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The All ORders Spectral Algorithm (AORSA) wave equation solver by Jaeger (Jaeger et al 2001 Phys. Plasmas 8 1573) solves the integro-differential wave equation relevant for the radio frequency (RF) domain and for fusion-relevant conditions in tokamaks or stellarators, retaining all finite Larmor radius corrections by substituting the continuous Fourier integrals by a sum over a discrete set of modes. Its strength is also its weakness: the simplicity of the method results in significant computational effort, a full matrix needing to be inverted to solve the associated linear system. Based on the notion that modes are gradually more independent if their eigenvalues differ, the present paper proposes a straightforward numerical method to partly alleviate this need, allowing to substitute the full system matrix by a banded one. The adopted method can be applied to a wide variety of equations. A few 1D examples—of relevance for solving the wave equation in the RF domain of frequencies—are provided: the tunneling equation is used to illustrate the potential of the method, and the all-FLR wave equation (retaining all Finite Larmor Radius corrections in the dielectric response) adopted by Jaeger is solved comparing the solutions found to those based on simpler models (a cold plasma and a 'tepid plasma' - i.e. a kinetic model truncated at zero order in Larmor radius—description). [ABSTRACT FROM AUTHOR]

Capacitively coupled plasmas (CCPs) are widely used in plasma processing applications, where efficient power coupling between the radio frequency (RF) source and the plasma is crucial. In practical CCP systems, impedance matching networks (IMNs) are employed to minimize power reflection. However, the presence of coaxial cables can significantly impact plasma impedance and matching performance. We develop a comprehensive simulation framework for the IMN design of CCPs, fully considering the effects of RF coaxial cables. The model self-consistently couples a distributed transmission line (TL) model, a lumped-element circuit model, and an electrostatic particle-in-cell model. This coupled model is used to investigate the impact of coaxial cables on matching performance under various discharge conditions and cable configurations. The simulation results indicate that the optimal power transmission efficiency was achieved after 6 matching iterations. The power coupled to the CCP increased from 2.7 W before matching to 180.9 W, and the reflection coefficient ultimately decreased to 0.003. The results also reveal that neglecting the cables will lead to a decrease in the power dissipated in the CCP. The proposed method demonstrates effectiveness in achieving impedance matching for different gas pressures (75–300 mTorr) and cable lengths. It can be concluded that the matching speed is faster for an appropriate cable length. This work provides valuable insights into the role of TLs in CCP impedance matching and offers a practical tool for optimizing power delivery in realistic CCP systems with RF coaxial cables. [ABSTRACT FROM AUTHOR]

The radio frequency current condensation effect reported in Reiman and Fisch (2018 Phys. Rev. Lett. 121 225001) is modeled in the nonlinear resistive magnetohydrodynamic code. A series of numerical investigations have been performed to investigate the enhancement of electron cyclotron current drive (ECCD) by the current condensation effect during the control of neoclassical tearing mode (NTM) in tokamak plasmas. In the numerical model, both the parallel transport and the perpendicular transport of electron temperature are considered. The EC driven current and driven perturbed electron temperature can nonlinearly evolve within the given magnetic configuration and eventually reach saturation states. The input power threshold of ECCD and the fold bifurcation phenomenon are numerically verified via nonlinear simulations. The numerical results show good agreements with the analytical results. Moreover, spatial distributions of EC current for the two solutions at different condensed level are displayed. The control effectiveness of ECCD for large NTM islands has been evaluated while considering the current condensation effect. While taking into account current condensation effect, for a sufficiently large input power, a larger island can be more effectively stabilized than a smaller one, which suggests a reassessment of the previous idea that the ECCD should always be turned on as early as possible. The potential physics mechanism behind the ECCD control have all been discussed in detail. Furthermore, the condensation effect is found to have favorable effects on the radial misalignment of ECCD. In the consideration of the situation for extremely localized control needs, a highly peaked heating profile is adopted to verify that the fold bifurcation phenomenon still exists and the current condensation effect can still take effect in this extreme condition. [ABSTRACT FROM AUTHOR]

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 ⋅ 10 14 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 ⋅ 10 15 cm−3 and 4 ⋅ 10 15 cm−3, respectively. [ABSTRACT FROM AUTHOR]

Wearable sensors have been implemented widely to provide comfortable and continuous long-term monitoring in many applications. Minimal requirements on maintenance is a main characteristic of wearable sensors, but unfortunately, many of them are still powered by battery with limited capacity which need to be charged or replaced regularly. Energy harvesting technologies are applied to provide a reliable solution to this issue. This paper presents several design considerations for self-powered wearable sensors. Suitable energy sources are discussed, such as ambient energy sources (solar, radio frequency, and ultrasonic energy), human body energy (mechanical, piezoelectric, triboelectric, electromagnetic, electrostatic, and thermal energy). Moreover, power management integrated circuits, energy storage options, and the material selection and conditioning circuit of triboelectric nanogenerator are discussed. Five case studies utilizing different energy harvesting techniques are discussed and evaluated in terms of their system implementation and performance to provide some deeper understandings of wearable sensors. [ABSTRACT FROM AUTHOR]

This paper presents a review of the radio frequency thin-film lithium niobate (LiNbO3) based acoustic microsystems. Thanks to their high electromechanical coupling (k 2), low loss, and great frequency scalability, thin-film LiNbO3 has outperformed state-of-the-art acoustic technologies in the past decade. This paper first reviews the acoustic wave transduction and propagation in thin-film LiNbO3 and then showcases the emerging acoustic applications. Outlook for further platform development and more functions is offered for future thin-film LiNbO3 based acoustic microsystems development. [ABSTRACT FROM AUTHOR]

High frequency (HF) capacitively coupled plasmas (CCPs) are ubiquitous, having several industrial applications, especially in the semiconductor industry. Inductive heating effects within these plasmas play an important role and therefore understanding them is key to improve industrial applications. For this purpose kinetic research, using particle-in-cell (PIC) codes, offers significant opportunity to study, and improve, industrial plasma processes that operate at the atomic level. However, PIC codes commonly used for CCPs are electrostatic and thus cannot be used to simulate electromagnetically induced currents. Therefore we have developed EPOCH-LTP, a 1D PIC code with a current heating model, that enables the simulation of inductive heating effects in HF CCPs. First simulation results, from an HF CCP (60 MHz) operated at 1 mTorr of argon, show that inductive currents couple most of their power to the electrons at the interface between the bulk plasma and the sheath. Furthermore, the simulation of a dual-frequency CCP, where a HF inductive current and a low-frequency (LF) voltage waveform at 400 kHz are applied, have shown a synergy between the HF and LF waveforms that increase the inductive heating rate. [ABSTRACT FROM AUTHOR]

