluid/Monte Carlo collision (fluid/MCC) hybrid model is developed based on the framework of multi-physics analysis of plasma sources. This hybrid model could be highly accurate in predicting the nonequilibrium phenomena in capacitively coupled plasmas and meanwhile avoid the limitation caused by the computational cost. Benchmarking against the well-established particle-in-cell/MCC (PIC/MCC) method and comparison with experimental data have been presented both in electropositive N2 discharges and electronegative O2 discharges. The results indicate that in N2 discharges, the ion density evolves from a uniform distribution to an edge-high profile as power increases. Besides, the electron energy distribution function (EEDF) at the bulk center exhibits a ‘hole’ at about 3 eV, and the ‘hole’ becomes less obvious at the radial edge, because more low energy electrons are generated there. In O2 discharges, the EEDF exhibits a Druyvesteyn-like distribution in the bulk region, and it evolves to a Maxwellian distribution in the sheath, indicating the dominant influence of the electric field heating there. The results obtained by the hybrid model agree well with those calculated by the PIC/MCC method, as well as those measured by double probe, except for a slight discrepancy in absolute values. The qualitative agreement achieved in this work validates the potential of this hybrid model as an effective tool in the deep understanding of plasma properties, as well as in the improvement of plasma processing.

The surface charging effect of positive ions at the trench bottom during the etching of dielectric materials has become a severe problem with the shrinkage of electronic feature dimensions. Pulsed plasmas have been widely used to overcome surface charging issues during plasma etching in industry. In this work, synchronized dual-level radio frequency pulsed modulation on capacitively coupled plasmas is investigated based on 1D3V particle-in-cell/Monte Carlo collision simulations. It is found that the plasma always remains a certain initial density during the dual-level pulse modulation, as the high frequency (HF) source still presents during pulse off time (with a reduced voltage amplitude), which may facilitate high processing efficiency. Meanwhile, many HF sheath oscillations are simulated at the initial stage of a pulse period, which induces vast high energy electrons to strike the electrode. When the HF voltage is maintained at a very low value during the pulse off (i.e. 0.1 V HF ), more electrons with higher energy bombard the bottom of the trench, effectively alleviating the charging effect. Furthermore, the nonlinear behavior of sheath oscillations is proved to be very sensitive to dual-frequency voltage amplitudes and pulse ramp-up times.

A bias power is usually applied in inductively coupled plasmas (ICP) to realize the separate control of the plasma density and the ion energy. In this research, a two-dimensional fluid/electron Monte Carlo hybrid model is developed to self-consistently investigate the bias effect on the stochastic heating and on the radial homogeneity in a biased argon ICP operated at low pressure (3 mTorr). The results show that the temporal evolution of the stochastic heating exhibits a plateau and a peak when the sheath collapses at high bias voltages, due to the limited sheath heating and the electron inertia. In addition, the plasma density in the diffusion chamber increases with bias voltage and bias frequency, because of the more pronounced stochastic heating both at the substrate and at the grounded wall. In the main discharge chamber, the plasma density decreases with bias voltage, due to the compression of the bulk plasma region, and this trend becomes less obvious at high bias frequency, because of the enhanced power absorption caused by the stochastic heating. Therefore, it is concluded that by tuning the bias voltage and bias frequency, the plasma radial uniformity could be modulated efficiently, which is very important for improving plasma processing.

The electron cyclotron resonance (ECR) effect in a weakly magnetized capacitively coupled radio frequency (RF) plasma was previously observed with optical emission spectroscopy (OES) in experiments and analyzed by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations (Zhang et al 2022 Plasma Sources Sci. Technol. 31 07LT01). When the electron cyclotron frequency equals the RF driving frequency, the electron can gyrate in phase with the RF electric field inside the plasma bulk, being continuously accelerated like microwave ECR, leading to prominent increases in the electron temperature and the excitation or ionization rate in the bulk region. Here, we study further the basic features of the RF ECR and the effects of the driving frequency and the gas pressure on the RF ECR effect by OES and via PIC/MCC simulations. Additionally, a single electron model is employed to aid in understanding the ECR effect. It is found that the maximum of the measured plasma emission intensity caused by ECR is suppressed by either decreasing the driving frequency from 60 MHz to 13.56 MHz or increasing the gas pressure from 0.5 Pa to 5 Pa, which shows a qualitative agreement with the change of the excitation rate obtained in the simulations. Besides, the simulation results show that by decreasing the driving frequency the electron energy probability function (EEPF) changes from a convex to a concave shape, accompanied by a decreased electron temperature in the bulk region. By increasing the gas pressure, the EEPF and the electron temperature show a reduced dependence on the magnitude of the magnetic field. These results suggest that the ECR effect is more pronounced at a higher frequency and a lower gas pressure, primarily due to a stronger bulk electric field, together wih a shorter gyration radius and lower frequency of electron–neutral collisions.

