Poloidal Wave Research Articles

Characterization of SOL profiles and turbulence in ICRF-heated plasmas in EAST

Scrape-off layer (SOL) profiles and turbulence in ion cyclotron range of frequency (ICRF)-heated plasmas are investigated by the reciprocating probe diagnostic system (FRPs) and gas puff imaging (GPI) diagnostic in EAST. A radio-frequency (RF) sheath potential reaching up to 100 V is identified proximate to the ICRF antennas. Notably, the amplitude of this RF sheath potential escalates in response to rising ICRF power and inversely with plasma density. When a RF sheath is present in the far SOL, a pronounced density ‘shoulder’ forms in front of the ICRF antennas, while the ‘shoulder’ fade away as the antenna and associated RF sheath shift outwards. A strong E r shear is revealed by measurements from both FRPs and GPI. Analysis of the poloidal wave number-frequency spectrum reveals suppression of high-frequency turbulence in the far SOL due to the RF sheath. This effect is manifested in the reduced autocorrelation time τ c and reduced average blob size δ blob of the SOL plasma. Intriguingly, the poloidal propagation direction of the low-frequency turbulence reverses from the electron to the ion diamagnetic drift direction at the RF sheath location. A surge of tungsten impurity is potentially attributed to the heightened interaction between the SOL plasmas and the wall material. Shifting the ICRF antennas outward, to alleviate heat spots, results in the relocation of the RF sheath to the shaded region of the main limiter. This shift amplifies the radial velocity of blobs in the far SOL and concurrently diminishes the SOL density when compared to conditions without ICRF injection. The properties of ion saturation current fluctuations are consistent with the stochastic model predictions.

High-frequency fluctuation and EHO-like mode in the H-mode pedestal on the EAST tokamak

In the pedestal region of the Experimental Advanced Superconducting Tokamak (EAST) during high confinement mode plasma operations with radio-frequency heating, two distinct fluctuations are observed: high-frequency fluctuations (HFFs) and edge harmonic oscillation-like (EHO-like) modes. The HFFs are characterized by intermittent fluctuations with a broadband frequency range of 1−3 MHz and a poloidal wave number ( kθ ) greater than 0.9 cm−1 . On the other hand, the EHO-like mode exhibits characteristics similar to magnetohydrodynamics (MHD)-like modes with n = 1−5 and lower poloidal wave numbers ( kθ⩽0.12 cm−1 ). During the pedestal establishing phase following the L–H transition, a significant concurrent presence of HFF and EHO-like modes in high-density pedestal regions has been noted. In this phase, the EHO-like mode not only modulates the amplitude of the HFF but also engages in nonlinear interactions. The occurrence of EHO-like mode and HFF is associated with particle transport toward the divertor, though it is notably less than that caused by edge coherent modes. During the inter-edge localized mode (ELM) period, a significant decrease in the Dα baseline is observed whenever the low frequency fluctuation (LFF) weakens and the HFF grows, prior to each large ELM. One possible explanation is that the rapid increase of E×B shear stabilizes the LFF and destabilizes the HFF, which lowers the pedestal transport and enables the further growth of the pedestal until the onset of the ELM.

Experimental observation of micro-tearing modes in the tokamak pedestal

A micro-tearing mode has been observed in electron temperature fluctuations and magnetic fluctuations with the frequency range of 80–120 kHz in the pedestal of the Experimental Advanced Superconducting Tokamak (EAST) using Electron Cyclotron Emission Imaging (ECEI) and Mirnov magnetic coil measurement. It appears in the type-I ELMs period after the L-H transition. The measured poloidal phase velocity of the electron temperature fluctuations is ∼40 km/s (f = 80–120 kHz) in the electron diamagnetic direction in the laboratory frame with the poloidal wave number around kΘ ∼0.15 /cm. The two-dimensional structure of MTMs is directly observed using two-dimensional Electron cyclotron emission imaging. The experiment confirmed that the electron temperature (ECEI) and radial electric field (Backward Doppler reflectometer) during MTM meet the pressure balance equation. Until now, the theory and simulation results showed that transport is in heat flux, small particle flux, while the experimental results of this paper indicate that the particle flux is small.