Reactive oxygen plasma treatment is an effective technique to eliminate hydrocarbon and improve the performance of superconducting radio frequency (SRF) cavities. This work investigated the reaction mechanism between reactive oxygen plasma and hydrocarbon, surface reaction kinetics, and cleaning process optimization through experiments, analytical models, and numerical simulations. The experimental results declare that the reaction between oxygen plasma and hydrocarbon is dominated by ion-assisted chemical sputtering, and the hydrocarbon attenuates exponentially, increasing the work function exponentially. To study the surface reaction kinetic process in-depth, we proposed a plasma cleaning rate model based on the Langmuir–Hinshelwood theory. This study found that the plasma cleaning rate primarily depends on the sheath potential, electron temperature, O atoms density, O+ ions, and Ar+ ions densities. Furthermore, we did a control-parameter simulation and found that increasing gas pressures or O2 ratios are conducive to enhancing the chemical reaction rate between O atoms and hydrocarbon. Also, the power increase can enhance the physical effect of ions. It shows that increasing the gas pressure and power and reducing the oxygen content can achieve a better cleaning effect while reducing the radio frequency power loss caused by the oxide. Those results provide valuable guidance for optimizing the cleaning process, deepening the understanding of the cleaning mechanism, and improving the performance of SRF cavities. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN) is becoming a mainstream semiconductor for power and radio-frequency (RF) applications. While commercial GaN devices are increasingly being adopted in data centers, electric vehicles, consumer electronics, telecom and defense applications, their performance is still far from the intrinsic GaN limit. In the last few years, the fin field-effect transistor (FinFET) and trigate architectures have been leveraged to develop a new generation of GaN power and RF devices, which have continuously advanced the state-of-the-art in the area of microwave and power electronics. Very different from Si digital FinFET devices, GaN FinFETs have allowed for numerous structural innovations based on engineering the two-dimensional-electron gas or p–n junctions, in both lateral and vertical architectures. The superior gate controllability in these fin-based GaN devices has not only allowed higher current on/off ratio, steeper threshold swing, and suppression of short-channel effects, but also enhancement-mode operation, on-resistance reduction, current collapse alleviation, linearity improvement, higher operating frequency, and enhanced thermal management. Several GaN FinFET and trigate device technologies are close to commercialization. This review paper presents a global overview of the reported GaN FinFET and trigate device technologies for RF and power applications, as well as provides in-depth analyses correlating device design parameters to device performance space. The paper concludes with a summary of current challenges and exciting research opportunities in this very dynamic research field. [ABSTRACT FROM AUTHOR]

We carry out a comprehensive analysis of supernova SN 2001em covering a period of 19 yr since discovery. SN 2001em is the oldest supernova known to have undergone a metamorphosis from a stripped envelope to an interacting supernova. An early spectrum indicates it exploded as a Type Ib supernova. Later, the ejecta caught up with a dense circumstellar H-shell, ejected a few thousand years before the explosion, triggering interaction between the supernova ejecta and the dense shell, producing radio, X-ray, and Hα emission. We use archival data with the Very Large Array in radio bands and with Chandra, XMM-Newton, and Swift-XRT in X-ray bands, along with published Hα measurements. We combine these data with our low radio frequency observations with the Giant Metrewave Radio Telescope at two epochs covering three frequencies. While the observations missed the phase when the shock entered the dense shell, the X-rays indicate that the shock came out of the dense shell at around 1750 days. The data suggest a forward shock origin of the X-ray emission. Radio data show a spectral inversion at late epochs (days) at around 3 GHz, which mimics the properties of the central absorbed component seen in SN 1986J. A possible explanation for this component is that the progenitor of SN 2001em was a massive binary system that had undergone a period of common-envelope evolution. The hydrogen envelope from the SN 2001em progenitor may have been lost as a result of binary interaction. [ABSTRACT FROM AUTHOR]

This paper details the design and simulation of a novel low-loss four-bit reconfigurable bandpass filter that integrates microelectromechanical system (MEMS) switches and comb resonators. A T-shaped reconfigurable resonator is reconfigured in a ‘one resonator, multiple MEMS switches’ configuration and used to gate the load capacitances of comb resonators so that a multiple-frequency filtering function is realized within the 7â€"16 GHz frequency range. In addition, the insertion loss of the filter is less than 1.99 dB, the out-of-band rejection is more than 18.30 dB, and the group delay is less than 0.25 ns. On the other hand, the size of this novel filter is only 4.4 mm Ă— 2.5 mm Ă— 0.4 mm. Our results indicate that this MEMS reconfigurable filter, which can switch 16 central frequency bands through eight switches, achieves a low insertion loss compared to those of traditional MEMS filters. In addition, the advantages of small size are obtained while achieving high integration. [ABSTRACT FROM AUTHOR]

The silicon heterojunction (SHJ) solar cell has long been considered as one of the most promising candidates for the next-generation PV market. Transition metal oxides (TMOs) show good carrier selectivity when combined with c-Si solar cells. This has led to the rapid demonstration of the remarkable potential of TMOs (especially MoO x ) with high work function to replace the p-type a-Si:H emitting layer. MoO x can induce a strong inversion layer on the interface of n-type c-Si, which is beneficial to the extraction and conduction of holes. In this paper, the radio-frequency (RF) magnetron sputtering is used to deposit MoO x films. The optical, electrical and structural properties of MoO x films are measured and analyzed, with focus on the inherent compositions and work function. Then the MoO x films are applied into SHJ solar cells. When the MoO x works as a buffer layer between ITO/p-a-Si:H interface in the reference SHJ solar cell, a conversion efficiency of 19.1% can be obtained. When the MoO x is used as a hole transport layer (HTL), the device indicates a desirable conversion efficiency of 17.5%. To the best of our knowledge, this current efficiency is the highest one for the MoO x film as HTL by RF sputtering. [ABSTRACT FROM AUTHOR]

Metal–oxide–semiconductor field-effect transistor (MOSFET) faces the major problem of being unable to achieve a subthreshold swing (SS) below 60 mV/dec. As device dimensions continue to reduce and the demand for high switching ratios for low power consumption increases, the tunnel field-effect transistor (TFET) appears to be a viable device, displaying promising characteristic as an answer to the shortcomings of the traditional MOSFET. So far, TFET designing has been a task of sacrificing higher ON state current for low subthreshold swing (and vice versa), and a device that displays both while maintaining structural integrity and operational stability lies in the nascent stages of popular research. This work presents a comprehensive analysis of a heterojunction plasma doped gate-all-around TFET (HPD-GAA-TFET) by making a comparison between Mg2Si and Si which serve as source materials. Charge plasma technique is employed to implement doping in an intrinsic silicon wafer with the help of suitable electrodes. A low-energy bandgap material, i.e. magnesium silicide is incorporated as source material to form a heterojunction between source and silicon-based channel. A rigorous comparison of performance between Si-based GAA-TFET and HPD-GAA-TFET is conducted in terms of electrical, radio frequency (RF), linearity, and distortion parameters. It is observable that HPD-GAA-TFET outperforms conventional Si-based GAA-TFET with an ON-state current (I ON), subthreshold swing (SS), threshold voltage (V th), and current switching ratio being 0.377 mA, 12.660 mV/dec, 0.214 V, and 2.985 × 1012, respectively. Moreover, HPD-GAA-TFET holds faster switching and is more reliable than Si-based device. Therefore, HPD-GAA-TFET is suitable for low-power applications. [ABSTRACT FROM AUTHOR]

