Multifaceted Asymmetric Radiation Research Articles

A dual-route optical emission spectroscopy diagnostic with wide spectral range and high wavelength resolution on HL-2A tokamak

A dual-route optical emission spectroscopy (D-OES) diagnostic is newly developed to monitor the optical emission from the X-point plasma region on the HL-2A tokamak. This diagnostic is composed of an imaging system, a beam-splitting system for dual-route measurements, fiber bundles, a spectrometer system, and a control and acquisition system. One route is used to obtain wide-spectral-range spectra, and the other route is used to acquire high-wavelength-resolution line shapes. The spectral resolution of the wide-range spectrometers is 0.8 nm with a coverage of 800 nm (@200–1000 nm). The spectral resolution of the high-resolution spectrometer is 0.01 nm with a coverage of 6 nm (@200–660 nm). The spatial resolution of each route of D-OES is about 4 cm with 11 channels. The temporal resolution is 16 ms at maximum in the single-channel mode. Wide-range spectra (containing Balmer series and a Fulcher band) and highly resolved line shapes are obtained by D-OES in the hydrogen glow discharge in the lab. D-OES measurements are carried out in the high-density deuterium experiments of HL-2A. The electron density and deuterium temperature in the X-point multifaceted asymmetric radiation from the edge (MARFE) region are derived simultaneously by fitting the measured shape. The density is observed to increase from ~ to ~ , and the temperature drops from ~14.4 eV to ~2.3 eV after the onset of MARFE in the discharge #38260.

Local current shrinkage induced by the MARFE in L mode discharges on EAST tokamak

In this study, the multifaceted asymmetric radiation from the edge (MARFE) was observed in high-density discharges or during impurity gas injection on the EAST tokamak. The MARFE onset indicated by spectral and radiation signals can also be detected by the POlarimeter-INTerferometer (POINT) diagnostic, which measures the horizontal line-integrated density and the Faraday rotation. The fluctuation amplitude of the density signal resulting from the MARFE oscillation increases with the edge safety factor, which is consistent with the thermal instability theory. By combining density and the Faraday rotation, the local current shrinkage in the MARFE region is observed during the MARFE movement. The density and the current profile calculated by the POINT become more peak during the MARFE, which may lead to a strong magnetohydrodynamic instability that can result in disruption.

Detection of MARFEs using visible cameras for disruption prevention

In metallic devices, various forms of radiation collapse are one of the major causes of plasma degradation leading to disruptions. Some of the most advanced scenarios, with impurity seeding or detachment, are prone to developing excessive edge cooling, which very often ends with the collapse of the discharge. This type of disruption is also typically preceded by the formation of an instability called Multifaceted Asymmetric Radiation From the Edge (MARFE). This manifests itself as a blob of radiation above the X point or in various other regions of the plasma edge. This work reports on the development of image processing algorithms to detect MARFE as early as possible, to contribute to disruption prediction. Particular attention has therefore been devoted to general techniques, which can identify the features typical of MARFEs in any region of the plasma cross-section. The developed tools are sufficiently flexible to allow selecting a good trade-off between sensitivity and specificity. The solutions adopted are also all compatible with the implementation in real time networks. The examples provided have been collected using JET cameras during the H and T campaigns, run in preparation of the recently concluded JET campaign in D-T. Given the warning times achieved, it is expected that the early and accurate detection of MARFEs can contribute significantly to the prevention of the disruptions caused or preceded by these instabilities.

Implementation of CO2 and PbLi as working fluids in RELAP5/MOD3.3 towards accident analysis of COOL blanket for CFETR

The supercritical CO2 cOoled Lithium-Lead (COOL) blanket is an advanced blanket candidate for the Chinese Fusion Engineering Testing Reactor (CFETR). The COOL blanket has dual coolant. Supercritical CO2 (S-CO2) is used to cool the structural components, while lithium-lead (PbLi) is self-cooling in the breeding zones. To verify the reliability of the COOL blanket, it is essential to conduct the accident analysis. The system code RELAP5/MOD3.3 is developed with implementation of the two working fluids of the COOL blanket, namely PbLi and CO2. The outboard blanket segment of the COOL blanket is modeled using the modified RELAP5. The plasma pulse operation and fusion power excursion transient caused by Multifaceted Asymmetric Radiation From the Edge (MARFE) are simulated to study the thermal dynamic behavior of the COOL blanket. Furthermore, sensitivity analysis of Loss of Flow Accident (LOFA) is carried out. The results show that the outlet temperature of PbLi under steady-state operation meets its design requirements, and the temperature of the structural material does not exceed its temperature limit. The thermal hydraulic parameters have pulsed characteristics under plasma pulse operation. The MARFE event can cause sharp temperature rise of the First Wall (FW) but the material remains below its temperature limit. In case of LOFA of the CO2 system, it is suggested to restore CO2 mass flow rate in time to prevent the FW from overheating when the outlet temperature of PbLi is 973 K. The results of the preliminary accident analysis can provide the basis for the design and optimization of the blanket structure and system.

MARFE movement and density fluctuations after deuterium pellet injections in H-mode plasmas on EAST tokamak

The multifaceted asymmetric radiation from the edge (MARFE), which is generally considered to be the result of a radiation thermal instability in the edge and usually occurs in high density operation, has been first observed to move up and down along the poloidal cross-section due to edge cooling after cryogenic deuterium pellet injections in EAST tokamak with tungsten divertor. A maximum electron density of 0.84 × n GW has been obtained using continuous cryogenic pellet fueling. In the meantime, MARFEs, initially located near the divertors of EAST, moves to the inner wall on high field side after each pellet injection. This movement should be attributed to the asymmetry of the power flow to the two sides of the MARFE after pellet injections. Accompanied with MARFE movement, two kinds of strong density fluctuations have been observed. The ones with continuous and regular frequency spectrum, which does not cause a reduction of main plasma density, are confirmed to be induced by MARFE. The others, appearing with magnetic fluctuations, have been determined to be induced by the m/n = 2/1 magnetohydrodynamic activities after pellet injections. All the investigations in this paper will be meaningful for the steady high density operation of future fusion reactors, such as ITER.