Global gyrokinetic simulations of electrostatic microturbulent transport in LHD stellarator with boron impurity

Global gyrokinetic simulations of electrostatic microturbulent transport for discharge # 166256 of the Large Helical Device stellarator in the presence of boron impurity show the co-existence of the ion temperature gradient (ITG) turbulence and trapped electron mode (TEM) turbulence before and during boron powder injection. ITG turbulence dominates in the core, whereas TEM dominates near the edge, consistent with the experimental observations. Linear TEM frequency increases from ∼80 kHz to ∼100 kHz during boron injection, and ITG frequency decreases from ∼20 kHz to ∼13 kHz, consistent with the experiments. The poloidal wave number spectrum is broad for both ITG (0–0.5 mm−1) and TEM (0–0.25 mm−1). The nonlinear simulations with boron impurity show a reduction in the heat conductivity compared to the case without boron. The comparison of the nonlinear transport before and during boron injection shows that the ion heat transport is substantially reduced in the region where the TEM is dominant. However, the average electron heat transport throughout the radial domain and the average ion heat transport in the region where the ITG is dominant are similar. The simulations with boron show the effective heat conductivity values qualitatively agree with the estimate obtained from the experiment.

Particle pump-out induced by trapped electron mode turbulence in electron cyclotron heated plasmas on XuanLong-50 spherical torus

Particle pump-out effects induced by low-frequency (<200 kHz) density fluctuations were observed in solely electron cyclotron wave (ECW)-heated plasmas on the spherical torus XuanLong-50 (EXL-50) without a central solenoid. The intensity of the relative density fluctuations increases with increasing ECW power and decays when the ECW is turned off while sustaining the plasma current. The electron densities are maintained relatively high and steady when the density fluctuations are completely absent, indicating that the outward transport of electrons is dominated by the particle pump-out effect of the ECW. The density fluctuations are modulated by a supersonic molecular beam injection pulse and the modulation amplitude decreases with increasing electron density at the same ECW injection power and decreasing ECW power at the same electron density, respectively. Analysis revealed that a critical value of electron temperature gradient (ETG) triggers the density fluctuations, and the intensity of the relative density fluctuations is positively correlated with the ETG and approximately inversely proportional to the effective collision frequency. With plasma parameters similar to those of EXL-50 experiments, the HD7 code simulations demonstrate that trapped electron mode (TEM) turbulence can be excited by ETG higher than the critical value observed in the experiment. In addition, the dependence of the mode growth rate (supposed to be proportional to the saturation level of fluctuations in quasi-linear theory) and the measured intensity of the density fluctuations is comparable. The simulated outward particle flux integrated over the poloidal wave number spectrum is significant and proportional to ETG. These observations demonstrate that the density fluctuation is TEM turbulence, which is driven by ETG and induces particle pump-out when the electron density/effective electron collision frequency is low. The potential relevance of this work with the controls of plasma profiles, impurities, helium ash, and heat transport in future reactors of similar low effective collision frequency is also discussed.

A new electromagnetic probe array diagnostic for analyzing electrostatic and magnetic fluctuations in EAST plasmas

A novel electromagnetic probe array (EMPA) diagnostic, which consists of a magnetic probe array and an electrostatic probe array, has recently been developed on EAST. The EMPA is fixed near the first wall at horizontal port P. The magnetic probe array of the EMPA consists of 24 identical magnetic probes, each of them capable of measuring toroidal, poloidal and radial magnetic fluctuations simultaneously, providing additional toroidal magnetic fluctuation measurements compared with the regular magnetic probes on EAST. With a higher sampling rate and self-resonant frequency, the EMPA magnetic probes can provide higher frequency magnetic fluctuation measurements. The magnetic probe array of the EMPA is composed of two parallel layers of magnetic probes with a radial distance of 63 mm, and each layer of magnetic probes is arranged in four poloidal rows and three toroidal columns. The compact arrangement of the EMPA magnetic probe array largely improves the toroidal mode number measurement ability from to and also improves the high poloidal wave number measurement ability of magnetic fluctuations compared with the regular high frequency magnetic probes on EAST. The electrostatic probe array of the EMPA consists of two sets of four-tip probes and a single-tip probe array with three poloidal rows and four toroidal columns. It complements the electrostatic parameter measurements behind the main limiter and near the first wall in EAST. The engineering details of the EMPA diagnostic, including the mechanical system, the electrical system, the acquisition and control system, and the effective area calibration, are presented. The preliminary applications of the EMPA in L-mode and H-mode discharges on EAST have demonstrated that the EMPA works well for providing information on the magnetic and electrostatic fluctuations and can contribute to deeper physical analysis in future EAST experiments.