In this paper we have studied AlGaN/GaN high electron mobility transistor devices with three different buffer layer structures and their effect on the RF performance of the devices. Detailed DC and pulsed IV measurements were carried out to investigate the effect of bulk traps on the performance of the devices. Activation energy of buffer traps is also investigated by low frequency S-parameter measurements of the devices at variable temperature. The effect of buffer structure and quality of buffer on RF gain has been observed for the devices. Using the S-parameters, the equivalent circuit model parameters of the devices were also extracted. All the characterizations revealed that the presence of low C-doped buffer shows less vulnerability towards traps and produces better RF performance. It was also observed that the device with only low C-doped thin buffer reproduces better RF characteristics. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of post-deposition annealing (PDA) temperature on H2S gas sensing and low-frequency noise characteristics of In2O3 gas sensors. In2O3 thin-films are deposited using the radio frequency (RF) sputtering method at an RF power of 150 W and post-annealed at various temperatures (200, 300, and 400 °C). The response of the In2O3 gas sensor to H2S decreases with increasing PDA temperature due to the increase of grain size. However, the In2O3 post-annealed at 200 °C shows the largest 1/f noise since the damaged sensing material-substrate interface is not fully recovered by the PDA. The sensors post-annealed at a higher temperature (300 °C and 400 °C) recover the damaged interface. Thanks to its moderate response and noise level, the sensor post-annealed at 300 °C shows the largest signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

This paper proposes a reasonable radiation-resistant composite channel structure for InP HEMTs. The simulation results show that the composite channel structure has excellent electrical properties due to increased modulation doping efficiency and carrier confinement. Moreover, the direct current (DC) and radio frequency (RF) characteristics and their reliability between the single channel structure and the composite channel structure after 75-keV proton irradiation are compared in detail. The results show that the composite channel structure has excellent radiation tolerance. Mechanism analysis demonstrates that the composite channel structure weakens the carrier removal effect. This phenomenon can account for the increase of native carrier and the decrease of defect capture rate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The plasma-beta effect on the n = 1 resonant magnetic perturbation (RMP) field penetration in purely radio-frequency (RF) wave heated discharges has been investigated in EAST. The experimental results show that the dependence of the threshold RMP coil current for field penetration, I RMP,th , on the total absorbed power P tot scales as approximately I RMP,th ∝ P tot 0.30 , indicating that the error-field tolerance is improved with increasing RF power. This is benefited by the increased electron perpendicular flow dominated by a counter-current electron diamagnetic flow with increasing RF power. However, theoretical scaling in cylindrical geometry overestimates the power index. Assuming an additional term β N α β N for the normalized beta in the scaling, it is shown that the fitted α β N from the experimental observation is around −1, indicating a degradation effect of plasma beta. To clarify the underlying physics of the plasma-beta effect that was not included in the theoretical scaling in cylindrical geometry, the MARS-Q code with full toroidal geometry is employed for simulation of nonlinear field penetration (Liu et al 2013 Phys. Plasmas 20 042503). The MARS-Q simulation results reproduce the β N dependence well, and hence the P tot scaling of the threshold current in experimental observations. The main reason for this is that the net total torque, which is mainly contributed by the neoclassical toroidal viscosity (NTV), increases with increasing plasma β N . The results demonstrate that the nonlinear toroidal coupling effect via NTV torque plays an important role in determining field penetration, even in cases with relatively low β N ∈ [ 0.3 , 0.6 ] , which is far less than the no-wall beta limit. [ABSTRACT FROM AUTHOR]

International guidelines/standards for human protection from electromagnetic fields have been revised recently, especially for frequencies above 6 GHz where new wireless communication systems have been deployed. Above this frequency a new physical quantity 'absorbed/epithelial power density' has been adopted as a dose metric. Then, the permissible level of external field strength/power density is derived for practical assessment. In addition, a new physical quantity, fluence or absorbed energy density, is introduced for protection from brief pulses (especially for shorter than 10 s). These limits were explicitly designed to avoid excessive increases in tissue temperature, based on electromagnetic and thermal modeling studies but supported by experimental data where available. This paper reviews the studies on the computational modeling/dosimetry which are related to the revision of the guidelines/standards. The comparisons with experimental data as well as an analytic solution are also been presented. Future research needs and additional comments on the revision will also be mentioned. [ABSTRACT FROM AUTHOR]

Metal-oxide (MO) semiconductor gas sensors based on chemical resistivity necessarily involve making electrical contacts to the sensing materials. These contacts are imperfect and introduce errors into the measurements. In this paper, we demonstrate the feasibility of using contactless broadband dielectric spectroscopy (BDS)-based metrology in gas monitoring that avoids distortions in the reported resistivity values due to probe use, and parasitic errors (i.e. tool–measurand interactions). Specifically, we show how radio frequency propagation characteristics can be applied to study discrete processes on MO sensing material, such as zinc oxide (i.e. ZnO) surfaces, when exposed to a redox-active gas. Specifically, we have used BDS to investigate the initial oxidization of ZnO gas sensing material in air at temperatures below 200 °C, and to show that the technique affords new mechanistic insights that are inaccessible with the traditional resistance-based measurements. [ABSTRACT FROM AUTHOR]

This paper describes a theoretical study of the optical responses of an atomic medium to the probe field in a four-level (Δ + ∇)-type closed-contour interaction system driven by two pump laser fields (P1, P2), one probe laser field (Pr) and two microwave or radio frequency (RF) coupling fields (L, U). Here, L and U are considered to connect the forbidden electric dipole transitions, i.e. two hyperfine ground states of the Δ subsystem and two hyperfine excited states of the ∇ subsystem, respectively. Since each subsystem is a closed-loop system, relative phases are thought to be present between the optical fields (pump and probe fields), and the microwave or RF fields. The manipulation of the quantum coherence of the system by the strengths of coupling fields along with the relative phases is demonstrated in terms of the changes occurring in the absorption of the probe field by the medium. The probe absorption and dispersion profiles are extracted by numerically solving the optical Bloch equations for the system under steady-state conditions. In this study, we start by illuminating the system with the probe only, and then, by applying the pump fields and the coupling fields one by one, the changes in the absorptive and dispersive probe line profiles are investigated for two distinct cases that depend on the relative strengths of the pump fields, e.g. when P2 is stronger than P1 and vice versa. Furthermore, the individual effects of the strengths and phases of the coupling fields on the probe absorption, transparency and amplification are also explored for both cases. All the changes that appear in the probe signal as a result of the application of different fields are explained by the modified interaction fields or the corresponding effective Rabi frequencies obtained from the partial dressed-state analysis. [ABSTRACT FROM AUTHOR]

