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The excitation of toroidicity-induced Alfvén eigenmodes (TAEs) using prescribed external electromagnetic perturbations (hereafter ‘antenna’) acting on a confined toroidal plasma, as well as its nonlinear couplings to other modes in the system, is studied. The antenna is described by an electrostatic potential resembling the target TAE mode structure along with its corresponding parallel electromagnetic potential computed from Ohm’s law. Numerically stable long-time linear simulations are achieved by integrating the antenna within the framework of a mixed representation and pullback scheme (Mishchenko et al 2019 Comput. Phys. Commun. 238 194). By decomposing the plasma electromagnetic potential into symplectic and Hamiltonian parts and using Ohm’s law, the destabilizing contribution of the potential gradient parallel to the magnetic field is cancelled in the equations of motion. Besides evaluating the frequencies and the growth/damping rates of excited modes compared to referenced TAEs, we study the interaction of antenna-driven modes with fast particles and indicate their margins of instability. Furthermore, we show the first nonlinear simulations in the presence of a TAE-like antenna exciting other TAE modes, as well as global Alfvén eigenmodes with different toroidal wave numbers from that of the antenna. [ABSTRACT FROM AUTHOR]

The application of the magnetically insulated baffled probe (MIBP) and MIBP cluster for studying properties of low-temperature and peripheral fusion plasmas is reviewed. MIBP operation principles, MIBP design strategy, and MIBP examples of measurement, data analysis, and interpretation are discussed. The implementation convenience and diagnostic usefulness, as well as the inconvenience and drawbacks, for studying plasma equilibrium and dynamics properties, are demonstrated. MIBP determination of oscillations of fluid observables, such as electron and ion temperatures, electrostatic plasma potential, and electron and ion density reveal plasma instabilities and waves. Ion and electron distribution functions, and the transport of charged-particle number, momentum, and energy can also be measured. [ABSTRACT FROM AUTHOR]

Accumulation of impurities in the core of the plasma is a potential problem for any fusion reactor, and is thought to be especially problematic for stellarators. We use the collisional transport code Sfincs to calculate, in a reactor stellarator scenario, the effects of flux-surface electrostatic potential variation on tungsten transport. The potential variation is found to have a large effect on the transport, reversing the sign of the transport coefficients related to the radial electric field and the bulk ion temperature gradient, and also increasing the magnitude of the former by two orders of magnitude. We optimize the electrostatic potential variation to minimize impurity accumulation, resulting in a 20%–50% reduction of the inward impurity flux. Furthermore, we investigate whether the electrostatic potential variation caused by fast particles due to ion-cyclotron resonance heating can be used to reduce the impurity accumulation, but find the effect to be small in the cases considered. [ABSTRACT FROM AUTHOR]

We present a combined experimental and numerical investigation of the plasma properties in an asymmetric capacitively coupled radio frequency plasma source using argon discharge. Besides driving the system in the conventional way, which results in a high negative self-bias voltage due to the asymmetric configuration, we also connect a 'quarter-wavelength filter' to the powered electrode, which lifts its DC potential to zero. At the powered side of the plasma, we employ electrodes with conducting and insulating surfaces, as well as electrodes combining both in different proportions ('hybrid electrodes'). Measurements are carried out for the plasma potential, the electron density and temperature in the bulk plasma, as well as for the flux-energy distribution of the ions at the grounded surface of the system. The nature of the surface of the powered electrode as well as the presence of the quarter-wavelength filter are found to highly influence the plasma potential,. For the electrode with a conducting surface 20 V and ∼150 V are found in the absence and the presence of the filter, respectively. For the electrode with an insulating surface, the self-bias voltage builds up directly at the plasma interface, thus the filter has no effect and a plasma potential of ∼20 V is found. For the electrodes with different conducting/insulating proportions of their surface, ranges between the above values. Particle-in-Cell/Monte Carlo Collisions calculations for identical conditions with hybrid electrodes predict double-peaked ion energy distribution at the powered electrode with peaks corresponding to and along with a lowering of the plasma potential (whencompared to wholly conducting electrode), a trend that is observed experimentally. These studies are of great importance for the application of similar plasma sources with in-situ cleaning of mirrors in fusion devices and the results can be extended to a variety of plasma processing applications. [ABSTRACT FROM AUTHOR]