Mechanism of low-temperature helical streamer discharge driven by pulsed electromagnetic field

Under the condition of specific pulsed discharge parameters, the discharge mode conversion of the low-temperature plasma jet discharge channel that originally propagates along a straight line will occur, forming a three-dimensional helical plasma channel. Unlike the traditional helical wave discharge, there are no factors such as an external constant magnetic field that destroys the poloidal symmetry of the dielectric tube, and the chiral direction of the helical streamer will change with the discharge parameters. In order to understand in depth the electromagnetic mechanism of the helical structure in the plasma jet, and the source and influencing factors of the poloidal electric field that leads to the helical shape and determines the chirality in this new type of discharge, we analyze the complex characteristics and electromagnetic mechanism of the helical streamer, such as the chiral direction, pitch, branching, by establishing a self-consistent plasma theoretical model. It is found that the phase of the poloidal wave mode has an effect on the chiral selection of the helical streamer, that the electron density has an influence on the pitch of the streamer, and that the repetition frequency has an effect on the bifurcation point. The above discharge characteristics and their influencing factors are of scientific significance in exploring the interaction mechanism of electromagnetic wave and plasma, and also in providing experimental and theoretical support for the chiral application of low-temperature plasma.

Distribution of ULF Wave Power in Magnetic Latitude and Local Time Using THEMIS and Arase Measurements

AbstractUltra‐low‐frequency (ULF) waves are known to radially diffuse hundreds‐keV to few‐MeV electrons in the magnetosphere, as the range of drift frequencies of such electrons overlaps with the frequencies of the waves, leading to resonant interactions. The theoretical framework for this process is described by analytic expressions of the resonant interactions between electrons and toroidal and poloidal ULF wave modes in a background magnetic field. However, most expressions estimate the radial diffusion rates based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground. In this study, using multiyear measurements from the THEMIS and Arase missions, we present a statistical analysis of the distribution of ULF wave power in magnetic latitude and local time and show that the wave power of the radial and azimuthal components of the magnetic field increases away from the magnetic equator. Our result could have significant implications for the radial diffusion rates as currently estimated.

The investigation of quasi coherent mode on EAST using Doppler reflectometry

An electrostatic Quasi coherent mode has been observed in density fluctuations and perpendicular velocity fluctuations with the frequency range of 3–80 kHz on the Experimental Advanced Superconducting Tokamak using multi-channel Doppler reflectometry. It appears in the edge localized mode (ELM)-free period after L-H transition or in the inter-ELM period. The mode rotates almost together with the plasma with the poloidal wave number around 0.6 and its frequency chirps with plasma poloidal velocity. The mode can exist in a large radial coverage (ρ = 0.75–0.98), and peaks near the top of pedestal, suggesting that it might be excited in the steep gradient pedestal region, and spread into the core area.

Nonresonant Diffusion in Alpha Channeling.

The gradient of fusion-born alpha particles that arises in a fusion reactor can be exploited to amplify waves, which cool the alpha particles while diffusively extracting them from the reactor. The corresponding extraction of the resonant alpha particle charge has been suggested as a mechanism to drive rotation. By deriving a coupled linear-quasilinear theory of alpha channeling, we show that, for a time-growing wave with a purely poloidal wave vector, a current in the nonresonant ions cancels the resonant alpha particle current, preventing the rotation drive but fueling the fusion reaction.