This paper presents the evolution of the electronegativity with the applied power during the E to H mode transition in a radio frequency (rf) inductively coupled plasma (ICP) in a mixture of Ar and O2. The densities of the negative ion and the electron, as well as their ratio, i.e., the electronegativity, are measured as a function of the applied power by laser photo-detachment combined with a microwave resonance probe, under different pressures and O2 contents. Meanwhile, the optical emission intensities at Ar 750.4 nm and O 844.6 nm are monitored via a spectrograph. It was found that by increasing the applied power, the electron density and the optical emission intensity show a similar trench, i.e., they increase abruptly at a threshold power, suggesting that the E to H mode transition occurs. With the increase of the pressure, the negative ion density presents opposite trends in the E-mode and the H-mode, which is related to the difference of the electron density and energy for the two modes. The emission intensities of Ar 750.4 nm and O 844.6 nm monotonously decrease with increasing the pressure or the O2 content, indicating that the density of high-energy electrons, which can excite atoms, is monotonically decreased. This leads to an increase of the negative ion density in the H-mode with increasing the pressure. Besides, as the applied power is increased, the electronegativity shows an abrupt drop during the E- to H-mode transition. [ABSTRACT FROM AUTHOR]

In this paper, the effects of n-type GaN deep level defects on the DC, small signal AC, and radio frequency (RF) characteristics of Ni/GaN Schottky barrier impact ionization avalanche transit time (IMPATT) diode are investigated. A double avalanche termination region (DATR) structural IMPATT diode is proposed to mitigate the influences caused by these deep level defects. Simulation results show that the internal electric field, carrier generation rate and carrier velocity of IMPATT diode are affected by these deep level defects. With the increase of deep level defects density, the maximum RF output power, DC-to-RF conversion efficiency and optimum frequency of the diode all show a tendency to degenerate correspondingly. Through adjusting the electric field property properly of the diode, the DATR structural IMPATT diode improves the performances of IMPATT diode. The negative peak conductance of the improved diode is 11.4 × 103 S cm−2 at 248 GHz, showing the lowest quality factor of 1.42, the improved maximum RF output power of 1.35 MW cm−2 and DC-to-RF conversion efficiency of 15.7% at 220 GHz under the same deep level defects density, these characteristics of the improved DATR structural IMPATT diode reach the level which the low deep level defects density original diode shows. [ABSTRACT FROM AUTHOR]

It is essential to investigate the electron dynamics, particularly electron heating mechanisms in order to elucidate the bidirectional E ⇌ H transition in a low-temperature inductively coupled plasma (ICP), sustained by a radiofrequency (RF) power. E ⇌ H transitions are fully 2D or 3D phenomena, and non-invasive optical spectroscopy is appropriate for such studies. One example is a 1D-t image during one RF period. Another example is a 2D or 3D snapshot at each phase of the transition, captured using in computerized emission tomography. Most studies have been performed in argon, oxygen, or their mixture in an ICP driven at 13.56 MHz. In the present paper we review and explore the past three decades of research into the electron dynamics related to plasma sustenance at each phase of the E–H and H–E transitions, including our new results. The hysteresis of the internal plasma parameters as a function of external parameter is discussed in terms of a change of species density, particularly long-lived metastables in plasma in the bidirectional E–H transition. [ABSTRACT FROM AUTHOR]

In order to understand, predict and control ion cyclotron range of frequency interactions with tokamak scrape-off layer plasmas, computational tools which can model radio frequency (RF) sheaths are needed. In particular, models for the effective surface impedance and DC rectified sheath potentials may be coupled with full wave RF simulation codes to predict self-consistent wave fields near surfaces and the resulting power dissipation and plasma–material interactions from ion sputtering. In this study, previous work assuming zero net DC current flow through the sheath is generalized to allow the surface to collect net positive or negative current, as is often observed in experiments. The waveforms, DC potential and RF admittance are investigated by means of analytical theory, nonlinear fluid and particle-in-cell codes. Cross-code comparisons provide detailed model verification and elucidate the roles of ion and electron kinetics. When the sheath draws negative (positive) DC current, the voltage rectification is reduced (increased) compared with the zero-current case, and the magnitude of both the real and imaginary parts of the admittance are increased (reduced). A previous four-input parametrization of the sheath rectification and admittance properties is generalized to include a fifth parameter describing the DC sheath current. [ABSTRACT FROM AUTHOR]

Low-temperature plasma (LTP) ignition concepts rely on the production of radical and charged species to speed up the onset of combustion in spark-ignition engines. These features are responsible for the superior performance of LTP igniters under extremely dilute combustion operation that is not achievable by conventional spark igniters. Additionally, LTP discharges extend the lifetime of the igniters, due to the avoidance of spark processes. For these reasons, the engine research community and the automotive industry have shown growing interest in this technology in the recent years. As of today, computational fluid-dynamics (CFD) codes typically used by the multi-dimensional engine modeling community do not have reliable models to describe LTP ignition processes. One key missing piece of information is the physical and chemical properties of the plasma and their effect on combustion ignition. Most non-equilibrium plasma simulations reported in literature are based on simplified, canonical geometries, with simple discharge excitation schemes. In this paper we conduct multi-dimensional modeling of the non-equilibrium plasma generated by an application-relevant radio-frequency (RF) corona discharge in air. Three test cases are simulated, characterized by different environmental pressure levels and peak electrode voltage values at room temperature. Streamer penetration, electron number density, atomic oxygen production, and bulk gas temperature distribution in the first 10 sinusoidal pulses are presented and discussed. This model can be used as a key tool for an in-depth understanding of RF-corona discharge for automotive applications and provides the basis for future implementations of dedicated LTP ignition models in CFD codes. [ABSTRACT FROM AUTHOR]

