Electron Cyclotron Emission Research Articles

Collective Thomson scattering with 77, 154, and 300 GHz sources in LHD

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A mega-watt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to ∼10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co- and counter- fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described.

Interaction of magnetic islands with turbulent electron temperature fluctuations in DIII-D and in GENE nonlinear gyrokinetic simulations

We present localized measurements of interactions between neoclassical tearing modes (NTMs) and turbulent electron temperature fluctuations () in the expected kθρs = 0–0.5 wave-number range of the ion temperature gradient (ITG) instability measured via correlation electron cyclotron emission (kθ and ρs are the poloidal wave-number and ion-sound Larmor radius, respectively). Comparison to GENE gyrokinetic simulations, shows qualitative agreement with being reduced (increased) inside (outside) of the islands due to modified local gradients, and being higher (lower) in the region outside the island where lower (higher) flow shear is expected due to radially asymmetric island shape. These results are consistent with previously reported modifications of long and intermediate wavelength turbulent density fluctuations and cross-field electron thermal diffusivity reduction at the O-point of magnetic islands. Interestingly, a 30% increase of is detected at the NTM rational surface when the island width is a few times ρs, which is replicated by GENE through zonal flow damping due to the destroyed magnetic field topology of the NTM rational surface.

Observation of the current driving due to electron temperature redistribution by anomalous Doppler instability in the Experimental Advanced Superconducting Tokamak

Density sweeping-down experiments have been performed in order to investigate the electron dynamics and the effects on instabilities such as magnetohydrodynamics (MHD) in the low collisionality regime in the Experimental Advanced Superconducting Tokamak (EAST). It is found that an electrostatic instability is always excited which is recognized as the anomalous Doppler instability (ADI). The ADI is characterized by the stair-stepping electron cyclotron emission (ECE), converting the parallel energy to the perpendicular component of energetic electrons in the direction of the magnetic field. The background electron temperature profile is redistributed which results in the gradient change in the core. Non-negligible bootstrap current causes the loop voltage decrease. The bootstrap current is dissipated by the bursting MHD instability which is explained as the Alfvén-acoustic mode. Analysis indicates that bulk electrons are easily trapped in the steepened temperature gradient region, intensifying the asymmetry of electron velocity distribution in the parallel direction and creating the bootstrap current in the low-collisionality regime. In particular, it is shown that the global Alfvén-acoustic mode dramatically removes trapped electrons and quenches the bootstrap current, which is the main limit for the bootstrap current life.

Intermittence and turbulence in fusion devices

In this work, we explore the response of the intermittence parameter C(1) to the magnetic topology of confined fusion plasmas, as well as to spontaneously generated zonal flows. To achieve this, we make extensive use of resistive Magneto-Hydrodynamic calculations. First, we explore single helicity calculations, where we observed clearly that the intermittence is due to the mixing between turbulence and (poloidal) flow fluctuations. The minima of C(1) were correlated with maxima of the mean transfer entropy, , noting that the latter have been identified with ‘trapping zones’ for radial transport in previous work. Thus, these ‘trapping zones’ are closer to monofractality than the surrounding plasma. In more realistic multiple helicity calculations, these results were confirmed, although somewhat less clear, as the mutual interaction between helicities now also contributed to the intermittence. We applied these techniques to experimental Electron Cyclotron Emission data from the W7-X stellarator and found that these observations also appear to hold in an experimental situation. Thus, we conclude that the intermittence parameter C(1) may provide valuable indirect information about the interaction between fluctuations, zonal flows, and the magnetic topology of fusion plasmas.

Radiometry for the vertical electron cyclotron emission from the runaway electrons at the COMPASS tokamak.

Due to an increased interest in runaway electron (RE) phenomena in tokamak research, the need for diagnostics of runaway electron population in plasma has emerged. A novel diagnostic of the nonthermal electron cyclotron emission from runaway electrons can be utilized for this purpose. It was designed and installed at the COMPASS tokamak based on the available heterodyne radiometer. The vertical ECE (V-ECE) system uses a 16-channel heterodyne radiometer with a vertically placed E2-band horn antenna with a 76.5-90 GHz frequency range front-end. Simulations with the ray-tracing SPECE code have shown a measurement feasibility of the runaway electrons with energies up to 1 MeV. Due to a low optical depth of the plasma in COMPASS during RE discharges, reflected waves from the tokamak wall can be detected as well. First results show strong connection with other RE diagnostics at COMPASS. The V-ECE can obtain important information about RE population evolution and primary generation mechanism.