Potassium ion channels are essential elements in cellular electrical excitability and help maintain a resting potential in nonexcitable cells. Their universality is based on a unique combination of strong selectivity for K^{+} ions and near-diffusion-limited permeation efficiency. Understanding how the channel regulates the ion conduction would be instructive to the treatment of ion channelopathies. In this work, by means of molecular dynamics simulations, we demonstrate the significantly enhanced permeation of KcsA channel in reaction to an external terahertz wave, due to the effective response of the K^{+} ions in the selectivity filter regions of the channel. Compared to the case without external terahertz wave, a fourfold increase in the ion current through the channel is found.

The effects of the gas pressure, the voltage amplitude and the afterglow duration on the ignition process over 300 mm-diameter in a pulse-modulated capacitively coupled radio-frequency argon discharge are experimentally investigated. A home-made optical probe is used to measure the optical emission intensity (OEI) as a function of time since the beginning of each pulse at various radial positions. A voltage and a current probe are used to measure the voltage and current waveforms at the power feeding point and then the time-dependent power deposition is also obtained. It was found that the radial profile of the OEI exhibits complex evolution when changing these external conditions. At lower pressures, the ignition occurs earlier, and the radial-integral OEI and the power deposition overshoot more slightly during the ignition. By increasing the pressure, these two quantities overshoot more significantly, and the OEI gradually evolves from an edge-peaked radial profile to a center-high radial profile for a given time when the OEI increases rapidly during the ignition. When increasing the voltage amplitude, the ignition tends to occur earlier, featuring a more significant overshoot of the OEI and power deposition at a higher voltage. Compared to that at high voltage, the OEI exhibits a significant center-high radial profile at low voltage amplitude during the ignition. By increasing the afterglow duration, T off, the ignition is delayed and the overshoot becomes more significant, due to a lower initial electron density when each pulse is turned on. During the phase when the growth rate of the OEI versus time is maximum, the OEI exhibits different radial distributions, i.e., it appears a center-high profile at short T off, an edge-peaked profile at intermediate T off, and a center-high profile at relatively long T off.

The pulsed inductively coupled plasma (ICP) has considerable potential to satisfy multiple stringent scaling requirements for use in the semiconductor industry. However, overshoot of plasma parameters during the rising period of the pulse affects the stability and uniformity of the plasma and can lead to a breakdown of the wafer and over-sputtering of the film. In this study, a step waveform modulation method is used to reduce the overshoot at the initial stage of the pulse. The behavior of the discharge is monitored by measuring (i) the modulated step waveform signal on the function generator, (ii) the input power (by a time-resolved VI-probe), and (iii) the amplitudes of the coil voltage and current (by voltage and current probes, respectively), as well as (iv) the plasma parameters including the electron density, the effective electron temperature, and the electron energy probability distribution function (by a time-resolved Langmuir probe). It was found that the state of the plasma can be controlled by changing the waveform, such as varying the time of the rising edge, varying the initial amplitude, and varying the duration of the low-high amplitude. The results indicated that the overshoot value of the electron density can be reduced by using a low-high step waveform. When the amplitude of the waveform was 500/550 mV and the duration was 200/300 μs, the overshoot value observed was 1/4 of that of the conventional ICP pulse discharge. In addition, increasing the duty cycle of the pulse could also reduce the overshoot value due to the high electron density that occurs during the afterglow period. Moreover, the plasma can reach a steady state more quickly at high pressure by using a step waveform of high amplitude.

The interactions of a fast-moving charged particle with two parallel layers, each containing a two-dimensional electron gas (2DEG), which are separated by an insulating material, are investigated b...

A high-intensity pulsed ion beam was used to irradiate Fe80Si7.43B12.57 metallic glass at different energy densities and numbers of pulses to study its structural thermal stability. A large number of round holes and a few bumps appeared on the surface of the glass after 300 pulses. No obvious damage was observed on the surface after irradiation at different energy densities. High-resolution transmission electron microscopy showed that this irradiation had a strong influence on the near-surface atomic arrangement of the glass. Irradiation induced strong migration and aggregation of atoms within the glass, causing an uneven atomic arrangement. Although there was an ordered arrangement of atoms, the metallic glass remained amorphous. The surface reflectivity of the metallic glass decreased slightly after irradiation. The number of irradiation pulses had greater impact on this Fe-based metallic glass than their energy density.