Tungsten impurity effects on the coupling of TEM and ITG mode in reversed magnetic shear tokamak plasmas

The trapped electron mode (TEM) and ion temperature gradient (ITG) mode may couple into one hybrid mode or coexist simultaneously, depending on the regime of tokamak plasma parameters. A gyrokinetic integral eigenvalue equation is applied to investigate the coupling of TEM and ITG mode in the presence of tungsten (W) impurity in tokamak geometry of reversed magnetic shear (RMS). Systematic analysis of W impurity effects on the mode characteristics and induced quasi-linear particle flux has been performed in a wide regime of plasma parameters such as density gradient , charge concentration , and charge number of impurity ions, and poloidal wave number spectrum. It is found that both RMS and the impurity ions with inwardly peaked density profile and high charge concentration have strongly stabilizing effects on the modes. Furthermore, the increasing tends to decouple the hybrid mode into coexisting modes or dominating TEM/ITG mode in different parameter regions. The significance of the results for the particle transport of impurity and main ions as well as electrons in advanced tokamaks with W components is discussed.

Nonlinear simulation and energy analysis of the EAST coherent mode

The EAST coherent modes (CMs) during the inter-ELM phase are simulated by the electromagnetic six-field two-fluid module in BOUT++ framework. The fluctuation level of the electrostatic potential, electron pressure and density perturbations are comparable to the experiments, and the simulated electrostatic perturbation is around two orders of magnitude larger than the magnetic one in EAST CM. The frequency and poloidal wave number are consistent with experiments in the simulations of EAST CM equilibriums. The energy transfer between three-wave coupling indicates that the energy tends to transfer from medium-n to low-n modes in the early nonlinear phase, and the modes coupling effect in the nonlinear saturation phase is larger than that in the early nonlinear phase. Both the energy transfer and bispectral analysis show that the Ni fluctuation tends to generate the ‘single-mode’ coupling and Te tends to be ‘multiple-mode’, which indicates that the collapse of the density profile is larger than the electron temperature. The relative phase analysis is applied to evaluate whether the turbulence can extract the energy from density and temperature profiles. The result indicates that the density profile provides much more energy to drive the turbulence than electron temperature. The kinetic and magnetic energy transfer rates are used to understand the instability and turbulence driving mechanisms of the EAST CM. In the linear phase of the nonlinear simulation, the instability is driven by the peeling-ballooning mode and drift-Alfven wave (DAW), and the radial electric field and shear Alfven wave have large suppressing effects. The turbulence of EAST CM is a predominantly electrostatic mode, which corresponds to the Reynolds stress seven times larger than Maxwell stress. In addition, the effect of the electrostatic part in DAW is much larger than the electromagnetic one.

Pc4‐5 Poloidal ULF Wave Observed in the Dawnside Plasmaspheric Plume

AbstractA localized Pc4‐5 ultralow‐frequency (ULF) wave event associated with a plasmaspheric plume was observed by THEMIS‐E on the dawnside near L = 6, which was identified as a second harmonic poloidal wave. The plume was identified as a sudden density enhancement during an outbound pass. The charged particle populations in the plume have a variety of periodic modulation characteristics at different energies. First, there is an antiphase relationship between magnetic field Br and particle flux across a wide energy range both for ions and electrons (~50 keV to 1 MeV). Second, there is a 180° phase shift in the modulated ion flux within an energy range of ~2–6 keV, with negative slope dispersions of ion pitch angle distributions at ~2–6 keV and ~50–75 keV, which are characteristic of drift‐bounce resonances. Third, the lower‐energy (&lt;32 eV) ion flux is modulated at double the wave frequency, which are the result of E × B effect. Considering the generation mechanism of this poloidal mode wave within the plume, we find that it is likely generated by drift‐bounce resonance from an unstable population of ions, due to an inward radial phase space density gradient. We suggest that the localization of waves to the plume is because the high plasma density reduces the local poloidal mode eigenfrequency, enabling a match to the drift‐bounce frequencies of these ions, and resonant energy transfer from these particles to the eigenfunction at this location. This generates the Pc4‐5 second harmonic poloidal waves at a much lower L region than would otherwise be expected.