Experimental characterization of a quasi-coherent turbulent structure in the edge plasma in Wendelstein 7-X

A quasi-coherent mode (QCM) near the plasma edge is characterized experimentally by measuring the density fluctuation via a poloidal correlation reflectometer (PCR) in Wendelstein 7-X. This QCM frequency descends from 25 kHz near the separatrix to 10 kHz as the cutoff position penetrates toward the core. A similar mode structure is also observed in the spectra of electron cyclotron emission edge channels and Mirnov coil signals, which shows a clear correlation with respect to the PCR measurement. The mode evaluation yields a range for the poloidal mode number of , and for the normalized mode scale , which suggest a drift type instability. The QCM structure appears in the standard and narrow-mirror configurations (both with the rotational transform in the boundary), but is absent in the high- configuration (boundary ). The impact of plasma parameters on the QCM is discussed. It is observed that the QCM frequency and amplitude can be affected by the edge magnetic topology and plasma parameters. The shifts upward during the negative and non-stellarator-symmetric control coil current scans, which may be attributed to the effect of the shear, Er, and ne in the plasma edge region.

Effects of collisionality and Te/Ti on fluctuations in positive and negative δ tokamak plasmas

The effects of negative triangularity () on confinement and fluctuations in plasmas covering a large range of parameters were investigated on the tokamak à configuration variable (TCV). The conditions explored in this paper include discharges where neutral beam (NB) heating was employed to obtain an electron-ion temperature ratio across a large fraction of the plasma profile. This significantly extended the range of negative plasmas studied on TCV towards conditions more relevant to future reactor-like tokamaks. Negative triangularity was found to improve confinement over the full range of collisionality studied () and (). The amplitude of radiative temperature fluctuations, measured using a correlation electron cyclotron emission diagnostic over the range , was found to be reduced, in negative with respect to positive plasmas, for all combinations of parameters explored. This was, in particular, verified for a pair of positive and negative plasmas with comparable density and under different conditions of NB heating. Linear gyrokinetic simulations found the dominant turbulence regime, in the strongly NB heated discharges, to be a mixture of trapped electron modes (TEMs) and ion temperature gradient driven modes. This is in contrast to ohmic or electron cyclotron heated discharges, for which the dominant turbulence regime was found to be pure TEM. Negative triangularity was found to lead to partial stabilization of the most unstable modes for low wavenumbers in both turbulence regimes. These findings demonstrate that negative triangularity could provide significant confinement improvement over a large range of parameters, that include conditions closer to future reactor-like machines (, low collisionality).

A multi-channel correlation ECE system for electron temperature fluctuation measurement on EAST tokamak

A multi-channel correlation electron cyclotron emission (CECE) system for electron temperature fluctuation measurement has been installed on Experimental Advanced Superconducting Tokamak (EAST). The frequency coverage of the CECE system is 106-120 GHz, and it is designed to cover the low field side of the plasma for Bt = 2.3 T. The system can measure four different radial positions at the same time. The measurable poloidal and radial wavenumber range is kθ < 1.85 cm−1, kr < 4.19 cm−1. Evolution of electron temperature measured by the CECE system is compared with the results of ECE system and the agreement is good. The experiment results of electron temperature fluctuation shows that the system can detect small amplitude electron temperature fluctuation in the ohmic shots when the electron density is above 2.8 × 1019 m−3 and the fluctuation level is seen to increase with density. Different fluctuation properties are obtained under various heating power in the L-mode plasmas, the electron temperature fluctuation level at ρ = 0.88 increases with auxiliary heating power by 16.7%. Obvious electron temperature fluctuation has been obtained in the region of ρ from 0.6 to 1, and the fluctuation level increases from 0.28% at ρ = 0.6 to 1.68% at ρ = 0.88.