Electromagnetic emission at the second harmonic of the plasma frequency produced by nonlinear interaction of counterpropagating laser-driven potential plasma waves are studied using particle-in-cell simulations. This process has been recently proposed as a method for generating high-power tunable THz radiation with a narrow spectral line-width (Timofeev et al 2017 Phys. Plasmas 24 103106). In the present paper, we find the optimal conditions for demonstrating this phenomenon in a laboratory experiment that implies excitation of colliding wakefields by axially symmetric 830 nm Gaussian laser pulses with the total energy 0.2 J in a supersonic gas jet. It is shown that the emission mechanism based on the collision of different-size wakes is always accompanied with the mechanism of plasma antenna which begins to radiate electromagnetic waves after the build-up of periodic ion density modulation. Such additional emission makes hydrogen more attractive for this generating scheme than gases with heavier atoms. [ABSTRACT FROM AUTHOR]

Dust production and its transport into the core plasma is an important issue for magnetic confinement fusion. Dust grains are charged by various processes, such as the collection of plasma particles and electron emissions, and their charge influences the dynamics of the dust. This paper presents the results of calculations of the surface potential of dust grains in a Maxwellian plasma. Our calculations include the charging balance of a secondary electron emission (SEE) from the dust. The numerical model that we have used accounts for the influence of backscattered electrons and takes into account the effects of grain size, material, and it is also able to handle both spherical and non-spherical grains. We discuss the role of the SEE under tokamak conditions and show that the SEE is a leading process for the grains crossing the scrape-off layer from the edge to core plasma. The results of our calculations are relevant for materials related to fusion experiments in ITER. [ABSTRACT FROM AUTHOR]

An extensive experimental survey of plasma potentials induced by ion cyclotron range-of frequency (ICRF) heating was carried out in the scrape-off layer (SOL) plasmas on the Alcator C-Mod tokamak. Enhanced plasma potentials >100 V are observed at locations where local magnetic fields map to active ICRF antennas. In these cases, the enhanced potential appears only when a local plasma density threshold is surpassed—a threshold that is quantitatively consistent with slow wave (SW) RF rectification theory. However, in many cases large potential enhancements are found in locations that do not map along magnetic field lines to active antennas without obstruction, i.e. locations that are inaccessible to SWs launched by the active antennas. Enhanced potentials in these ‘unmapped’ locations are correlated with local plasma parameters, ICRF electromagnetic fields associated with the fast wave (FW) and SW, launched wave spectra, and the boundary surface geometry. It is found that enhanced plasma potentials in unmapped locations correlate with the FW field strength. These observations are qualitatively consistent with a model that accounts for the conversion of FWs to SWs at conducting surfaces oriented at an oblique angle with respect to the magnetic field, with the SW leading to sheath rectification. In addition, enhanced plasma potentials are found far into the shadow of passive limiter structures. These are correlated with the magnitude of the local FW field strength, yet the effect does not follow any present model. Overall, ICRF-induced plasma potentials may appear in regions far removed from the active antennas, yet due to the complex response of the SOL potentials at a variety of boundary surfaces, it remains unclear what part of the plasma-facing wall should be targeted to mitigate ICRF-induced impurities. The results also suggest that operating active ICRF antennas in a high single pass absorption regime is crucial in minimizing the effects of the FW fields on plasma–material interactions. [ABSTRACT FROM AUTHOR]

The accurate measurement of plasma potentials at high pressure is investigated with an emissive probe. The emissive probe I-V characteristic of argon plasma obtained at 100 Pa is interpreted in detail, showing two inflection points in the I-V trace. Accurate plasma potentials at high pressure can be determined from the first inflection point potential (the higher potential) by using the improved inflection point method of the emissive probe, while the second inflection point is a result of the additional ionizing phenomenon caused by the collision between the emitted electrons and the neutral argon atoms. Additionally, the accuracy of the plasma potential measurement at high pressure is verified to be within 0.3 V, confirmed by the measurement of space potential distribution between two parallel plates in neutral gas at 100 Pa. The accurate measurement of space potentials in high pressure plasma and neutral gas indicates that the improved inflection point method of the emissive probe approach is promising in the study of collisional sheath structures and electrostatic probe diagnostics of high pressure plasmas. [ABSTRACT FROM AUTHOR]