Based on particle-in-cell and test-particle simulations, an α- to γ-mode transition of the electron power absorption dynamics is found to be induced as a function of an externally applied transverse magnetic field in low pressure capacitively coupled radio frequency discharges operated in argon. This transition is caused by the increased magnetic confinement of energetic ion-induced secondary electrons (γ-electrons) at higher magnetic fields. If a transverse magnetic field is applied, γ-electrons, that are accelerated to high energies by the sheath electric field, are confined in the vicinity of the sheath. Due to their gyromotions they are returned periodically to the adjacent sheath and interact repetitively with the sheath electric field. Such confined γ-electrons are, thus, effectively multiplied by ionization collisions in the vicinity of the sheath, thereby resulting in a significant enhancement of the local ionization rate and the plasma density. In such cases, the energy coupling efficiency of γ-electrons is greatly enhanced, implying that the discharge operates in a magnetized γ-mode. Moreover, the effects of the magnetic field on ion properties and ionization dynamics at different driving frequencies and operating pressures are studied.

In low-temperature inductively coupled radio frequency (rf) plasmas, electrons and ions that gain energy from the electric field can transfer a portion of energy to neutral particles. The resulting radial variation of the neutral gas temperature [Formula: see text] can significantly influence the radial distributions of reaction rates and radical densities on the substrate, thus affecting the etching/film deposition uniformity. In this work, we perform an experimental study on the dependence of the neutral gas temperature [Formula: see text] on external parameters (i.e., rf power, pressure, and gas component) in inductively coupled Ar and Ar/O2 plasmas by using a fiber Bragg grating sensor. To analyze the correlation between [Formula: see text] and the plasma characteristics, a Langmuir probe is used to measure the electron density [Formula: see text], effective electron temperature [Formula: see text], and ion density [Formula: see text] under the same discharge conditions. It is found that in both Ar and Ar/O2 plasmas, neutral gas heating is sensitive to plasma density. As the plasma density increases with the pressure/power, the collisions of ions and electrons with neutral particles are enhanced so that [Formula: see text] increases monotonically. With the increase of O2 content, [Formula: see text] and [Formula: see text] are observed to decrease due to enhanced dissociation and excitation of O2, leading to a decrease in [Formula: see text]. The radial profile of [Formula: see text] exhibits a parabolic distribution in pure Ar discharges, whereas it evolves through a center-flat shape into a saddle shape with the increase of O2 content. The variation of [Formula: see text] with rf power during the E-to-H mode transition is also presented and discussed.

Two classic radio-frequency (RF) plasmas, i.e., the capacitively and the inductively coupled plasmas (CCP and ICP), are widely employed in material processing, e.g., etching and thin film deposition, etc. Since RF plasmas are usually operated in particular circumstances, e.g., low pressures (mTorr–Torr), high-frequency electric field (13.56 MHz–200 MHz), reactive feedstock gases, diverse reactor configurations, etc., a variety of physical phenomena, e.g., electron resonance heating, discharge mode transitions, striated structures, standing wave effects, etc., arise. These physical effects could significantly influence plasma-based material processing. Therefore, understanding the fundamental processes of RF plasma is not only of fundamental interest, but also of practical significance for the improvement of the performance of the plasma sources. In this article, we review the major progresses that have been achieved in the fundamental study on the RF plasmas, and the topics include 1) electron heating mechanism, 2) plasma operation mode, 3) pulse modulated plasma, and 4) electromagnetic effects. These topics cover the typical issues in RF plasma field, ranging from fundamental to application.

An enhanced electron heating mechanism based on a resonance between the cyclotron motion of electrons and radio frequency (rf) electric field in the plasma bulk is reported in weakly magnetized capacitively coupled argon plasmas at low pressure. When the electron cyclotron frequency coincides with the applied power source frequency, the bulk electrons can continuously acquire energy from the background electric field within certain rf periods during the cyclotron motion, inducing overall distinct increase of excitation rate and electron temperature in the plasma bulk. This enhanced electron heating effect has been examined by a combination of kinetic particle simulations, experimental measurements, and an analytical model, and the dynamics of electrons are revealed at resonant conditions.

An accurate understanding of ion-beam transport in plasmas is crucial for applications in inertial fusion energy and high-energy-density physics. We present an experimental measurement on the energy spectrum of a proton beam at 270 keV propagating through a gas-discharge hydrogen plasma. We observe the energies of the beam protons changing as a function of the plasma density and spectrum broadening due to a collective beam-plasma interaction. Supported by linear theory and three-dimensional particle-in-cell simulations, we attribute this energy modulation to a two-stream instability excitation and further saturation by beam ion trapping in the wave. The widths of the energy spectrum from both experiment and simulation agree with the theory.

