Electron Cyclotron Emission Signals Research Articles

Numerical investigation of the 2/1 double tearing mode in EAST with the CLT code

The pressure crashes observed in shot No. 71326 in the Experimental Advanced Superconducting Tokamak are numerically investigated with the three-dimensional, toroidal, and full resistive-magnetohydrodynamics code (CLT). Based on the experimental observations, the pressure crash is caused by the nonlinear evolution of the m/n = 2/1 double tearing mode (DTM), where n and m are the toroidal and poloidal mode numbers, respectively. However, we find that the m/n = 2/1 DTM is stable based on the safety factor (q) profile from the equilibrium code EFIT, which indicates that the original q profile is somewhat inconsistent with the actual profile due to q measurement uncertainty. Since there is no motional Stark effect diagnostic for this shot, the local information of the magnetic field is missing, which leads to the largest contribution to the discrepancy. If other information is perfectly known and the q profile is the only uncertainty, then we could provide some information for the EFIT reconstruction by comparing our simulation results with electron cyclotron emission signals to constrain the uncertainty of the q profile to a much smaller region. The influence of plasma rotation and the two-fluids effect is also discussed.

Role of magnetohydrodynamic activity in sawtooth induced heat pulse propagation in ADITYA tokamak

Fast propagation of sawtooth induced heat pulse is observed in the magnetohydrodynamic (MHD) active plasmas of ADITYA tokamak. The sawtooth crash deposits heat beyond the inversion radius, which gets transported to the edge region of the plasma, and is reflected as inverted sawtooth modulation in the edge channels of electron cyclotron emission (ECE) and spectral line emission. The time-lag analysis on ECE signal reveals the propagation time from plasma core to edge of ∼150 µs. From this analysis, the estimated transient electron heat diffusivity is found to be ∼50–60 m2 s−1, which is ten times higher than that of power balance heat diffusivity , in the MHD active discharges of ADITYA. It has been observed that the presence of MHD (m/n = 2/1, 3/1) activity in the intermediate region between the q = 1 and the edge radii, significantly influences the heat transport from the plasma core to the edge region. Stochastic magnetic field region formation with overlapping m/n = 2/1 and 3/1 MHD islands facilitates the fast heat-pulse propagation during a sawtooth crash in ADITYA tokamak. The disparity between the measured and the power-balance estimated diffusivity is significantly reduced by considering the electron heat diffusivity due to stochastization of magnetic field in the intermediate region.

Analysis of synthetic electron cyclotron emission from the high field side of HL-2M tokamak plasmas.

A synthetic electron cyclotron emission (ECE) diagnostic is used to interpret ECE signals from preset plasma equilibrium profiles, including magnetic field, electron density, and electron temperature. According to the simulation results, the electron temperature (Te) profile covering the harmonic overlap region can be obtained by receiving ECE signals at the high field side (HFS) of the HL-2M plasma. The third harmonic ECE at the low field side (LFS) cannot pass through the second harmonic resonance layer at the HFS unless the optical thickness (τ) of the second harmonic becomes gray (τ ≤ 2). In addition, the impact of the relativistic frequency down-shift has been evaluated and corrected. The measurable range of the HFS ECE has been calculated by scanning different parameters (electron density, temperature, and magnetic field). Higher plasma parameters allow a wider radial range of electron temperature measurements. The minimum inner measurable position can reach R = 120cm (r/a = -0.89) when the product of core temperature (Te0) and density (ne0) is greater than 35 × 1019keVm-3, which is extended by more than 30cm inward compared with that of the LFS measurement. The HFS ECE will greatly improve the diagnostic ability of ECE systems on the HL-2M tokamak.