The Material Plasma Exposure eXperiment (MPEX) is a steady state linear device with the goal to perform plasma material interaction studies at future fusion reactor relevant conditions. A prototype of MPEX referred as 'Proto-MPEX' is designed to carry out research and development related to source, heating and transport concepts on the planned full MPEX device. The auxiliary heating schemes in MPEX are based on cyclotron resonance heating with radio frequency (RF) waves. Ion cyclotron heating (ICH) and electron cyclotron heating in MPEX are used to independently heat the ions and electrons and provide fusion divertor conditions ranging from sheath-limited to fully detached divertor regimes at a material target. A hybrid particle-in-cell code- PICOS++ is developed and applied to understand the plasma parallel transport during ICH in MPEX/Proto-MPEX to the target. With this tool, evolution of the distribution function of MPEX/Proto-MPEX ions is modeled in the presence of (a) Coulomb collisions, (b) volumetric particle sources and (c) quasi-linear RF-based ICH. The code is benchmarked against experimental data from Proto-MPEX and simulation data from B2.5 EIRENE. The experimental observation of 'density-drop' near the target in Proto-MPEX and MPEX during ICH is demonstrated and explained via physics-based arguments using PICOS++ modeling. In fact, the density drops at the target during ICH in Proto-MPEX/MPEX to conserve the flux and to compensate for the increased flow during ICH. Furthermore, sensitivity scans of various plasma parameters with respect to ICH power are performed for MPEX to investigate its role on plasma transport and particle and energy fluxes at the target. [ABSTRACT FROM AUTHOR]

Experimental campaigns were undertaken to understand and improve the coupling efficiency of Radio frequency (RF) power into the plasma in three accelerator-based ion sources. Different matching circuit and mechanical engineering setups were used and the network resistance calculated. The efficiency was then measured for a range of RF frequencies and input gas flows. Coupling efficiencies of around 60% were measured in setups using RF-coils mounted external to the plasma chamber. The efficiency is improved to 80% when the coil is immersed in the plasma, allowing closer coupling. As well as the coil geometry, the isolation transformer required for beam production contributes to the overall losses. [ABSTRACT FROM AUTHOR]

High-voltage direct current (HVDC) cables are increasingly being installed to connect new offshore wind farms. Unplanned outages of these connectors can cause high economic impacts. Hence, there are requirements for condition-based maintenance that can improve operational reliability. Partial discharge (PD) is indicative of insulation defects. PD monitoring for AC cables is well established, but before applying the technique to HVDC cable connections, it is important to characterise PD behaviour under DC conditions and the attenuation in HVDC cables. This paper investigates PD activity under non-ideal DC stress, PD signal attenuation in HVDC cables, and electromagnetic noise in converter stations. Under the voltage of superimposed DC and harmonics, PD pulses tend to synchronise with the phase of harmonics. Therefore, synchronised recording of PD pulses can produce phase resolved patterns as an additional tool for insulation diagnostics. Modelling of attenuation in a HVDC transmission cable indicates that a detection bandwidth of tens of kHz to a few MHz may improve detection sensitivity when measuring PD current pulses over very long cable runs is carried out through sensors such as high frequency current transformers (HFCTs) installed at cable ends. Additionally, the RF spectrum measured in a converter station cable hall did not include any switching-related signals, demonstrating the viability of RF sensors based PD monitoring for the HV components associated with the cable connections. [ABSTRACT FROM AUTHOR]

A one-dimensional self-consistent calculation model of capacitively coupled plasma (CCP) discharge and electromagnetic wave propagation is developed to solve the plasma characteristics and electromagnetic wave transmission attenuation. Numerical simulation results show that the peak electron number density of argon is about 12 times higher than that of helium, and that the electron number density increases with the augment of pressure, radio frequency (RF) power, and RF frequency. However, the electron number density first increases and then decreases as the discharge gap increases. The transmission attenuation of electromagnetic wave in argon discharge plasma is 8.5-dB higher than that of helium. At the same time, the transmission attenuation increases with the augment of the RF power and RF frequency, but it does not increase or decrease monotonically with the increase of gas pressure and discharge gap. The electromagnetic wave absorption frequency band of the argon discharge plasma under the optimal parameters in this paper can reach the Ku band. It is concluded that the argon CCP discharge under the optimal discharge parameters has great potential applications in plasma stealth. [ABSTRACT FROM AUTHOR]

Similarity laws (SLs) are useful for correlation and prediction of plasma parameters at different scales, which have been verified for geometrically symmetric capacitive radio-frequency (rf) discharges in nonlocal kinetic regimes. In this work, we demonstrate the applicability of SLs to dual-frequency rf discharges and confirm that similarity relations still hold considering the electrical asymmetry effect (EAE). By simultaneously tuning the control parameters (the gas pressure p, discharge gap d, and driving frequency f), we examine the similarity relations in rf plasmas via fully kinetic particle-in-cell simulations with the external circuits coupled and solved self-consistently. The validity of the SL scalings in dual-frequency rf plasmas with the EAE is confirmed for parameters such as the electron/ion density, ion flux, dc self-bias, ion energy distribution function, and power absorption. Although adjusted by the EAE, the dc self-bias and ion energy distribution functions are identified as similarity invariants under similar discharge conditions. Furthermore, the plasma series resonance phenomenon, filamentation of power depositions of electrons and ions in bulk plasma, and electric field reversal are observed in dual-frequency discharges with the EAE, which can also be exactly replicated under similar discharge conditions. The results further extend the application of SL scaling to dual-frequency rf plasmas, providing a more comprehensive understanding of the scaling characteristics in rf plasmas. [ABSTRACT FROM AUTHOR]

Based upon the new designed helical resonator, the resonant radio frequency (RF) for trapping ions can be consecutively adjusted in a large range (about 12 MHz to 29 MHz) with high Q -factors (above 300). We analyze the helical resonator with a lumped element circuit model and find that the theoretical results fit well with the experimental data. With our resonator system, the resonant frequency near magic RF frequency (where the scalar Stark shift and the second-order Doppler shift due to excess micromotion cancel each other) can be continuously changed at kHz level. For 88Sr+ ion, compared to earlier results, the measurement accuracy of magic RF frequency can be improved by an order of magnitude upon rough calculation, and therefore the net micromotion frequency shifts can be further reduced. Also, the differential static scalar polarizability Î" α 0 of clock transition can be experimentally measured more accurately. [ABSTRACT FROM AUTHOR]