Measurements of visible bremsstrahlung and automatic Bayesian inference of the effective plasma charge Zeff at W7-X

The effective charge Zeff indicates the overall impurity contamination of a plasma. Zeff can be derived experimentally from the intensity of the plasma bremsstrahlung emission. We describe here the diagnostic set-ups and the Bayesian modeling allowing the inference of Zeff at W7-X. First results from the operational campaigns in 2017 and 2018 are shown. Measurements of the visible plasma radiation along a single line-of-sight traversing the core plasma has been carried out using a compact USB-spectrometer with a time resolution of 100 ms. A spectral region (627–641 nm) that is free from line emission is selected for the analysis of the bremsstrahlung emission, which also depends on electron temperature and density profiles. Electron temperature profiles are derived from either the electron cyclotron emission or the Thomson scattering diagnostic. Electron density profiles, however, have their shape information derived from Thomson scattering measurements and absolute values from single line-of-sight interferometry measurements. The Minerva framework is used to infer the profiles with Gaussian processes and develop a Bayesian model of the bremsstrahlung emission to infer line averaged Zeff. The sensitivity of the diagnostic enables Zeff measurements down to the lowest core electron densities observed in the last campaign of 0.75×1019 m−3 with a statistical relative error of ≈50% (Zeff = 3.2, 100 ms integration time). The analysis is automated to routinely compute Zeff after every plasma discharges.

Intense intermittent radiation at the plasma frequency on EAST

Intense intermittent radiation has been observed regularly in EAST plasmas. The duration of radiation bursts is with a characterization time of a few microseconds. The radiation is very intense, with an equivalent radiation temperature higher than the electron cyclotron emission by a few orders of magnitude. An electron density threshold exists to terminate the bursts, and it is strongly dependent on the toroidal magnetic field. The radiation frequency f is at the plasma frequency, and the frequency bandwidth is very narrow (∼1.5 MHz FWHM, ). The fine structure of the spectrum in the frequency domain has been observed, and the frequency spacing is around 3 MHz. The cavity modes model is capable of explaining the radiation frequency and the frequency spacing. However, a quantitative and complete description of this phenomenon is still lacking.

Design of Surface Defects Loaded Selectively Notched W-Band Waveguide Filter for Millimeter Wave Diagnostic in Fusion Reactor

In the fusion experiment, sensitive radio frequency signal diagnostic instruments need to be protected against high power stray radiation coming from the Electron Cyclotron Resonance Heating. The design of a stopband notch filter in the transmission path of the diagnostic system is helpful to avoid the stray radiations. This work presents the study and design methodology of the notch filter based on a circular corrugated waveguide Bragg reflector that provides a single stopband between 75 and 105 GHz. The existence of the HE11 mode, i.e. the only propagating mode in the W-band notch filter is the key aspect in the design of the filter. The proposed notch filter is compact and spans only 46.62 cm in length with an inner diameter of 16.4 mm. By optimizing the number of corrugation to 140, their period to λ through parametric analysis and unconventionally introducing uniformly spaced surface defects within the filter structure, a single stopband at 90 GHz is obtained corresponding to the LP12 Bragg resonance. The notch filter may be useful in wideband diagnostics like Electron Cyclotron Emission and reflectometry in the fusion plasma experiments.

Current Profile Reconstruction Using Motional Stark Effect Polarimeter Data on HL-2A Tokamak

In order to reconstruct the plasma current density, the Current Profile Fitting (CPF) code has been successfully developed on the HL-2A tokamak. A seven-channel motional Stark effect (MSE) diagnostic based on dual photoelastic modulators is installed to measure the pitch angle of the magnetic field, which can be used as an internal magnetic field constraint for the CPF code. Recently, the MSE polarimeter was upgraded with a real-time wavelength matching system to improve the signal-to-noise ratio. The magnetic field angle (γpitch) with a temporal resolution of 10 ms can be provided. In the CPF code, the plasma current density is described as a polynomial, and the Least-Squares method is used to determine the coefficients of the polynomial. The Finite Difference method and the Strongly Implicit Procedure method are used to solve the Grad-Shafranov equation. The code operation is stable. With the improved-quality MSE data, the CPF calculation result of shot 30782 suggests that the safety factor q profile is monotonic. The minimum q value is less than 1 on-axis during sawtooth oscillations in shot 30782. And, the position of the q = 1 surface is consistent with the sawtooth inversion radius measured by electron cyclotron emission and soft X-ray diagnostics.