A model for the initial phase of unipolar arcing has been developed with account of an external energy source which triggers the arcing, the vaporization of the atoms from the heated surface, the ions and electrons produced by ionization of the vapor, the electron emission from the metal surface, and melt motion and surface deformation. Current transfer outside the arc attachment is taken into account and the potential difference between the plasma and the metal surface (the plate) is evaluated from the condition that the net current transferred to the plate is zero at each moment. The model is used for simulation of the interaction of an external energy load (laser beam) with a tungsten plate immersed in a helium background plasma. The results revealed the formation of a crater, but no jet formation or droplet detachment. If the plate is large (), the peak temperature attained is 5200 K, and the plate potential remains below the plasma potential. If the plate is small (), a peak temperature of 7500 K is reached, the potential of the plate surpasses the plasma potential, circulation of the melt at the pool periphery occurs, and the erosion (which is mainly due to the vaporization of the metal atoms in the spot) reaches the value of 37 μg. [ABSTRACT FROM AUTHOR]

We present a new diagnostic tool for the determination of various plasma parameters in the edge region of medium-size tokamaks (MST) and stellarators (specifically Wendelstein 7-X) which is under development under the EUROfusion project task force MST2 and S1. This will be a probe head (called the new probe head-NPH) which will carry two cold Langmuir probes, one electron-emissive probe (EEP), two retarding field analysers (RFA) facing upstream and downstream and two magnetic pickup coils. By various adaptors, the same NPH will be used on all three European MSTs (ASDEX Upgrade, TCV and MAST-U) and on Wendelstein 7-X. For the first time the plasma potential in the edge region of MSTs and comparable toroidal fusion experiments will be directly determined by an EEP that will be permanently heated during the measurements. After the introduction and the theoretical background especially of the EEP, the NPH and its components are described in detail. The NPH will be able to measure electron and ion temperature, electron and ion density, cold floating potential, plasma potential and magnetic fluctuations in all three directions of space at two radial positions. [ABSTRACT FROM AUTHOR]

Generation of electromagnetic (EM) radiation near the second harmonic of the plasma frequency during the injection of an electron beam into a rippled-density plasma channel is investigated using both analytical theory and particle-in-cell simulations. The generating scheme is based on nonlinear interaction of the most unstable beam-driven potential plasma wave with its satellite arising due to scattering on the longitudinal modulation of plasma density. Resulting superluminal oscillations of electric current in a finite-size plasma channel radiate EM waves via the same mechanism which has been recently studied for the fundamental harmonic emission and reffered as a beam-driven plasma antenna. It is shown that theoretical predictions for the optimal plasma width and modulation period are confirmed by simulation results and the power conversion efficiency of the second harmonic emission reaches several percent. Such an efficient mechanism opens the path to explanation of laboratory experiments with a thin electron beam at the GOL-3 mirror trap as well as to developing the scheme of terahertz generation at the gigawatt power level. [ABSTRACT FROM AUTHOR]

Impurity transport in the T-10 tokamak plasma with ohmic heating is studied in this paper. The values of various impurities densities, measured with the use of passive spectral diagnostics in the visible (Zeff), active charge exchange measurements (He, C, O), and integral bolometric measurements with absolute extreme ultraviolet detectors (Fe, W) are shown. The experimental data show that accumulation level is growing with impurity nuclear charge and determined by the parameter , which is common for all sorts of impurities. Accumulation process is determined by neoclassical processes and begins with the increase of impurity content in the plasma and ends with the formation of density profiles more peaked than the ne(r). In discharges with low γ anomalous transport completely dominates. So it prevents the impurity accumulation and flattens their density profiles down to the ne(r). These observations correlates with measured negative (positive) plasma potential in discharges with high γ (low γ). 1D modelling using ASTRA and STRAHL transport codes is performed to describe the behaviour of impurities in a wide range of T-10 ohmic regimes. It is shown that the coefficients of anomalous transport Dan and Van established in Krupin et al (1983 Sov. J. Plasma Phys.9 529–36) and Krupin et al (1985 12th EPS Conf. on Plasma Physics) by describing the density dynamics of injected argon and potassium ions are applicable for the modelling of the He, C, O, W impurity density profiles and their sources. The analysis of the obtained results allows us to state the existence of a common dependence of the anomalous transport for all ions (impurities and deuterons) on the discharge parameters in the T-10 ohmic regimes. [ABSTRACT FROM AUTHOR]