Superimposition of dual frequencies (DFs) is one of the methods used for controlling plasma distribution in an inductively coupled plasma (ICP) source. The effects of a superimposed DF on the argon plasma characteristics have been investigated using a two-dimensional self-consistent fluid model. When both currents are fixed at 6 A, the plasma density drops with decrease in one of the source frequencies due to less efficient heating and the plasma uniformity improves significantly. Moreover, for ICP operated with superimposed DFs (i.e., 4.52 MHz/13.56 MHz and 2.26 MHz/13.56 MHz), the current source exhibits the same period as the low frequency (LF) component, and the plasma density is higher than that obtained at a single frequency (i.e., 4.52 and 2.26 MHz) with the same total current of 12 A. However, at superimposed current frequencies of 6.78 MHz/13.56 MHz, the plasma density is lower than that obtained at a single frequency of 6.78 MHz due to the weaker negative azimuthal electric field between two positive maxima during one period of 6.78 MHz. When the superimposed DF ICP operates at 2.26 and 13.56 MHz, the rapid oscillations of the induced electric field become weaker during one period of 2.26 MHz as the current ratio of 2.26 MHz/13.56 MHz rises from 24 A/7 A to 30 A/1 A, and the plasma density drops with the current ratio due to weakened electron heating. The uniformity of plasma increases due to sufficient diffusion under the low-density condition.

Effects of chamber size on electron bounce-resonance heating (BRH) and power deposition profile are numerically studied in a finite inductive Ar discharge under a low-pressure range of 0.3–3 Pa. The BRH characterized by a plateau formation in the electron energy probability function (EEPF) exists only at the small chamber radius and relatively low pressure and is enhanced at a larger chamber height. It is attributed to a remarkable increase in the energy diffusion coefficient caused by electron heating at the first bounce resonance condition. As increasing chamber radius and pressure, the enhancement in the energy diffusion coefficient caused by electron–electron collisions tends to Maxwellianize the EEPF, thus resulting in weakness and even disappearance of the BRH. For relatively low pressure, the number of positive and negative power deposition regions increases with increasing chamber radius, and there is no region of negative power deposition at the small chamber radius where the skin depth becomes closer to the chamber radius.

In electronegative radiofrequency plasmas, striations (STRs) can appear if the bulk plasma is dominated by positive and negative ions that can react to the driving frequency. Here, we investigate such self-organized structures in dual-frequency (2/10 MHz) capacitively coupled CF4 plasmas by phase-resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collision simulations. This choice of the frequencies is made to ensure that the ions can react to both the lower (2 MHz, ‘low frequency’, LF) and the higher (10 MHz, ‘high frequency’, HF) components of the excitation waveform. A strong interplay of the two excitation components is revealed. As the STRs appear in the plasma bulk, their number depends on the length of this region. By increasing the LF voltage, ϕ LF, the sheath widths at both electrodes increase, the bulk is compressed and the number of STRs decreases. The maximum ion density decreases slightly as a function of ϕ LF, too, due to the compressed plasma bulk, while the minimum of the ion density remains almost constant. The spatio-temporal distributions of the excitation and ionization rates are modulated both by the LF and HF with maxima that occur at the first HF period that follows the complete sheath collapse at a given electrode. These maxima are caused by a high local ambipolar electric field. At a given phase within a HF period the current density is different at different phases within the LF period because of frequency coupling. The LF components of the F− ion velocity and of the electric field are much lower than the respective HF components due to the lower LF component of the displacement current in the sheaths. The LF component of the total current is dominated by the ion current at low values of ϕ LF but by the electron current at high values. The HF component of the total current is dominated by the electron current and decreases slightly as a function of ϕ LF.

A gas-discharge plasma device is simulated with COMSOL software, and the obtained plasma density profile is input into a two-dimensional particle-in-cell code, in which the transport of relativistic electron beams in the plasma with an actual density profile is investigated. The results show that the device can produce a wide range of high-density plasmas with the maximum density approaching [Formula: see text] m−3. With the relativistic electron beams produced from a linear electron accelerator, the gas-discharge plasma is shown to be an ideal medium for the investigation of coupled two-stream and current filamentation instability.