Investigation of Optically Grey Electron Cyclotron Harmonics in Wendelstein 7-X

For a magnetic field of 2.5T, the electron cyclotron harmonics are spectrally well separated in W7-X as it has a large aspect ratio. Because of this advantage from the geometry of W7-X stellarator, it is easier to scan these higher harmonics (70,140,210…..GHz) compared to tokamaks with small aspect ratio. For confinement reasons, W7-X is planned to work at high plasma densities applying O2 electron cyclotron resonance heating (ECRH). For such plasmas, which already have been demonstrated in experimental campaign OP1.2a of W7-X, electron cyclotron emission (ECE) from second harmonic extraordinary mode (X2) is in cutoff. In that case, optically grey higher harmonics provide the only access to ECE signal and hence electron temperature profiles. A Michelson interferometer will be used in the next operational campaign of W7-X for broadband (50-500 GHz) ECE scan and that will be compared to modeling of ECE emission at different plasma parameters from radiation transport calculations (TRAVIS). The strong stray radiation from non-absorbed ECRH is a major concern in scanning these higher harmonics using an interferometer because this stray radiation will dominate the spectra thus masking the ECE signal. For this purpose, a multimode notch filter design based on a multilayer dielectric structure is constructed to attenuate the stray radiation at 140GHz by 50dB.

Low-frequency whistler waves in quiescent runaway electron plasmas

In quiescent runaway electron plasmas in the DIII-D tokamak, whistler waves with frequencies between 90 and 200 MHz are driven unstable in plasmas with appreciable hard x-ray and non-thermal electron cyclotron emission (ECE). Narrow (δf < 50 kHz) discrete modes with erratically spaced frequencies are observed. Unstable modes often extend over a range Δf ≃ 50 MHz but lower frequency unstable modes are usually most intense. The dependency of the frequency on field and density implies a wavenumber k ≃ 150 m−1 with parallel wavenumber k∥ ≪ k. Reducing the gap between the plasma and the wall increases the number of detected modes. Lowering the magnetic field promotes instability. Nonlinear limit-cycle-like oscillations in the whistler amplitude occur on a 10 ms timescale. The ECE signals often jump at whistler bursts, suggesting that the modes pitch-angle scatter the runaways. Sawteeth cause transient stabilization of the whistlers.

The effects of kinetic instabilities on the electron cyclotron emission from runaway electrons

In this paper we show that the kinetic instabilities associated with runaway electron beams play an essential role for the production of high-level non-thermal electron–cyclotron-emission (ECE) radiation. Most of the non-thermal ECE comes from runaway electrons in the low-energy regime with large pitch angle, which are strongly scattered by the excited whistler waves. The power of ECE from runaway electrons is obtained using a synthetic diagnostic model based on the reciprocity method. The electron distribution function is calculated using a kinetic simulation model including the whistler wave instabilities and the quasilinear diffusion effects. Simulations based on DIII-D low-density discharge reproduces the rapid growth of the ECE signals observed in DIII-D experiments. Unlike the thermal ECE where radiation for a certain frequency is strongly localized inside the resonance region, the non-thermal ECE radiation from runaway electrons is nonlocal, and the emission-absorption ratio is higher than that of thermal electrons. The runaway electron tail is more significant for ECE with higher frequencies, and the ECE spectrum becomes flatter as RE population grows. The nonlinear behavior of the kinetic instabilities is illustrated in the oscillations of the ECE waves. The good agreement with the DIII-D experimental observations after including the kinetic instabilities clearly illustrate the significance of the scattering effects from wave-particle interactions, which can also be important for runaway electrons produced in disruptions.

Interpretation of the electron cyclotron emission signal from optically marginal plasmas

The interpretation of electron cyclotron emission (ECE) signal from the optically marginal region of magnetized plasma is presented. The density and the temperature fluctuations associated with the edge localized modes (ELMs) observed in the KSTAR tokamak are estimated by assuming an ELM filamentary structure as a flux tube bulges out like a ballooning mode instability. Synthetic ECE signals from the rotating ELM are calculated based on the measured electron temperature profile and an assumed electron density profile constrained by the measured line-averaged density, yielding an ~ 0.02 relative fluctuation level in agreement with the experimental observations. The measured ECE signal is nearly in phase with the density modulation associated with the rotating ELM. This implies that the ECE signal corresponding to the ELM filaments has a significant contribution from the density fluctuations.