Radio frequency ablation and microwave hyperthermia are powerful tools for destroying dysfunctional biological tissues. However, wireless application of these techniques is hindered by their inability to focus the electromagnetic energy to small targets. The use of locally injected radio frequency- or microwave-absorbing nanomaterials can help to overcome this challenge by confining heat production to the injected region. Previous theoretical work suggests that high-aspect-ratio conducting nanomaterials, such as carbon nanotubes, offer powerful radio frequency and microwave absorption. While carbon nanotubes have been demonstrated as radiothermal agents, common solubilization methods may reduce their absorption efficiency, yielding undesirable nonspecific heating in the biological tissue background. In this manuscript, we hypothesize that pristine carbon nanotubes can act as efficient absorbers at radio frequencies, thus providing differential heating over the tissue background. Specifically, we use a sonication-free preparation technique to preserve both the high aspect ratio and local concentration of pristine carbon nanotubes. We validate the differential heating of these samples by 4.5-fold at 2 GHz compared to the heating of saline at a physiological concentration using infrared thermography. In addition, we successfully achieved local heating of pristine carbon nanotubes within a three-dimensional biological tissue phantom. Numerical simulations further aid in producing a temperature map within the phantom and confirming localized heating. Due to their significant differential and local heating, we believe that pristine carbon nanotubes may facilitate region-specific radio frequency ablation and microwave hyperthermia while keeping nonspecific heating to a low level in the normal tissue background. [ABSTRACT FROM AUTHOR]

We present measurements of the time evolution of plasma and electrical parameters in a pulsed capacitively coupled argon discharge operated at a radio frequency of 12.5 MHz, whose amplitude is ‘step-up’ and ‘step-down’ modulated. The ‘step-up (-down)’ amplitude-modulated waveform consists of three segments, i.e., a low (high)-voltage, a high (low)-voltage, and a zero-voltage stage. Here, we focus on the effect of the ratio (ζ = V L/ V H ⩽ 1) of the low-(V L) to high-voltage (V H) amplitude (measured at the end of the respective segment) on the time evolution of discharge parameters. We monitor the behavior of the discharge by measuring (i) the optical emission intensity (OEI) of a selected Ar-I spectral line, (ii) the electron density at the center of the plasma (using a hairpin probe) as well as (iii) the electrical characteristics (by voltage and current probes). It is found that at relatively large ζ (i.e., at low disparity between the two voltage amplitudes), for both the ‘step-up’ and ‘step-down’ cases, these parameters evolve relatively smoothly with time upon changing the voltage amplitude, and the ignition process strongly depends on the duration of the zero-voltage period. At low ζ (i.e., at high disparity between the voltage amplitudes), an abnormal evolution of the parameters can be observed during the low-voltage period for both cases. Specifically, the voltage amplitude and the modulus of the system impedance increase to a higher value, while the relative phase, φ vi, between the voltage and the current approaches 90°, resulting in a reduction of the power deposition and the OEI. The enhanced voltage amplitude decreases to a steady-state value, accompanied by a decline of φ vi, and an abnormal increase of the current amplitude and the electron density after some time, of which the duration increases with the decrease of ζ. The ζ -dependent evolution of the electron density during the low-voltage period was found to significantly affect the subsequent ignition process and electron power absorption mode at the beginning of the high-voltage period. [ABSTRACT FROM AUTHOR]

In many areas of research, from neurobiology to nuclear fusion, which investigate complex dynamical systems involving numerous, different and interconnected physical quantities, the application of advanced analysis tools based on Chaos theory and Information theory has provided significant improvements both by supporting theoretical models and by highlighting hidden relationships between quantities characterizing the observed phenomena. The present article is therefore devoted to the analysis of synchronization experiments in magnetically controlled plasmas at JET, involving a ‘target’ quantity (sawteeth) and a ‘driver’ (modulated injected radio frequency) one. The typical approach to such analysis would start from assuming that the coupling between driver and target is free from external influences. However, since sawteeth can occur naturally and constitute a confounding factor in the analysis, by relaxing such an assumption a more realistic description of the system can emerge. The use of the conditional joint recurrence plots, complemented by the conditional transfer entropy, has provided further evidences supporting the effective influence of the pacing on the fast ions populations of both the minority and of the main plasma species in H-mode plasmas. The results highlight specific physical factors affecting the efficiency of the pacing and are in agreement with modelling estimates. The analysis performed then paves the way for future studies on more recent DT pulses performed at JET, and on data from other synchronization experiments. [ABSTRACT FROM AUTHOR]

Modulated lower hybrid waves, are injected into the Experimental Advanced Superconducting Tokamak (EAST), to investigate the modulated evolutions of pedestal structure and unstable edge-localized modes (ELMs). The spectrum patterns of show that the ELM amplitude only slightly decreases while the ELM frequency becomes almost 3–5 times larger as the lower hybrid radio frequency (LHRF) jumps into the lower power level. Pedestal structure analysis indicates that higher LHRF power injection can increase the plasma stored energy and core plasma parameters, accompanying with the pedestal shift and gradient increase. Ion saturation contour current plots suggest that LHRF power can deposit in the plasma edge region to profoundly change the three-dimensional (3D) magnetic topology, similarly to the effect of resonant magnetic perturbations (RMPs), by inducing the helical current filaments (HCFs) following along the magnetic field lines in the scrape-off layer (SOL), which may cause the density pump-out effect by increasing the radial plasma transport. The modulated higher temperature and lower density of pedestal top-values can reduce the pedestal collisionality, which will strongly increase the bootstrap current density and gradually shift the Peeling-Ballooning mode (PBM) boundary. ELITE code stability analysis indicates that LHRF injection can lead to the relatively unstable ELMs with a higher pedestal gradient, simultaneously accompanying with the normalized pressure gradient and edge bootstrap current density increase. All of these results imply that there is a significant correlation between the ELM behavior and pedestal structure modulation because of LHRF dissipation at the plasma edge for the unstable ELM modulated case. [ABSTRACT FROM AUTHOR]

AlfvĂ©n eigenmode (AE) instabilities driven by alpha-particles have been observed in D-3He fusion experiments on the Joint European Torus (JET) with the ITER-like wall. For the efficient generation of fusion alpha-particles from D-3He fusion reaction, the three-ion radio frequency scenario was used to accelerate the neutral beam injection 100 keV deuterons to higher energies in the core of mixed D-3He plasmas at high concentrations of 3He. A large variety of fast-ion driven magnetohydrodynamic modes were observed, including the elliptical AlfvĂ©n eigenmodes (EAEs) with mode numbers n = âˆ'1 and axisymmetric modes with n = 0 in the frequency range of EAEs. The simultaneous observation of these modes indicates the presence of rather strong alpha-particle population in the plasma with a ‘bump-on-tail’ shaped velocity distribution. Linear stability analysis and Fokkerâ€"Planck calculations support the observations. Experimental evidence of the AEs excitation by fusion-born alpha-particles in the D-3He plasma is provided by neutron and gamma-ray diagnostics as well as fast-ion loss measurements. We discuss an experimental proposal for the planned full-scale D-T plasma experiments on JET based on the physics insights gained from these experiments. [ABSTRACT FROM AUTHOR]