The effects of gas pressure and voltage amplitude on the ignition process of a pulse capacitively coupled RF argon discharge are experimentally investigated. The electron density is measured by a hairpin probe, the spatiotemporal distribution of the electron impact excitation dynamics is determined by phase resolved optical emission spectroscopy, and the electrical parameters are obtained by analyzing the measured current and voltage waveforms. In this work, the pulse plasma is ignited with few initial electrons, so the ignition process behaves like gas breakdown. Based on the measured RF breakdown curve, the gas pressures and voltage amplitudes are selected, and then different characteristics of ignition processes are compared and discussed in detail. Particularly, the spatiotemporal pattern of the electron impact excitation rate obtained within the selected pressure range, as well as other results, aid the intuitive understanding of a typical “V-shaped” RF breakdown curve. At lower pressures, the excitation pattern exhibit shorter and tilted regions, ending at electrodes during the early ignition stage, implying a substantial electron energy loss, while at relatively high pressures, the excitation pattern becomes wider and less tilted, and the proportion of electron energy consumed by excitation processes increases. In addition, by increasing the voltage amplitude, the ignition is advanced and becomes more significant, manifesting a faster increase in discharge current and a stronger overshoot of RF power deposition. Meanwhile, at high voltage amplitude, the excitation pattern exhibits complex spatiotemporal distribution due to enhanced local electric field when the plasma emission intensity overshoots.

The effects of gas pressure and gas component on the power transfer efficiency [Formula: see text] and transition power threshold [Formula: see text] during the E–H mode transition in inductively coupled plasmas are studied. The evolutions of [Formula: see text] during the E–H mode transition in pure Ar and Ar/O2 discharges are similar, i.e., in E-mode discharge, [Formula: see text] slightly increases with raising the applied power ([Formula: see text] is below 30%), whereas, an abruptly upward jump of nearly two to four times for [Formula: see text] occurs when the discharge transits to H-mode, and then [Formula: see text] monotonously increases with increasing the applied power. In addition, as the pressure rises, [Formula: see text] increases rapidly first and then slowly in pure Ar discharge. However, [Formula: see text] in Ar/O2 mixture gases discharge shows a non-monotonic behavior. It reveals a tendency of first increase at low pressures and then decreases at high pressures, which is because the resistive component of the plasma varies with the electron density [Formula: see text] and the electron effective collision frequency [Formula: see text]. Furthermore, a non-monotonic behavior of transition power threshold [Formula: see text] with the pressure is found, i.e., with increasing the pressure, [Formula: see text] first decreases and then increases. It may be caused by the threshold electron density, which is constant at low pressures [Formula: see text] but increases at high pressures [Formula: see text].

We have conducted a systematic experimental investigation on the electron heating mechanism named as electron sheath resonance (ESR) effect, with a parallel plate discharge configuration under various experimental conditions. In all conditions, a clear plasma density peak was observed at the magnetic field for ESR, providing a direct evidence for the effect. Further analysis suggests that the more significant ESR effect should appear at higher frequency, lower pressure discharges with larger electrodes. The results form a basis for further studies of the ESR effect, which is also meaningful to practical applications such as etching and thin film deposition processes.

The mechanism of resonance heating between the gyrating electrons and the oscillating sheath induced by a small transverse magnetic field in low-pressure capacitively coupled plasmas is investigated. The gyrating electrons will coherently collide with the expanding sheath if the electron gyro-frequency coincides with half the driving frequency. These electrons will gain substantial energy from collision-less heating, which strongly enhances the electron power absorption and the plasma density at a constant driving voltage. The electron kinetics is revealed at resonant conditions by particle simulations. Our numerical results demonstrate that the relation between the magnetic field and the driving frequency determines this resonance effect. Besides, it is found that the operating pressure, electrode gap, and driving voltage all strongly affect this electron resonance mechanism. The resonance effect is more pronounced at the conditions of low pressure, large gap, and high voltage.

An electron heating mechanism based on a resonance between the cyclotron motion of electrons and the radio frequency sheath oscillations is reported in weakly magnetized capacitively coupled plasmas at low pressure. If half of the electron cyclotron period coincides with the radio frequency period, then electrons will coherently collide with the expanding sheath and gain substantial energy, which enhances the plasma density. A relation between the magnetic field and the driving frequency is found to characterize this resonance effect and the kinetics of electrons are revealed at resonance conditions for various driving frequencies.

In this work, the electron power absorption mode transition in capacitively coupled Ar/CF4 discharges is investigated by using a one-dimensional fluid/electron Monte Carlo hybrid model. Different electron power absorption modes are observed under various external discharge conditions, which could be explained by examining the contribution of bulk electrons and secondary electrons respectively. The results indicate that as the gap increases, the electron power absorption mode changes from the drift-ambipolar (DA) mode to a α-γ-DA hybrid mode. This is ascribed to the enhanced ionization process of secondary electrons due to their sufficient collisions when the discharge region expands, as well as the weakened drift and ambipolar electric fields. By increasing the secondary electron emission coefficient, the number density of secondary electrons grows, and thus the discharge experiences a transition from a α-DA hybrid mode over a α-γ-DA hybrid mode and finally into the γ mode. Moreover, when the proportion of CF4 increases, the discharge tends to be more electronegative. As a consequence, the discharge gradually transits from a α-γ hybrid mode over a α-γ-DA hybrid mode, and finally to the DA mode. The results obtained in this work are important for improving the high aspect ratio etching process by secondary electrons.

