2D Electron Cyclotron Emission Research Articles

Investigation of MHD instabilities and related dynamic interactions during the major disruption in the HL-2A tokamak

Avoiding the major disruption is of paramount importance in future reactor-level devices, for which understanding the disruption mechanism is essential. In this work, MHD instabilities and related dynamic interactions during the major disruption have been investigated in the ohmically heated plasmas in the HL-2A tokamak. It is reported that the interaction between a kind of edge-oscillating-mode (EOM) perturbation and a rotating m/n = 2/1 tearing mode (TM) inside the plasma plays an important role in inducing the mode locking and the subsequent disruption. The EOM perturbation is oscillating in the laboratory frame, which is proposed to be originally generated by the penetrated error field at the plasma edge and is modulated by the rotating 2/1 TM mode. Before mode locking, the 2D electron cyclotron emission imaging shows that the momentary coupling of the EOM and the 2/1 mode can be decoupled each other and the mode structure does not alter significantly. After the mode locking, the EOM and the 2/1 mode expand and couple each other and induce the heat transfer from the core to the edge. The influence of the TM instability and the EOM perturbation on surrounding plasmas prior to the mode locking has also been presented. The results deepen our understanding of the disruption dynamics related to the external field perturbations, especially in the presence of TMs inside the plasma.

Behavior of multiple modes before and during minor disruption with the external resonant magnetic perturbations on J-TEXT tokamak

The behavior of multiple modes before and during minor disruption with the external resonant magnetic perturbations (RMPs) has been studied on a J-TEXT tokamak. The main component of RMPs is m/n = 2/1, where m and n are the numbers of the poloidal and toroidal modes, respectively. During the mode-locking caused by RMPs, it is found that before a minor or a major disruption (if there is no minor disruption), strong oscillations in both electron temperature and density occur if the edge safety factor q a > 3. The analysis shows that the oscillations are caused by the m/n = 3/1 mode. In addition, using the ECE, Mirnov coils and 2D electron cyclotron emission imaging diagnostic systems, it is found that a thermal collapse occurs on the inner side of the 2/1 magnetic island during the minor disruption, and before the collapse, a 3/2 island increases, after the collapse, the 3/2 island may disappear. Moreover, the study also shows that these 3/1, 2/1 and 3/2 modes play roles in the thermal collapse of disruptions.

Distinct stages of radio frequency emission at the onset of pedestal collapse in KSTAR H-mode plasmas

Using a high-speed and broadband radio frequency (RF) (0.1–1 GHz) spectrum analyzer developed on the KSTAR tokamak, it is found that several distinct stages of RF emission appear at the pedestal collapse in high confinement discharges. Comparison with 2D electron cyclotron emission (ECE) images has revealed that each stage is related to the instantaneous condition at the outboard mid-plane edge. First, high-harmonic ion cyclotron emissions (ICE) are intensified with the appearance of a non-modal filamentary perturbation in the edge within several tens of microseconds before the collapse. Then, the RF emission becomes broad toward high-frequency range (<500 MHz) at the burst onset of the non-modal filament. During the pedestal collapse initiated by the filament burst, rapid chirping (1–3 μs) appear with additional filament bursts. The strong correlation between the RF spectra and the perturbation structure provides important clues on the stability of edge-localized modes and on the ion dynamics in the plasma boundary.

Observation of the double e-fishbone instability in HL-2A ECRH/ECCD plasmas

Two m/n = 1/1 kink modes excited by energetic electrons (called double e-fishbone) have been observed near the q = 1 flux surfaces in the HL-2A discharges. The negative magnetic central shear configuration was achieved with localized electron cyclotron resonance heating and electron cyclotron current drive during plasma current ramp-up. The features of the modes have been first shown by advanced 2D electron cyclotron emission imaging (ECEI) system. From ECEI, two m/n = 1/1 modes propagating in the opposite directions poloidally have been clearly observed. These modes can be found only in low density discharge, and their frequencies are close to the precessional frequency of the trapped energetic electrons. More interestingly, the thermal energy transfer between the two modes was revealed by this new diagnostic, which is found to be related to the nonlinear interaction of the two modes and local electron thermal transport.

Design of the 2D electron cyclotron emission imaging instrument for the J-TEXT tokamak.

A new 2D Electron Cyclotron Emission Imaging (ECEI) diagnostic is being developed for the J-TEXT tokamak. It will provide the 2D electron temperature information with high spatial, temporal, and temperature resolution. The new ECEI instrument is being designed to support fundamental physics investigations on J-TEXT including MHD, disruption prediction, and energy transport. The diagnostic contains two dual dipole antenna arrays corresponding to F band (90-140 GHz) and W band (75-110 GHz), respectively, and comprises a total of 256 channels. The system can observe the same magnetic surface at both the high field side and low field side simultaneously. An advanced optical system has been designed which permits the two arrays to focus on a wide continuous region or two radially separate regions with high imaging spatial resolution. It also incorporates excellent field curvature correction with field curvature adjustment lenses. An overview of the diagnostic and the technical progress including the new remote control technique are presented.

Observations of compound sawteeth in ion cyclotron resonant heating plasma using ECE imaging on experimental advanced superconducting tokamak

The spatial and temporal evolutions of compound sawteeth were directly observed using 2D electron cyclotron emission imaging on experimental advanced superconducting tokamak. The compound sawtooth consists of partial and full collapses. After partial collapse, the hot core survives as only a small amount of heat disperses outwards, whereas in the following full collapse a large amount of heat is released and the hot core dissipates. The presence of two q = 1 surfaces was not observed. Instead, the compound sawtooth occurs mainly at the beginning of an ion cyclotron resonant frequency heating pulse and during the L-H transition phase, which may be related to heat transport suppression caused by a decrease in electron heat diffusivity.