Update on the status of the ITER ECE diagnostic design

Considerable progress has been made on the design of the ITER electron cyclotron emission (ECE) diagnostic over the past two years. Radial and oblique views are still included in the design in order to measure distortions in the electron momentum distribution, but the oblique view has been redirected to reduce stray millimeter radiation from the electron cyclotron heating system. A major challenge has been designing the 1000 K calibration sources and remotely activated mirrors located in the ECE diagnostic shield module (DSM) in the equatorial port plug #09. These critical systems are being modeled and prototypes are being developed. Providing adequate neutron shielding in the DSM while allowing sufficient space for optical components is also a significant challenge. Four 45-meter long low-loss transmission lines transport the 70–1000 GHz ECE from the DSM to the ECE instrumentation room. Prototype transmission lines are being tested, as are the polarization splitter modules that separate O-mode and X-mode polarized ECE. A highly integrated prototype 200–300 GHz radiometer is being tested on the DIII-D tokamak in the USA. Design activities also include integration of ECE signals into the ITER plasma control system and determining the hardware and software architecture needed to control and calibrate the ECE instruments.

Temperature gradient scale length measurement: A high accuracy application of electron cyclotron emission without calibration.

Calibration is a crucial procedure in electron temperature (Te) inference from a typical electron cyclotron emission (ECE) diagnostic on tokamaks. Although the calibration provides an important multiplying factor for an individual ECE channel, the parameter ΔTe/Te is independent of any calibration. Since an ECE channel measures the cyclotron emission for a particular flux surface, a non-perturbing change in toroidal magnetic field changes the view of that channel. Hence the calibration-free parameter is a measure of Te gradient. BT-jog technique is presented here which employs the parameter and the raw ECE signals for direct measurement of electron temperature gradient scale length.

Runaway electron generation as possible trigger for enhancement of magnetohydrodynamic plasma activity and fast changes in runaway beam behavior

Peculiar phenomena were observed during experiments with runaway electrons: rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the electron cyclotron emission (ECE) signal (cyclotron radiation of suprathermal electrons). These phenomena were initially observed in TEXTOR (Tokamak Experiment for Technology Oriented Research), where these events only occurred in the current decay phase or in discharges with thin stable runaway beams at a q = 1 drift surface. These rapid changes in the synchrotron spot were interpreted by the TEXTOR team as a fast pitch angle scattering event. Recently, similar rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the non-thermal ECE signal were observed in the EAST (Experimental Advanced Superconducting Tokamak) runaway discharge. Runaway electrons were located around the q = 2 rational magnetic surface (ring-like runaway electron beam). During the EAST runaway discharge, stepwise ECE signal increases coincided with enhanced magnetohydrodynamic (MHD) activity. This behavior was peculiar to this shot. In this paper, we show that these non-thermal ECE step-like jumps were related to the abrupt growth of suprathermal electrons induced by bursting electric fields at reconnection events during this MHD plasma activity. Enhancement of the secondary runaway electron generation also occurred simultaneously. Local changes in the current-density gradient appeared because of local enhancement of the runaway electron generation process. These current-density gradient changes are considered to be a possible trigger for enhancement of the MHD plasma activity and the rapid changes in runaway beam behavior.

Microwave imaging reflectometry for density fluctuation measurement on KSTAR

A dual-frequency microwave imaging reflectometry system was commissioned to measure both coherent and turbulent electron density fluctuations in KSTAR plasmas. Imaging of the density fluctuations is achieved with an array of 16 vertically aligned detectors and two X-mode probe beam frequencies (tunable over 78–92 GHz between plasma discharges). The system provides the capability of fluctuation measurements with poloidal wavenumbers (kθ) up to ∼3 cm−1 at the maximum sampling rate of 2 MHz. Following extensive laboratory tests, the system was further tested with known coherent density fluctuations during the precursor oscillation of the m/n = 1/1 internal kink mode. The phase information of the reflected beam was compared with the precursor oscillation of the electron temperature measured by an electron cyclotron emission (ECE) radiometer. Density fluctuation levels (δne/ne) at two radial positions separated by the inversion radius (inside and outside) were comparable to temperature fluctuation levels (δTe/Te) from ECE signals. Subsequently, two correlation analysis methods were applied to turbulent fluctuation measurements in a neutral beam heated L-mode plasma to determine the mean poloidal rotation velocities of density fluctuations at two radial positions. The measured mean poloidal velocities were ∼8.4 km s−1 at r/a ∼ 0.6 and ∼5 km s−1 at r/a ∼ 0.7 in the clockwise direction, which differed by 1–2 km s−1 with the projected poloidal velocities from the toroidal rotation velocity measured by charge exchange recombination spectroscopy.