Plasma potentials computed from Langmuir probe data rely on a method to account for secondary electron emission (SEE) from the electrodes. However, significant variations exist among published models for SEE and the reported experimental parameters used to evaluate them. As a means to critically assess SEE computation methods, two of four tungsten electrodes on a Langmuir–Mach probe head were replaced with molybdenum and exposed to Alcator C-Mod boundary plasmas where electron temperatures exceed 50 eV and SEE becomes significant. In this situation, plasma potentials computed for either material should be identical—the SEE evaluation method should properly account for the differences in SEE yields. Of the six methods used to compute SEE, two are found to produce consistent results (Sternglass model with Bronstein experimental parameters and Young–Dekker model with Bronstein experimental parameters). In contrast, the method previously used for C-Mod data analysis (Sternglass model with Kollath parameters) was found to be inconsistent. We have since adopted Young–Dekker–Bronstein as the preferred method. [ABSTRACT FROM AUTHOR]

The radial distributions of the main plasma parameters in the scrape-off-layer of the COMPASS tokamak are measured during L-mode and H-mode regimes by using both Langmuir and ball-pen probes mounted on a horizontal reciprocating manipulator. The radial profile of the plasma potential derived previously from Langmuir probes data by using the first derivative probe technique is compared with data derived using ball-pen probes. A good agreement can be seen between the data acquired by the two techniques during the L-mode discharge and during the H-mode regime within the inter-ELM periods. In contrast with the first derivative probe technique, the ball-pen probe technique does not require a swept voltage and, therefore, the temporal resolution is only limited by the data acquisition system. In the electron temperature evaluation, in the far scrape-off layer and in the limiter shadow, where the electron energy distribution is Maxwellian, the results from both techniques match well. In the vicinity of the last closed flux surface, where the electron energy distribution function is bi-Maxwellian, the ball-pen probe technique results are in agreement with the high-temperature components of the electron distribution only. We also discuss the application of relatively large Langmuir probes placed in parallel and perpendicularly to the magnetic field lines to studying the main plasma parameters. The results obtained by the two types of the large probes agree well. They are compared with Thomson scattering data for electron temperatures and densities. The results for the electron densities are compared also with the results from ASTRA code calculation of the electron source due to the ionization of the neutrals by fast electrons and the origin of the bi-Maxwellian electron energy distribution function is briefly discussed. [ABSTRACT FROM AUTHOR]

Gas-puff imaging techniques are employed to determine the far SOL region radial electric field and the plasma potential in ICRF heated discharges in the Alcator C-Mod tokamak. The two-dimensional velocity fields of the turbulent structures, which are advected by RF-induced flows, are obtained via the time-delay estimation (TDE) techniques. Both the magnitude and radial extension of the radial electric field Er are observed to increase with the toroidal magnetic field strength Bφ and the ICRF power. In particular, the RF-induced Er extends from the vicinity of the ICRF antenna to the separatrix when and . In addition, low-Z impurity seeding near the antenna is found to substantially reduce the sheath potential associated with ICRF power. The TDE techniques have also been used to revisit and estimate ICRF-induced potentials in different antenna configurations: (1) conventional toroidally aligned (TA) antenna versus field-aligned (FA) antenna; (2) FA monopole versus FA dipole. It shows that FA and TA antennas produce similar magnitude of plasma potentials, and the FA monopole induced greater potential than the FA dipole phasing. The TDE estimations of RF-induced plasma potentials are consistent with previous results based on the poloidal phase velocity. [ABSTRACT FROM AUTHOR]