Radio-frequency-driven atmospheric pressure plasma jets (RF APPJs) play an essential role in many technological applications. This work studies the characteristics of these discharges in the so-called non-neutral regime where the conventional structure of a quasi-neutral bulk and an electron depleted sheath does not develop, and the electrons are instead organized in a drift-soliton-like structure that never reaches quasi-neutrality. A hybrid particle-in-cell/Monte Carlo collisions (PIC/MCC) simulation is set up, which combines a fully kinetic electron model via the PIC/MCC algorithm with a drift-diffusion model for the ions. In addition, an analytical model for the electron dynamics is formulated. The formation of the soliton-like structure and the connection between the soliton and the electron dynamics are investigated. The location of the electron group follows a drift equation, while the spatial shape can be described by Poissonâ€"Boltzmann equilibrium in a co-moving frame. A stability analysis is conducted using the Lyapunov method and a linear stability analysis. A comparison of the numerical simulation with the analytical models yields a good agreement. [ABSTRACT FROM AUTHOR]

Almost all existing studies on inter-satellite radio-frequency (RF) measurement have focused on two-satellite formations. Although some frequency division multiple access and code division multiple access multisatellite RF measurement schemes have been proposed, their poor scalability does not satisfy the inter-satellite measurement requirements of multisatellite formations, especially large-scale formations. Two-way ranging (TWR), which is based on a time division mechanism, is an effective solution that has been used for inter-satellite links in the global positioning system and Beidou navigation constellations. However, the high measurement accuracy achieved with TWR in these navigation constellations is heavily reliant on high-performance atomic clocks and the assistance of navigation ephemeris, which are not available on microsatellite platforms. This work focuses on a scalable multisatellite measurement scheme that adopts a distributed broadcast-based time division multiple access mechanism as the media access control layer and uses an asymmetric double-side TWR method as the physical layer. The measurement performance of the proposed scheme is evaluated through in-depth theoretical modeling, simulation verification, and experimental validation, along with a comprehensive comparison with the conventional TWR method. The experimental results show that centimeter-level measurement accuracy can be achieved with the proposed scheme when only a common miniaturized frequency source is used. This accuracy level is two orders of magnitude better than that of the TWR method, and thus satisfies the application requirements of general large-scale microsatellite formations. [ABSTRACT FROM AUTHOR]

A scheme for instantaneous frequency measurement (IFM) using two parallel I/Q modulators based on optical power monitoring is proposed. The amplitude comparison function (ACF) can be constructed to establish the relationship between the frequency of radio frequency (RF) signal and the power ratio of two optical signals output by two I/Q modulators. The frequency of RF signal can be derived by measuring the optical power of the optical signals output by two I/Q modulators. The measurement range and measurement error can be adjusted by controlling the delay amount of the electrical delay line. The feasibility of the scheme is verified, and the corresponding measurement range and measurement error of the system under different delay amounts of the electrical delay line are given. Compared with previous IFM schemes, the structure of this scheme is simple. Polarization devices, a photodetector and an electrical power meter are not used, which reduces the impact of the environmental disturbance on the system and the cost of the system. In simulation, the measurement range can reach 0 GHzâ€"24.5 GHz by adjusting the delay amount of the electrical delay line Ď„ = 20 ps. The measurement error of the scheme is better at low frequency, and the measurement error of low frequency 0 GHzâ€"9.6 GHz can reach â€"0.1 GHz to +0.05 GHz. [ABSTRACT FROM AUTHOR]

Soliton crystal micro-combs are powerful tools as sources of multiple wavelength channels for radio frequency (RF) signal processing. They offer a compact device footprint, a large number of wavelengths, very high versatility, and wide Nyquist bandwidths. Here, we demonstrate integral order RF signal processing functions based on a soliton crystal micro-comb, including a Hilbert transformer and first, second and third-order differentiators. We compare and contrast the results and the trade-offs involved with varying the comb spacing, and tap design and shaping methods. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics plasma simulations of a capillary capacitively coupled radio frequency thruster discharge with front powered electrode was performed. Good agreement with experimental results was obtained for the global parameters of the thruster. The spatial characteristics of the discharge were analyzed. DC bias is found to develop in accordance with classical theory of asymmetric capacitive discharge. The electrons temperature in the plasma bulk is ∼2 eV and increasing the sheath near the powered electrode to ∼6 eV. The plasma density is 1019 m−3 whereas excited argon species are 1020 m−3. The thrust can be completely attributed to gas dynamic acceleration. [ABSTRACT FROM AUTHOR]

The particle-in-cell/Monte Carlo collisions (PIC/MCC) simulation approach has become a standard and well-established tool in studies of capacitively coupled radio frequency (RF) plasmas. While code-to-code benchmarks have been performed in some cases, systematic experimental validations of such simulations are rare. In this work, a multi-diagnostic experimental validation of 1d3v electrostatic PIC/MCC simulation results is performed in argon gas at pressures ranging from 1Â Pa to 100Â Pa and at RF (13.56Â MHz) voltage amplitudes between 150Â V and 350Â V using a custom built geometrically symmetric reference reactor. The gas temperature, the electron density, the spatio-temporal electron impact excitation dynamics, and the ion flux-energy distribution at the grounded electrode are measured. In the simulations, the gas temperature and the electrode surface coefficients for secondary electron emission and electron reflection are input parameters. Experimentally, the gas temperature is found to increase significantly beyond room temperature as a function of pressure, whereas constant values for the gas temperature are typically assumed in simulations. The computational results are found to be sensitive to the gas temperature and to the choice of surface coefficients, especially at low pressures, at which non-local kinetic effects are prominent. By adjusting these input parameters to specific values, a good quantitative agreement between all measured and computationally obtained plasma parameters is achieved. If the gas temperature is known, surface coefficients for different electrode materials can be determined in this way by computationally assisted diagnostics. The results show, that PIC/MCC simulations can describe experiments correctly, if appropriate values for the gas temperature and surface coefficients are used. Otherwise significant deviations can occur. [ABSTRACT FROM AUTHOR]