Investigation of ring-like runaway electron beams in the EAST tokamak

In the EAST tokamak, asymmetrical ring-like runaway electron beams with energy more than 30 MeV and pitch angle about 0.1 were investigated. Those runaway beams carried about 46% of the plasma current and located around the q = 2 rational surface when m/n = 1/1 and m/n = 2/1 MHD modes existed in the plasma. Those runaway beams changed from a hollow to a filled structure during the slow oscillations in the discharge about every 0.2 s, which correlated with a large step-like jump in electron cyclotron emission (ECE) signals, a big spike-like perturbation in Mirnov signals and a large decrease in runaway energy. Between those slow oscillations with large magnitude, fast oscillations with small magnitude also existed about every 0.02 s, which correlated with a small step-like jump in ECE signals, a small spike-like perturbation in Mirnov signals, but no clear decrease in runaway energy and changes in the runaway beam structure. Resonant interactions occurred between runaway electrons and magnetohydrodynamic instabilities, which gave rise to fast pitch angle scattering processes of those resonant runaway electrons, and hence increased the synchrotron radiation. Theoretical calculations of the resonant interaction were given based on a test particle description. Synchrotron radiation of those resonant runaway electrons was increased by about 60% until the end of the resonant interaction.

Lower hybrid current drive at high density on Tore Supra

Lower hybrid current drive (LHCD) experiments with line-averaged density varying between 1.5 × 1019 and 6 × 1019 m−3 are performed on the Tore Supra tokamak under quasi-steady-state conditions with respect to the fast electron dynamics. The LHCD efficiency is analysed from the fast electron bremsstrahlung (FEB) and electron cyclotron emission (ECE). The effect of plasma equilibrium and particle fuelling is documented. It is concluded that the fast decay of FEB with plasma density could be consistent with simple scaling of the current drive efficiency and FEB. Plasma edge measurements are presented looking for the effect on fast electron emission. In a specific case of particle fuelling, an anomalous decay of the hard x-ray and ECE signals suggests deleterious interaction of the wave with edge plasma.

Oblique electron-cyclotron-emission radial and phase detector of rotating magnetic islands applied to alignment and modulation of electron-cyclotron-current-drive for neoclassical tearing mode stabilization

A two channel oblique electron cyclotron emission (ECE) radiometer was installed on the DIII-D tokamak and interfaced to four gyrotrons. Oblique ECE was used to toroidally and radially localize rotating magnetic islands and so assist their electron cyclotron current drive (ECCD) stabilization. In particular, after manipulations operated by the interfacing analogue circuit, the oblique ECE signals directly modulated the current drive in synch with the island rotation and in phase with the island O-point, for a more efficient stabilization. Apart from the different toroidal location, the diagnostic view is identical to the ECCD launch direction, which greatly simplified the real-time use of the signals. In fact, a simple toroidal extrapolation was sufficient to lock the modulation to the O-point phase. This was accomplished by a specially designed phase shifter of nearly flat response over the 1-7 kHz range. Moreover, correlation analysis of two channels slightly above and below the ECCD frequency allowed checking the radial alignment to the island, based on the fact that for satisfactory alignment the two signals are out of phase.

Interpretation of tearing mode physics from ECE temperature fluctuations associated with magnetic islands and Mirnov signals

The experimental identification of the physical mechanisms behind the rotating magnetic helical modes with low numbers m,n which appear at magnetic surfaces where q = m/n is an elusive task. We discuss here a theoretical principle of a combined analysis of magnetic Mirnov signals and of electron cyclotron emission signals that can shed light on the underlying physics of these perturbations.