In recent years, high entropy alloys (HEAs) have attracted significant attention due to their excellent physical, chemical, mechanical properties, and good irradiation resistance, thus are considered as potential candidates for fission and fusion structural applications. CoCrFeMnNi HEA was irradiated by high intensity pulsed ion beam (HIPIB) to investigate the effects of thermal shock irradiation on their microstructure, surface morphology and mechanical properties. It was found that CoCrFeMnNi HEA maintained the face-centered cubic single-phase structure after HIPIB irradiation. The ion beam effect of irradiation produced numerous defects such as vacancies and stacking faults within the range of carbon ions. While the thermal effect reduced vacancy concentration beyond the ion range by promoting the recombination of vacancies with interstitials, and decreased the nano-hardness of CoCrFeMnNi HEA. The thermal effect and shock wave effect promoted the migration of vacancies and formed defects such as stacking faults etc. far beyond the ion range in CoCrFeMnNi HEA. Because of the very compositional complexity, the high-level chemical disorder and local lattice distortion of CoCrFeMnNi HEA, its lattice parameter was almost unchanged after HIPIB irradiation. Even if the temperature reached the melting point of CoCrFeMnNi HEA, there was no crack on the surface after surface remelting and rapid cooling. CoCrFeMnNi HEA showed good thermal stability and thermal shock irradiation resistance.

Radio frequency capacitively coupled plasmas (RF CCPs) sustained in fluorocarbon gases or their mixtures with argon are widely used in plasma-enhanced etching. In this work, we conduct studies on instabilities in a capacitive CF4/Ar (1:9) plasma driven at 13.56 MHz at a pressure of 150 mTorr, by using a one-dimensional fluid/Monte-Carlo (MC) hybrid model. Fluctuations are observed in densities and fluxes of charged particles, electric field, as well as electron impact reaction rates, especially in the bulk. As the gap distance between the electrodes increases from 2.8 cm to 3.8 cm, the fluctuation amplitudes become smaller gradually and the instability period gets longer, as the driving power density ranges from 250 to 300 W m−2. The instabilities are on a time scale of 16–20 RF periods, much shorter than those millisecond periodic instabilities observed experimentally owing to attachment/detachment in electronegative plasmas. At smaller electrode gap, a positive feedback to the instability generation is induced by the enhanced bulk electric field in the highly electronegative mode, by which the electron temperature keeps strongly oscillating. Electrons at high energy are mostly consumed by ionization rather than attachment process, making the electron density increase and overshoot to a much higher value. And then, the discharge becomes weakly electronegative and the bulk electric field becomes weak gradually, resulting in the continuous decrease of the electron density as the electron temperature keeps at a much lower mean value. Until the electron density attains its minimum value again, the instability cycle is formed. The ionization of Ar metastables and dissociative attachment of CF4 are noticed to play minor roles compared with the Ar ionization and excitation at this stage in this mixture discharge. The variations of electron outflow from and negative ion inflow to the discharge center need to be taken into account in the electron density fluctuations, apart from the corresponding electron impact reaction rates. We also notice more than 20% change of the Ar+ ion flux to the powered electrode and about 16% difference in the etching rate due to the instabilities in the case of 2.8 cm gap distance, which is worthy of more attention for improvement of etching technology.

The protons with an energy of 250 keV were used to irradiate metallic glass Ni62Ta38 and metal W, the changes in structure and surface morphology of both the materials were compared, and their performances against the irradiation were evaluated, in order to provide the reference for the possibility of application of metallic glass Ni62Ta38 in irradiation environment. The results showed that after the proton irradiation, metallic glass Ni62Ta38 still maintained its good amorphousness without obvious irradiation damage on its surface. Meanwhile, stress was generated in metal W, and its crystalline size, micro-strain and dislocation line density varied undulately with the increase of fluence. When the irradiation fluence was 1.0 × 1018 ions/cm2, large area of blisters were formed on the surface of metal W. The surface roughness of metal W increased with the fluence. Compared with metal W, the proton irradiation resistance of metallic glass Ni62Ta38 was better.

The irradiation resistance of CLF-1 steel was investigated by using He ion irradiation with an energy of 300 keV. After the irradiation, the structure of CLF-1 steel mainly remained martensitic structure, while the lattice distortion and grain refinement occurred in the irradiation damaged layer. As irradiation fluence increased, the number of vacancies in CLF-1 steel increased, and the vacancies combined with the He atoms to form a bubble layer with the thickness of about 300 nm which located at the end of the He ions range. In the bubble layer, local amorphous region formed and the carbide particles showed a tendency to decompose. When the irradiation fluence was up to 1 × 1018ions/cm2, extensive blisters and a small amount of peelings appeared on the surface of CLF-1 steel. Due to grain refinement and dispersion of small carbide particles, the hardness of CLF-1 steel increased after the irradiation.