Purpose. To characterize and quantify the induced radiofrequency (RF) electric (E)-fields and B1+rms fields in patients undergoing magnetic resonance (MR) examinations; to provide guidance on aspects of RF heating risks for patients with and without implants; and to discuss some strengths and limitations of safety assessments in current ISO, IEC, and ASTM standards to determine the RF heating risks for patients with and without implants. Methods. Induced E-fields and B1+rms fields during 1.5 T and 3 T MR examinations were numerically estimated for high-resolution patient models of the Virtual Population exposed to ten two-port birdcage RF coils from head to feet imaging landmarks over the full polarization space, as well as in surrogate ASTM phantoms. Results. Worst-case B1+rms exposure greater than 3.5 μT (1.5 T) and 2 μT (3 T) must be considered for all MR examinations at the Normal Operating Mode limit. Representative induced E-field and specific absorption rate distributions under different clinical scenarios allow quick estimation of clinical factors of high and reduced exposure. B1 shimming can cause +6 dB enhancements to E-fields along implant trajectories. The distribution and magnitude of induced E-fields in the ASTM phantom differ from clinical exposures and are not always conservative for typical implant locations. Conclusions. Field distributions in patient models are condensed, visualized for quick estimation of risks, and compared to those induced in the ASTM phantom. Induced E-fields in patient models can significantly exceed those in the surrogate ASTM phantom in some cases. In the recent 19ε2 revision of the ASTM F2182 standard, the major shortcomings of previous versions have been addressed by requiring that the relationship between ASTM test conditions and in vivo tangential E-fields be established, e.g. numerically. With this requirement, the principal methods defined in the ASTM standard for passive implants are reconciled with those of the ISO 10974 standard for active implantable medical devices. [ABSTRACT FROM AUTHOR]

Radiofrequency emission in the 0.4–3 GHz range from FTU tokamak in presence of runaway electrons (REs) has been measured in various plasma regimes. Rapid emission bursts associated with enhanced RE pitch-angle scattering reveal kinetic instabilities affecting evolution of the RE population from the buildup phase. Such measurements also provide a sensitive monitor for instabilities during early RE formation. The leading edge of radio bursts is much shorter than interleaving periods of low emission; spectral broadening during growth indicates nonlinear wave coupling as an explanation for the observed intermittency. Both broadband and coherent spectra have been observed. Radio emission disappears at the beginning of post-disruption RE plateaus, and subsequently reappears in the shape of very intense bursts accompanied by macroscopic magnetic perturbations. [ABSTRACT FROM AUTHOR]

In the 2018 EAST experimental campaign, a very high central electron heating, fully-non-inductive discharge with the core electron temperature over 9 keV has been achieved. Such high central electron heating was realized by injecting radio frequency waves, including 1.8 MW lower hybrid wave (LHW) and 0.8 MW electron–cyclotron waves (ECW). Experimental diagnosis indicates two different time scales characterizing the electron heating process, a rapid and a slow rise of the central electron temperature after the injection of ECW. In this work, integrated modeling is performed to investigate the physical mechanisms of such high electron heating. Five characteristic phases during the increase of the electron temperature are chosen for modeling. In phase 1 , the electron heating is by LHW alone. The modeling confirms that the LHW can only sustain the core electron temperature Te ≈ 5.5 keV, which is consistent with the experiment. In phase 2 , the electron temperature increases rapidly after the first 0.4 MW ECW is injected. The result shows that the rapid increase of the electron temperature is due to the interaction between the ECW and the electrons. With the increase of the electron temperature, the electron flux induced by the trapped electron modes (TEMs) and the electron temperature gradient driven modes (ETGs) is enhanced in the core region. In phase 3 , the electron temperature increases slowly after phase 2. It is found that the slow increase is mainly due to the flattening of the density profile. The flattening of the density profile can decrease the thermal diffusivity of the electrons mainly induced by the TEMs leading to a higher electron temperature for a given heating source. In phase 4 , the electron temperature again increases rapidly after the second 0.4 MW ECW is injected. The physical mechanism is similar to that in phase 2. In phase 5 , the LHW power deposition of the LHW remains almost unchanged compared to that in phase 4 since the electron temperature is sufficiently high. The slow rise of the electron temperature is caused by the improvement of the electron energy confinement as thermal diffusivity of the electrons is decreased due to the flattening of the electron density profile, which is similar to the main reason in phase 3. [ABSTRACT FROM AUTHOR]

This paper is motivated by the recent measurement of large (>100 V) plasma potentials in Alcator C-Mod during ion cyclotron range of frequency (ICRF) heating. The plasma potential is measured on field lines that intersect a limiter but do not pass near a powered ICRF antenna. The measured potential correlates with the local ICRF fast wave electric field and is a prime candidate to cause increased Mo sputtering from the limiter surface. In this paper, it is shown that a theory of 'far-field' radio-frequency (rf) sheaths can qualitatively explain this experimental observation. The theory describes rf-sheath formation when unabsorbed fast ICRF waves are incident on a conducting boundary far from the antenna. It is shown that the rf-sheath drive is sensitive to the angle between the surface normal and the equilibrium magnetic field. The main conclusion of this work is that the rapid tangential variation in the B field-limiter geometry near the tip of the limiter promotes the formation of large sheath potentials of the same order as the measured ones. [ABSTRACT FROM AUTHOR]

The various advantages of extended-source (ES), broken gate (BG), and hetero-gate-dielectric (HGD) technology are blended together for the proposed tunnel field-effect transistor (ESBG TFET) in order to enhance the direct-current and analog/radio-frequency performance. The source of the ESBG TFET is extended into channel for the purpose of increasing the point and line tunneling in the device at the tunneling junction, and then, the on-state current for the ESBG TFET increases. The influence of the source region length on the direct-current and radio-frequency performance parameters of the ESBG TFET is analyzed in detail. The results show that the proposed TFET exhibits a high on-state current to off-state current ratio of 1013, large transconductance of 1200 μS/μm, high cut-off frequency of 72.8 GHz, and high gain bandwidth product of 14.3 GHz. Apart from these parameters, the ESBG TFET also demonstrates high linearity distortion parameters in terms of the second- and third-order voltage intercept points, the third-order input interception point, and the third-order intermodulation distortion. Therefore, the ESBG TFET greatly promotes the application potential of conventional TFETs. [ABSTRACT FROM AUTHOR]

Heteroatom doping is an effective method to improve the capacitive performance of graphene-based materials. In this work, a facile and efficient radio-frequency (RF) plasma treatment strategy has been employed to achieve simultaneous doping and reduction of graphene oxide (GO). As a result, boron-doped and nitrogen-doped reduced graphene oxide (denoted as B-rGO and N-rGO) have been synthesized rapidly under relatively low temperatures compared with conventional thermal methods. The B-rGO and N-rGO present significantly improved specific capacitances as high as 345 F g−1 and 365 F g−1 at 0.2 A g−1, respectively, exhibiting a fourfold increase compared to that of GO before plasma treatment. Interestingly, the N-rGO shows better rate capability than the B-rGO. Furthermore, the mechanism of simultaneous doping and reduction by RF plasma treatment is discussed based on the diagnosis of emission spectroscopy. The high energy electrons and plasma-excited ions and radicals render effective reduction, etching, and doping of GO at the same time. Compared with high-temperature carbonization and wet chemical methods, our plasma treatment method is more energy-saving and eco-friendly. We believe this rapid and straightforward plasma treatment method reported here can be extended to the incorporation of various heteroatoms into graphene lattice for broad applications. [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]