Characteristics and control of the type I edge localized mode in JT-60U

The detailed characteristics of the precursor of the type I edge localized mode (ELM) have been studied in JT-60U using diagnostics with high temporal and spatial resolution such as a microwave reflectometer, electron cyclotron emission (ECE) heterodyne radiometer and grating polychromator. Coherent density and temperature precursors have been observed before the collapse phase of type I ELM. The growth rate of the precursor is evaluated to be γ/ωA ∼ 10−3 for several edge pedestal conditions. From the phase delay between ECE signals measured at two toroidal locations and the frequency of the precursor, the toroidal mode number is experimentally evaluated as n = 8–10 or 14–16 assuming that the precursor rotates toroidally with the same toroidal rotation speed of carbon impurity. It is found that the dominant n varies with each ELM under the same plasma condition. The ratio of the pressure gradient inside the pedestal (∇pin) to the pressure gradient within the pedestal (∇pped) has been confirmed as an important parameter in determining the ELM energy loss (ΔWELM) normalized to the pedestal stored energy (Wped), ΔWELM/Wped. From the comparison of the reduction rate in the ion temperature profile due to ELMs, a larger reduction rate within the pedestal and a wider ELM affected area are observed in the plasma with larger ∇pin/∇pped. When the plasma near the top of the pedestal on the high-field side is heated by an electron cyclotron wave (ECW) power of 1.57 MW, the ΔWELM/Wped is reduced by ∼35%, together with an increase in the ELM frequency. The increasing rate of the ELM frequency with the heating power is about four times larger in the ECW injection case than the natural power dependence observed in the neutral beam injection case.

Investigation of fast pitch angle scattering of runaway electrons in the EAST tokamak

This paper reports that an experimental investigation of fast pitch angle scattering (FPAS) of runaway electrons in the EAST tokamak has been performed. From the newly developed infrared detector (HgCdTe) diagnostic system, the infrared synchrotron radiation emitted by relativistic electrons can be obtained as a function of time. The FPAS is analysed by means of the infrared detector diagnostic system and the other correlative diagnostic systems (including electron-cyclotron emission, hard x-ray, neutrons). It is found that the intensity of infrared synchrotron radiation and the electron—cyclotron emission signal increase rapidly at the time of FPAS because of the fast increase of pitch angle and the perpendicular velocity of the energetic runaway electrons. The Parail and Pogutse instability is a possible mechanism for the FPAS.

Development of electron cyclotron emission imaging system on Large Helical Device

A combined system of microwave imaging reflectometry and electron cyclotron emission (ECE) imaging has been developed for the Large Helical Device. This system includes a wide-band two-dimensional horn-antenna mixer array (HMA). The HMA consists of horn antennas, waveguides, mixers, and intermediate frequency circuits. The frequency response of the HMA is between 50 and 110 GHz. The ECE signal is selected using a 95 GHz local oscillator and a 93 GHz high-pass filter.

Identification of anomalous Doppler resonance effect during current ramp down in HT-7 tokamak

The abrupt steep jump of electron cyclotron emission (ECE) signals during current ramp-down has been observed and explained by an anomalous Doppler resonance effect (ADR). The identifying process of ADR was presented based on the fast Fourier transform (FFT) technique. The threshold value for triggering a steep jump on ECE signals has been identified under different discharge conditions.

Initial result of collective Thomson scattering using 77 GHz gyrotron for bulk and tail ion diagnostics in the Large Helical Device

The collective Thomson scattering (CTS) technique has been utilized with the backscattering configuration in the collective scattering regime to diagnose the velocity distribution functions in the Large Helical Device (LHD). The receiver was equipped with 16 channels and the first test has been carried out using the eight channels for scattered radiation and these channels cover a few GHz frequency shift from the 76.95 GHz probe beam. During the discharge, the electron density and temperature at the central region of the LHD are 1×1019m−3, and 1.0 keV, respectively. The probing beam with rectangular wave modulation is injected by 50 Hz in order to be distinct from the background electron cyclotron emission (ECE). The scattered radiation is resolved successfully at each channel of CTS receiver system. The detected signals of bulk ion and electron components are by a factor of 10 ~ 102 larger than the background ECE signal. We found that the measured spectra are in reasonably agreement with the theoretical spectra calculated by using the reliable measured electron temperature and density for input parameters. The CTS receiver system will be improved to obtain more accurate velocity distributions in high temperature plasmas.