Modeling the electron cyclotron emission below the fundamental resonance in ITER

The electron cyclotron emission (ECE) in fusion devices is non-trivial to model in detail at frequencies well below the fundamental resonance where the plasma is optically thin. However, doing so is important for evaluating the background for microwave diagnostics operating in this frequency range. Here we present a general framework for estimating the ECE levels of fusion plasmas at such frequencies using ensemble-averaging of rays traced through many randomized wall reflections. This enables us to account for the overall vacuum vessel geometry, self-consistently include cross-polarization, and quantify the statistical uncertainty on the resulting ECE spectra. Applying this to ITER conditions, we find simulated ECE levels that increase strongly with frequency and plasma temperature in the considered range of 55–75 GHz. At frequencies smaller than 70 GHz, we predict an X-mode ECE level below 100 eV in the ITER baseline plasma scenario, but with corresponding intensities reaching keV levels in the hotter hybrid plasma scenario. Benchmarking against the SPECE raytracing code reveals good agreement under relevant conditions, and the predicted strength of X-mode to O-mode conversion induced by wall reflections is consistent with estimates from existing fusion devices. We discuss possible implications of our findings for ITER microwave diagnostics such as ECE, reflectometry, and collective Thomson scattering.

Spatial localization of ECE measurement in EAST LHW-heated plasmas

In this work, electron cyclotron emission (ECE) is simulated by using the code SPECE to study the spatial localization of ECE measurement in EAST plasmas heated by lower hybrid wave (LHW). The results indicate that generally there are two emission layers for an individual frequency in plasmas with non-thermal electrons, and they are separately attributed to the thermal electrons and non-thermal electrons. The emission layer due to the thermal electrons is nearly identical to that for the case with Maxwellian distribution. The emission layer due to non-thermal electrons is well localized in the location of the non-thermal electrons. Even though the non-thermal emission layer is broader, the emission intensity is smaller than that from the thermal emission layer for the cases studied in this work. Localized electron temperature fluctuations can still be distinguished by ECE measurement as long as it does not coexist with the non-thermal electrons. Sawtooth inversion radii and tearing mode island location determined respectively by the ECE measurement and the soft x-ray measurement for a LHW-heated plasma show a good agreement, and this indicates that the ECE measurement in the plasma core region is not seriously polluted.

Enhanced radial energy transport induced by radially curved Alfvén eigenmode wavefronts

Radially curved Alfvén eigenmode wavefronts as observed on the electron cyclotron emission imaging diagnostic indicate that the modes are associated with a net radial energy flow. This energy flow is due to a radial mismatch in the location of the drive and damping of the mode. Energy transport is created by an enhanced parallel fluctuating magnetic field that is generated by the radial wavefront curvature.

Alfvén eigenmodes and fast ion transport in negative triangularity DIII-D plasmas

The first energetic particle experiments in negative triangularity tokamak plasmas have been carried out on DIII-D. Alfvén eigenmode (AE) activity and associated fast ion transport comparable to that in positive triangularity is observed during the current ramp portion of all plasmas with early beam heating indicating negative triangularity does not confer a special advantage with respect to AE induced transport. In these discharges, a range of mode activity driven by the sub-Alfvénic () 80 kV neutral beams is found including beta induced Alfvén acoustic eigenmodes, beta induced Alfvén eigenmodes, reversed shear Alfvén eigenmodes (RSAEs) and toroidicity induced Alfvén eigenmodes (TAEs). Mode intermittency and possibly chirping appears to be more common than comparable positive triangularity and/or oval discharges but overall, the unstable spectra and mode amplitudes observed on magnetics, CO2 interferometry, and electron cyclotron emission in a set of matched positive and negative triangularity cases at moderate beam power is similar. Large levels of Alfvén eigenmode induced fast ion transport are found in both positive and negative triangularity with up to central fast ion pressure deficits relative to classical predictions early during the current ramp phase for discharges with 3 MW injected 80 kV neutral beam power. The deficit in both cases is reduced toward zero as the current penetrates and eventually reaches classical levels during current flattop with . Fast ion transport in negative triangularity plasmas measured using a beam modulation technique show very similar levels to those measured in oval plasmas over the range of tested beam powers (–7 MW). This similarity is found despite quite different unstable spectra in the highest beam power case where a mixture of coherent and quasi-coherent modes in the TAE frequency range are observed in negative triangularity and a spectrum of more typical narrowband TAEs and RSAEs is observed for the oval plasmas.