The major emissive probe techniques for the measurement of vacuum space potentials are compared to find the best emissive probe method in a vacuum. An overview of the inflection point method, the floating point method, and the vacuum current bias method is given, addressing how each method works in a vacuum. A comparison of the experimental data measured in a vacuum shows that the improved inflection point method has a best accuracy of 0.1 V in vacuum space potential measurements, while the original inflection point method can yield about half of a volt below the real space potential due to the inappropriate linear fitting in the method. Although the floating point method and the vacuum current bias method are convenient and rapid measurement techniques, the floating point method can only measure the vacuum potentials more than 1 V, and the space charge effect is not considered in the vacuum current bias method.

Metallic glass Ni 62 Ta 38 was irradiated with the high-intensity pulsed ion beams (HIPIB) at an energy density of 0.32 J/cm 2 and different impulse times to study the structural thermal stability. Metallic glass Ni 62 Ta 38 remained an amorphous structure after HIPIB irradiation at different impulse times. However, the arrangement of atomic structure in the depth of about 700 nm from the surface of metallic glass became more disordered after irradiation. When the number of pulses increased to 300, a small amount of “bumps” appeared on the surface of the metallic glass, and no radiation damage such as cracks occurred. It shows that the metallic glass Ni 62 Ta 38 has good resistance to thermal damage from HIPIB irradiation and good structural stability.

We summarize our theoretical studies for stopping power of energetic heavy ion, diatomic molecular ions and small clusters penetrating through plasmas. As a relevant research field for the heavy ion inertial confinement fusion (HICF), we lay the emphasis on the dynamic polarization and correlation effects of the constituent ion within the molecular ion and cluster for stopping power in order to disclose the role of the vicinage effect on the Coulomb explosion and energy deposition of molecules and clusters in plasma. On the other hand, as a promising scheme for ICF, both a strong laser field and an intense ion beam are used to irradiate a plasma target. So the influence of a strong laser field on stopping power is significant. We discussed a large range of laser and plasma parameters on the coulomb explosion and stopping power for correlated-ion cluster and C60 cluster. Furthermore, in order to indicate the effects of different cluster types and sizes on the stopping power, a comparison is made for hydrogen and carbon clusters. In addition, the deflection of molecular axis for diatomic molecules during the Coulomb explosion is also given for the cases both in the presence of a laser field and laser free. Finally, a future experimental scheme is put forward to measure molecular ion stopping power in plasmas in Xi'an Jiaotong University of China. Keywords: Molecules, Stopping power, Coulomb explosion, Vicinage effect, Laser, PACS Codes: 34.50.Bw, 52.40.Mj, 61.85.+p, 34.50.Dy

The radiation-driven tearing mode (RTM) in tokamaks was numerically studied in our previous investigation (Xu et al 2020 Plasma Phys. Control. Fusion 62 105009), and it was noticed that radiation cooling could drive magnetic island growth and could even cause disruption due to the nonlinear mode coupling. In this work, the suppression of the radiation-driven m / n = 2 / 1 tearing mode by an externally driven current is investigated numerically by using a three-dimensional toroidal code CLT (Ci-Liu-Ti means magnetohydrodynamics in Chinese) based on a full set of resistive magnetohydrodynamic equations. It is found that an externally driven current with appropriate turn-on time and strength can effectively stabilize the RTM. The effects of the asymmetry current and multi-mode coupling can be stabilized effectively by the externally driven current. In the nonlinear stage, under the influence of the radiation, a strong m / n = 1 / 0 mode is generated due to the hot-core shift caused by the decrease of the core thermal pressure, which is irreversible even if the m / n = 2 / 1 magnetic island could be effectively controlled by the externally driven current. When the externally driven current is turned on in the linear stage, it very efficiently and quickly suppresses the dominant modes, and the hot-core shift effect also becomes weaker because of the smaller m / n = 1 / 0 mode. Moreover, it is found that external auxiliary heating with appropriate intensity in the core region can effectively balance the reduced pressure and can further control the hot-core shift caused by radiative cooling.