Development of electron cyclotron emission imaging system on the HL-2A tokamak

A 2D electron cyclotron emission imaging (ECEI) system has been developed for measurement of electron temperature fluctuations in the HL-2A tokamak. It is comprised of a front-end 24 channel heterodyne imaging array with a tunable RF range spanning 75-110 GHz, and a set of back-end ECEI electronics that together generate 24 × 8 = 192 channel images of the 2nd harmonic X-mode electron cyclotron emission from the HL-2A plasma. The simulated performance of the local oscillator (LO) optics and radio frequency (RF) optics is presented, together with the laboratory characterization results. The Gaussian beams from the LO optics are observed to properly cover the entire detector array. The ECE signals from the plasma are mixed with the LO signal in the array box, then delivered to the electronics system by low-loss microwave cables, and finally to the digitizers. The ECEI system can achieve temporal resolutions of ~μs, and spatial resolutions of 1 cm (radially) and 2 cm (poloidally).

Design of the reflective optics for Tore Supra ECEI system

A 2D electron cyclotron emission (ECE) imaging system for Tore Supra is under design for studying the MHD physics of the magnetically confined plasma such as sawteeth, tearing modes, and turbulent fluctuations. Complex beam path due to the tight access in Tore Supra led to the design of reflective optics made of 6 or more large cylindrical∕flat mirrors. The total path length of the ECE beam is about 11 m, including almost 4 m inside the vacuum vessel. The imaging property of the optics has been estimated using the Gaussian beam simulation and ray transfer analysis. The possible setups for the optical alignment of the diagnostic and the operation scenarios with single- or dual-array measurement system are discussed.

Investigation of fast particle driven instabilities by 2D electron cyclotron emission imaging on ASDEX Upgrade

Detailed measurements of the 2D mode structure of Alfvén instabilities in the current ramp-up phase of neutral beam heated discharges were performed on ASDEX Upgrade, using the electron cyclotron emission imaging (ECEI) diagnostic. This paper focuses on the observation of reversed shear Alfvén eigenmodes (RSAEs) and bursting modes that, with the use of the information from ECEI, have been identified as beta-induced Alfvén eigenmodes (BAEs). Both RSAEs with first and second radial harmonic mode structures were observed. Calculations with the linear gyro-kinetic code LIGKA revealed that the ratio of the damping rates and the frequency difference between the first and second harmonic modes strongly depended on the shape of the q-profile. The bursting character of the BAE type modes, which were radially localized to rational q surfaces, was observed to sensitively depend on the plasma parameters, ranging from strongly bursting to almost steady state.

2D ECE measurements of type-I edge localized modes at ASDEX Upgrade

The installation of a 2D electron cyclotron emission imaging (ECEI) diagnostic on ASDEX Upgrade has provided a new means to observe the nature of edge localized modes (ELMs). For a series of ELMs in a typical type-I ELMy H-mode (with q95 = 4.7), the 2D dynamics have been characterized. Firstly, a clear distinction between so-called ‘fast’ and ‘slow’ ELMs was found to be the occurrence of an off-mid-plane fluctuation in case of the latter. This mode has its amplitude strongest off-mid-plane and its poloidal and toroidal mode numbers are m ∼ 110 and n ∼ 30. Secondly, prior to the onset of the ELM's temperature collapse, a mode is observed that covers the whole ECEI-observation window. Here, the estimated poloidal and toroidal mode numbers are m ∼ 75 and n ∼ 20. These have been seen to increase towards the ELM crash, simultaneously with a velocity increase of the mode (in poloidal direction). Finally, filaments have been identified during the temperature collapse phase and their motion could be followed in the vertical direction. In contrast to both the off-mid-plane fluctuation and the ELM-onset mode, which only have been seen rotating in the electron diamagnetic drift direction, the first few filaments have sometimes been observed to move in the opposite direction as well.

2D electron cyclotron emission imaging at ASDEX Upgrade (invited)

The newly installed electron cyclotron emission imaging diagnostic on ASDEX Upgrade provides measurements of the 2D electron temperature dynamics with high spatial and temporal resolution. An overview of the technical and experimental properties of the system is presented. These properties are illustrated by the measurements of the edge localized mode and the reversed shear Alfvén eigenmode, showing both the advantage of having a two-dimensional (2D) measurement, as well as some of the limitations of electron cyclotron emission measurements. Furthermore, the application of singular value decomposition as a powerful tool for analyzing and filtering 2D data is presented.

Two-dimensional correlation measurements of electron cyclotron emission fluctuations on the stellarator Wendelstein 7-AS

The miniaturization of receiver arrays permits the arrangement of numerous poloidally staggered radial sightlines of an electron cyclotron emission (ECE) diagnostic for the measurement of the electron temperature and its fluctuations, making two-dimensional (2D) imaging of the electron temperature and its fluctuations caused by plasma turbulence possible. For the stellarator W7-AS, the development of a fully monolithic microwave integrated circuit 150 GHz subharmonic mixer array is under development. As a first step, a 2D ECE system for the measurement of electron temperature fluctuations using four individual horn-reflector arrangements in conjunction with multichannel heterodyne radiometers was installed and set into operation. With Gaussian beam optics and four poloidally staggered sightlines, electron temperature fluctuations could be characterized in radial and poloidal directions simultaneously. First observations in purely electron cyclotron resonance heated stellarator plasmas reveal a broadband drift-wave feature. Earlier experiments showing a decrease of the electron temperature fluctuation level with increasing heating power were confirmed. Additionally, it was revealed, using the 2D ECE correlation radiometer, that an increased velocity shear might account for the decrease of the coherence length and thus for the reduction of the electron temperature fluctuation level.