Interpretation of quasi-periodic frequency sweeping in electron cyclotron emission of nonequilibrium mirror-confined plasma sustained by high-power microwaves

In Viktorov et al (2016 Eur. Phys. Lett. 116 55001) we reported on chirping frequency patterns that have been observed in the electron cyclotron emission from strongly nonequilibrium plasma confined in a table-top mirror magnetic trap. These patterns were interpreted qualitatively as the first experimental evidence for so-called ‘holes and clumps’ mechanism acting at such high frequencies (1–10 GHz). In the present paper, we prove this statement on a more rigorous basis, considering the formation of nonlinear phase space structures in proximity of the wave-particle resonances within the framework of the Berk–Breizman theory adopted for a kinetically unstable mirror-confined plasma.

Electron cyclotron emission based q-profile measurement and concept for equilibrium reconstruction

Measuring the plasma equilibrium is essential for real time plasma control and for the investigation of an abundance of physics questions. However, many existing kinetic equilibria reconstruction techniques rely heavily on integrated magnetic measurements and neutron-sensitive diagnostics, which will be difficult to design and operate in ITER and DEMO. Here we present and test a conceptual design for a non-magnetic equilibrium reconstruction method using data from a radial electron cyclotron emission diagnostic and an image of the plasma boundary—diagnostics which can be robustly designed in high-neutron environments. This technique is based on the measurement of a discrete q-profile and the symmetry mapping of the electron temperature profile, both of which can be acquired with ECE.

An eight-channel correlation electron cyclotron emission diagnostic for turbulent electron temperature fluctuation measurement in HL-2A tokamak.

A new correlation electron cyclotron emission (CECE) diagnostic has recently been installed on the HL-2A tokamak in order to study electron temperature fluctuations. Eight radial locations are measured simultaneously through eight pairs of correlated channels. Multiplexers are employed in the intermediate frequency section instead of the conventional separated filter banks to meet strict cross-isolation specifications and lower insertion loss. Relative electron temperature fluctuations are observed by CECE for the first time on the HL-2A by using the spectral decorrelation method. The achieved minimum detectable fluctuation level is up to (T̃e/Te)min∼0.5%. When studying electron temperature fluctuations in the core region with gas puffing, the cross-power spectra show that the amplitude of the electron temperature fluctuation increases in a high temperature and low density plasma. Further analysis demonstrates that the electron temperature gradient ∇Te drives the electron temperature fluctuations together with electron heat transport.

Direct evidence of E × B flow changes at the onset of resonant magnetic perturbation-driven edge-localized mode crash suppression

The bifurcation of perpendicular mode velocity near the pedestal top () at the onset of RMP-driven edge-localized mode (ELM)-crash suppression has been directly measured by using an electron cyclotron emission imaging diagnostic on KSTAR. The ELM crashes are suppressed along with a sudden reduction of , which synchronizes with the transition into and out of the ELM-crash suppression. The change of flow appears mainly responsible for the change of , whose magnitude is the smallest near the normalized flux surface during the ELM-crash suppression. The plasma response to the RMP field was most enhanced in the vicinity of during the ELM-crash suppression and showed a hysteresis behavior with respect to resonant field strength. With these changes, a strong nonlinear coupling between turbulent eddies was observed in the ELM-crash suppression, which is an important feature of the ELM-crash suppression.