Ball-pen probes (BPP) have been deployed in the SOL of numerous tokamak experiments and low-temperature magnetised plasmas to make direct measurements of the plasma potential and electron temperature. Despite strong empirical evidence for the success of the BPP it lacks a theoretical underpinning of its collection mechanism. In this paper we investigate the capability of the probe to measure the plasma potential by means of particle-in-cell simulations. The BPP is found to float at a potential offset from the plasma potential by a factor . By simulating BPPs and Langmuir probes, excellent agreement has been found between the measured electron temperature and the specified source temperature. The transport mechanism for both ions and electrons has been determined. E × B drifts are observed to drive electrons and ions down the tunnel. This mechanism is sensitive to the diameter of the probe. [ABSTRACT FROM AUTHOR]

The edge topology of helically symmetric experiment (HSX) in the quasi-helically symmetric configuration is characterized by an 8/7 magnetic island remnant embedded in a short connection length scrape-off layer (SOL) domain. A 2D mapping of edge plasma profiles within this heterogeneous SOL has been constructed using a movable, multi-pin Langmuir probe. Comparisons of these measurements to edge simulations using the EMC3-EIRENE 3D plasma fluid and kinetic neutral gas transport model have been performed. The measurements provide strong evidence that particle transport is diffusive within the island region and dominantly convective in the SOL region. Measurements indicate that phenomenological cross-field diffusion coefficients are low in the SOL region between the last closed flux surface and edge island (i.e. m2 s−1). This level of transport was found to increase by a factor of two when a limiter is inserted almost completely into the magnetic island. A reduction in gradients of the edge electrostatic plasma potential was also measured in this configuration, suggesting that the reduced electric field may be linked to the increased cross-field transport observed. [ABSTRACT FROM AUTHOR]

The influence of long-scale length radial electric fields on zonal flows-like structures has been studied in the TJ-II stellarator. This relation has been investigated in the edge plasma using two electrical rake probes. The results presented here show an empirical correlation between the properties of long-range correlations (LRCs) with zonal flow-like structures and the magnitude of radial (neoclassical, NC) electric fields in TJ-II neutral beam heated plasmas. These experimental findings show that the enhancement of the NC radial electric field increases the magnitude of LRCs, considered as a proxy of zonal flows, while the radial correlation length of the plasma potential fluctuations was found to decrease by about 40%. A strong relation between the magnitude of electric field structures with long and short radial scales was found. The calculated shearing rate corresponding to the short scale length structures of the radial electric field may be sufficient to regulate turbulence. [ABSTRACT FROM AUTHOR]

Neoclassical theory provides usable expressions for studying transport in toroidal plasmas and computing the associated radial electric field. An algebraic and three semi-analytical models are used here to study the radial electric field in TJ-II plasmas and compare it with experimental data from a heavy ion beam probe (HIBP) and with DKES calculations. Good qualitative agreement as well as reasonable quantitative agreement is found which allows us to validate the models for describing TJ-II radial electric fields. Furthermore, a simple algebraic formulation (2005 Plasma Phys. Rep. 31 14) provides physical insight for the interpretation of experimental data from the TJ-II heliac in spite of its complicated geometry, like the place of the transition from the electron to the ion root of the radial electric field, which occurs at the maximum value of collisionality, for example. [ABSTRACT FROM AUTHOR]

The impact of applied magnetic perturbations (MPs) on tokamak edge parameters has been investigated in ASDEX Upgrade low collisionallity L-mode discharges using a flexible set of in-vessel saddle coils (capable of generating n = 0, 1, 2 and 4 toroidal modes) and an extensive set of high resolution edge diagnostics. Doppler reflectometry is used principally to probe the MP field penetration and structure via the radial electric field Er and density fluctuation δne behaviour. Different MP response behaviour are observed for the near scrape-off-layer, SOL, (where Er flattens) and in the confinement region (where the negative Er well reverses). The radial structure of Er and δne are particularly sensitive to the degree of MP resonance with the edge rational field-lines. Specifically, the edge turbulence is enhanced for strongly resonant MPs and reduced when non-resonant. The toroidal structure of the MP response has also been probed for various MP configurations by rotating the MP field toroidally and is found to be different for the edge and near SOL regions. [ABSTRACT FROM AUTHOR]