A synergistic combination of multi-diagnostic methods are proposed to investigate temporal evolution of electrical and plasma parameters at various radial positions over 300 mm-diameter electrodes during the pre-ignition, ignition, and post-ignition phases of a pulsed capacitively coupled radio-frequency (RF) argon discharge. The electron density, n e, and the optical emission intensity (OEI) of argon at 750.4 nm at different radial positions are measured time-resolved by using a hairpin probe and an optical probe, respectively. A B-dot probe is employed to determine the waveforms of the azimuthal magnetic field at different radii, from which the waveforms of the axial current density at corresponding radial positions are derived based on Ampere’s law. Then, the time evolution of the power density at various radii can be calculated, provided that the voltage drop between the electrodes is independent of radius. Meanwhile, the time-dependent total power deposited into the reactor is calculated with the voltage and the current waveforms measured by a voltage and a current probe at the power feeding point. It was found that during pre-ignition phase, the OEI and n e cannot be measurable due to extremely low power deposition when the system exhibits pure capacitive impedance. During the ignition phase, the OEI, the power density, and the current density exhibit the most significant increase at the electrode center, while the time evolution of n e measured by the hairpin probe seems to exhibit a relatively weak radial dependence during this phase. In particular, at r ⩽ 8 cm, the OEI at every radius was observed to change with time in the same manner as the power density during the ignition phase, because the RF power is absorbed primarily by electrons, which dissipate their energy via inelastic collisions. Shortly, the profile of n e becomes edge-high during the post-ignition phase and remains thereafter until the end of the pulse-on period. Methodologically, the synergistic diagnostics lay the foundation for extensive studies on spatiotemporal evolution of plasma ignition process under broader conditions, e.g. electronegative gas, lower working gas pressure and very high driving frequency, widely used by practical etching process.

It is recognized that in large-area, very-high-frequency capacitively coupled plasma (VHF CCP) reactors, the higher harmonics generated by nonlinear sheath motion can lead to enhanced standing wave excitation. In this work, a self-consistent electromagnetic model, which couples a one-dimensional, radial nonlinear transmission line model with a bulk plasma fluid model, is employed to investigate the nonlinear standing wave excitation in a VHF driven, geometrically asymmetric capacitive argon discharge operated at low pressure. By considering a radially non-uniform plasma density profile (case I) calculated self-consistently by the nonlinear electromagnetic model and the corresponding radially-averaged, uniform plasma density profile (case II), we first examine the effect of the plasma density non-uniformity on the propagation of electromagnetic surface waves in a 3 Pa argon discharge driven at 100 MHz and 90 W. Compared to case II, the higher plasma density at the radial center in case I determines a higher plasma series resonance frequency, yielding stronger high-order harmonic excitations and more significant central peak in the harmonic current density J z,n and the harmonic electron power absorption p n profiles. Therefore, under the assumption of the radially uniform plasma density in a CCP discharge, the self-excitation of higher harmonics at the radial center should be underestimated. Second, using the self-consistent electromagnetic model, the effect of the rf power on the excitation of nonlinear standing waves is investigated in a 3 Pa argon discharge driven at 100 MHz. At a low power of 30 W, the discharge is dominated by the first two harmonics. The higher harmonic excitations and the nonlinear standing waves are observed to be enhanced with increasing the rf power, resulting in a more pronounced central peak in the radial profiles of the total electron power absorption density p e, the electron temperature T e, and the electron density n e. For all rf powers, the calculated radial profiles of n e show good agreement with the experimental data obtained by a floating double probe.

Spectra and emission efficiencies of x-rays produced by hot electrons (hot-e’s) with drift Maxwellian distribution are studied using both analytical and numerical methods. The analytic expressions are particularly useful for the analysis of x-ray spectra produced by hot-e’s in laser-produced plasma. Both analytical and numerical results show that when hot-e’s have a nonzero drift velocity, the bremsstrahlung spectrum is still approximately a linear function in logarithmic coordinates, which indicates that the process of inferring hot-e’s temperature from the slope of x-ray spectrum needs to be revisited under certain circumstances.

Plasma parameters (electron density, electron temperature and relative light intensity) often follow an unstable state at the initial pulse stage in pulsed radio frequency discharges. For instance, the electron temperature and the excitation rate often experience overshoot phenomena within a short time after the pulsed power is turned on, while the electron density first shows a sharp increase and then continues to increase more slowly before reaching a stable value within a relatively longer time. A systematic interpretation of the evolution of plasma parameters in synchronized dual-level radio frequency pulsing capacitively coupled plasmas is reported based on 1D3V particle-in-cell simulations with Monte Carlo Collisions in argon. The low electron density and rapid sheath expansion accompanied by a strong electric field in the bulk, are found to be of great importance for the overshoot of the electron temperature and excitation rate. In addition, a time-varying ambipolar electric field develops at the initial pulse stage, which compresses the plasma bulk toward the discharge center, contributing to a sharp increase of the plasma density.