Experimental observation of coexisting electromagnetic fluctuations correlating with the inter-ELM pedestal evolution on EAST

Correlations between the edge fluctuations and the pedestal evolution during the relatively large edge localized mode (ELM) cycles at high pedestal normalized electron collisionality (νe,ped* &amp;gt; 1) on the EAST tokamak are investigated. Not only the edge electrostatic coherent mode (ECM, ∼50 kHz) and the low frequency magnetic coherent mode (MCM, ∼32 kHz) but also a high frequency electromagnetic mode (HFM, &amp;gt;150 kHz) are observed to be coexisting between ELMs. After the ELM crash, the pedestal electron temperature recovered faster than the pedestal electron density. It is found that the saturation of the ECM coincides more with the saturation of the pedestal electron density, while the saturation of the HFM and MCM coincides more with the saturation of the pedestal electron temperature. In addition, the characteristics of the electromagnetic fluctuations (the HFM and MCM) are studied in detail: the HFM propagates in the electron diamagnetic drift direction in the laboratory frame with an average poloidal wave number of k¯θHFM≈0.17 cm−1, while the MCM propagates in the ion diamagnetic drift direction in the laboratory frame with k¯θMCM ≈ 0.12 cm−1 and the toroidal mode number of n = 1. Furthermore, both the HFM and MCM have inward average radial wave numbers of k¯RHFM≈0.13 cm−1 and k¯RMCM≈4.64 cm−1. The bispectral analysis shows that the HFM and MCM have strong nonlinear interactions. The HFM is clearly observed on both low and high field side Mirnov coils, which might suggest a feature beyond a ballooning type instability, e.g., the kinetic ballooning mode. These studies may contribute to a better understanding of the pedestal evolution.

Destabilization of field-line localized density fluctuation with a 1/1 internal kink mode in the EAST tokamak

A high frequency (HF) density fluctuation, ranging from 100 kHz to 400 kHz, is locally destabilized with an m/n = 1/1 internal kink mode before a sawtooth crash in the experimental advanced superconducting tokamak (EAST). The location is just around the region with a high electron pressure gradient due to the flux compression from the 1/1 mode. And this HF fluctuation can be triggered at both the low field side and high field side. This fluctuation has a poloidal wave number (kθ) from 3 to 5 cm−1, corresponding to a poloidal mode number (m) of 36–60. A threshold of the local electron pressure gradient (|∇p e|) exists for the stimulation of the HF fluctuation, and its amplitude increases with |∇p e|. The results could be beneficial for the understanding of a sawtooth crash, and also physics in 3D equilibrium.

Impurity effects on ion temperature gradient driven multiple modes in transport barriers

The impurity effects on ion temperature gradient (ITG) driven instability in transport barriers (TBs) are numerically investigated with the gyrokinetic integral eigenmode equations in tokamak plasmas. In particular, the effects of temperature and density gradients of the main ions ( and ) are analyzed independently to understand the physical mechanisms better, instead of keeping their ratio as carried out in previous works, when the parameters of impurity ions vary. It is found that the effect of impurity ions with outwardly peaked density profiles on ITG modes depends on the competition between the destabilizing effect of the impurity density gradient and the stabilizing effect induced by the dilution of main ions from impurity ions when is fixed, which is in significant contrast with the results for a fixed . The destabilizing effects include enhancement of ITG modes and coupling to the impurity mode (IM) in weak ITGs (big ) and strong impurity density gradient regimes. In addition, the stability boundaries for ITG modes, including high-order modes, are discussed in detail, and compared with previous works (Fröjdh et al 1992 Nucl. Fusion 32 419). Furthermore, the impurity ions with either inwardly or slightly outwardly peaked density profiles have weaker and stronger stabilizing effects on small and big poloidal wave vector modes, respectively. However, the impurity ions with steeper outwardly peaked density profiles have stronger stabilizing effects on big modes. Moreover, the inwardly peaked impurity ion density profiles are beneficial for main ion confinement and impurity decumulation, due to the main (impurity) ions flowing inwardly (outwardly). Finally, analyses of eigenmode structure and the quasi-linear particle flux are performed in detail. The results show that impurity ions have non-negligible effects, especially on higher-order ITG modes.

Structure of Magnetic Plasma Fluctuations in the Uragan-3M Torsatron at Rare Collision Frequencies

Fluctuations of the poloidal component of the plasma magnetic field in the frequency range of 0.5–50 kHz are studied in the Uragan-3M (U-3M) torsatron. Hydrogen plasma is produced and heated by RF fields at frequencies close to that of the ion cyclotron. The studies are carried out using a set of 15 magnetic sensors installed in one of the torus cross sections. RF heating provided the plasma with rare collision frequencies and the presence of the bootstrap current. The study is carried out when the maximum amplitude of magnetic fluctuations is observed and their connection with the plasma energy content is noticeable. Two types of vibrations are observed. In the first type, the current structure rotates with a certain frequency mainly in the direction of the rotation of electrons in the magnetic field, and the amplitude varies slowly with time (the rotating structure). For the second type, the spatial structure does not rotate, but its amplitude changes with a certain frequency (the standing structure). The frequencies of fluctuations and rotations are close for structures with a given poloidal wave number. The standing vibration structures with different poloidal wave numbers in this frequency range are correlated. The maximum amplitude of the rotating structures is observed with m = 2, and for the standing structures with m = 3 and reaches the values of $$\tilde {B}$$ ≤ 0.3 G in the confinement region. The vibration frequency does not depend on poloidal wave number m for the studied cases; m = 0, 1, 2, 3.

Multiple satellites observation evidence: High-&lt;i&gt;m&lt;/i&gt; Poloidal ULF waves with time-varying polarization states

We report multi-spacecraft observations of ULF waves from Van Allen Probes (RBSP), Magnetospheric Multiscale (MMS), Time History of Events and Macroscale Interactions during Substorm (THEMIS), and Geostationary Operational Environmental Satellites (GOES). On August 31, 2015, global-scale poloidal waves were observed in data from RBSP-B, GOES and THEMIS from L=4 to L=8 over a wide range of magnetic local time (MLT). The polarization states varied towards purely poloidal polarity. In two consecutive orbits over 18 hours, RBSP-A and RBSP-B recorded gradual variation of the polarization states of the poloidal waves; the ratio (|Ba|/|Br|) decreased from 0.82 to 0.13. After the variation of polarization states, the poloidal ULF waves became very purely poloidal waves, localized in both L and MLT. We identify the poloidal wave as second harmonic mode with a large azimuthal wave number (m) of –232. From RBSP particle measurements we find evidence that the high-m poloidal waves during the polarization variations were powered by inward radial gradients and bump-on-tail ion distributions through the N=1 drift-bounce resonance. Most of the time, the dominant free energy source was inward radial gradients, compared with the positive gradient in the energy distribution of the bump-on-tail ion distributions.

Структура магнитных флуктуаций плазмы в торсатроне Ураган-3М при редких частотах соударений

AbstractFluctuations of the poloidal component of the plasma magnetic field in the frequency range of 0.5–50 kHz are studied in the Uragan-3M (U-3M) torsatron. Hydrogen plasma is produced and heated by RF fields at frequencies close to that of the ion cyclotron. The studies are carried out using a set of 15 magnetic sensors installed in one of the torus cross sections. RF heating provided the plasma with rare collision frequencies and the presence of the bootstrap current. The study is carried out when the maximum amplitude of magnetic fluctuations is observed and their connection with the plasma energy content is noticeable. Two types of vibrations are observed. In the first type, the current structure rotates with a certain frequency mainly in the direction of the rotation of electrons in the magnetic field, and the amplitude varies slowly with time (the rotating structure). For the second type, the spatial structure does not rotate, but its amplitude changes with a certain frequency (the standing structure). The frequencies of fluctuations and rotations are close for structures with a given poloidal wave number. The standing vibration structures with different poloidal wave numbers in this frequency range are correlated. The maximum amplitude of the rotating structures is observed with m = 2, and for the standing structures with m = 3 and reaches the values of $$\tilde {B}$$ ≤ 0.3 G in the confinement region. The vibration frequency does not depend on poloidal wave number m for the studied cases; m = 0, 1, 2, 3.

Quasi-coherent mode simulation during inter-ELM period in HL-2A

We have performed linear and nonlinear simulations to study the drive mechanism of the quasi-coherent modes (QCM) during the inter-edge localized mode period in HL-2A. With an unstable Resistive-Ballooning mode from linear simulations, the nonlinear simulation has successfully reproduced the QCM inside the separatrix at the outer mid-plane. The poloidal wave number and frequency from fluctuation analysis are almost the same as the experimental results. The phase shift between the electrical potential and the density fluctuation matches. Linear and nonlinear global profile scans show that the QCM is triggered by the density gradient. Based on these scans, some other experimental observations of the QCM are explained.