Development of High-Speed Image Acquisition and Processing System for Real-Time Plasma Control on EAST

In order to realize advanced plasma control based on visible cameras, a high-speed image acquisition and processing system has been developed recently on Experimental Advanced Superconducting Tokamak (EAST). This system is optimized in many ways to achieve high-speed, real-time, low-latency performance, and to load multiple acquisition cards simultaneously. The acquisition rate of this system can be close to 10 000 FPS when the frame size is set as 320 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times240$ </tex-math></inline-formula> and the pixel depth is set as 8 bits. DMA is used for high-speed data transmission; the memory copy function is optimized for reducing the time cost on data memory reading and writing. Besides, the visualization subsystem based on the Python web can communicate with the acquisition machines and also can synthesize data of multiple acquisition machines in real time to perform image fusion and access display. In addition, a thermal event recognition function based on visible imaging is also included in this system. The convolutional neural network (CNN) model of hot spots and multifaceted asymmetric radiation from the edge (MARFE) detection for EAST plasma discharge has been developed so far. The detection results can be visualized in quasi-real time. In terms of data storage, a new data storage format is designed; a GUI for off-line data analysis and processing based on MATLAB is provided.

Observation of the high-density front at the high-field-side in the J-TEXT tokamak

This article reports the observation of the high-density front at the high field side (HFS) region in the limiter J-TEXT tokamak. In the J-TEXT high density discharges, the high-density front at the HFS scrape-off layer (SOL) region has been clearly identified by the far-infrared laser polarimeter-interferometer. The high-density front forms at the HFS SOL when the plasma density reaches a critical value, it can be up to seven times higher than the density in the low field side region and it stays stable as long as the density remains. In this stable phase, the local radiation is at a low level and it shares common characteristics with the high-field-side high-density (HFSHD) phenomena observed in divertor devices. If density increases continuously, the high-density front expands poloidally and evolves into an unstable ‘moving’ phase while propagating into the main plasma region, which eventually triggers the disruption. The maximum achievable density on J-TEXT seems to be correlated with the evolution of the high-density front, suggesting that the region may play a role in setting the operational limit on J-TEXT. The unstable phase of high-density front seems to share some common characteristics with the well-known multifaceted asymmetric radiation from the edge (MARFE), except that the radiation level in J-TEXT stays at a low level. The density threshold of the high-density front formation and the maximum density of the region itself increase with , consistent with the observation on both HFSHD and MARFE. The MARFE-like behavior of the high-density front in its final stage, suggests that MARFEs might be the result of these fronts forming in or moving into regions where the plasma temperature and impurity concentration are conducive for them to be radiatively unstable.

Investigation of the double peak in the visible-light range of radiative divertor emission profiles on the EAST tokamak

The double-peaked emission profile has been identified by tomographic reconstruction from the signals of the tangentially viewing visible-light imaging system on EAST. In the upper-single-null discharge with neon seeding, two light emission peaks are located near the X-point and in the high-field-side (HFS) scrape-off layer (SOL) on the poloidal cross-section while the outer divertor target is still attached. The double-peaked emission phenomenon has not been reported for other tokamaks. In particular, the existence of the emission peak near the X-point under the attached condition is unexpected because it is usually an indication of X-point multifaceted asymmetric radiation from the edge under the detached condition. Therefore, it is of fundamental importance for the divertor physics to make clear the mechanism of the double-peaked emission phenomenon. The computational simulations with SOLPS-ITER and DIVIMP are performed in this work. The simulated results show the contribution of W impurity to the double-peaked emission profile is negligible with the upper tungsten divertor. Considering the existence of a lower carbon divertor in EAST, the influence of the possible deposition of the carbon on the upper divertor is further investigated. The simulating CIII (465 nm) and CII (514 nm) emissions, which make a significant contribution to the visible-light emission in channel B (blue) and G (green), respectively, are found to be similar tothe experimental results. The emission peak near the X-point is related to the carbon deposited on the dome, while that in the HFS SOL is contributed by the carbon on the inner baffle. The ionization source of the carbon impurity and the net force pointing to the X-point are analyzed to understand the concentration of the carbon impurity near the X-point, which in turn causes the emission peak.

PHAD: a phase-oriented disruption prediction strategy for avoidance, prevention, and mitigation in JET

The ideal operational scenario for the future tokamak reactor is disruption-free operation. However, so far all the experimental evidence indicates that disruptions are unavoidable and can occur with alarming frequency when approaching reactor conditions (low q 95, high radiated fraction, divertor detachment, etc). In this article, a unified strategy for disruption avoidance, prevention, and mitigation is proposed and validated on JET data. The approach is based on three phase-oriented predictors to detect the main instabilities leading to the undesired and sudden end of the discharge. The first model detects dangerous profiles as an early indication of a critical situation. The second is designed to identify multifaceted asymmetric radiation from the edge and other abnormal radiative events. The third model is devoted to mitigation, and triggers alarms around few tens of ms before the beginning of the current quench. The models have been trained and tested with a database of almost 1000 JET discharges of recent campaigns with the ITER-like wall. The overall performances are very close to 100% of successful detections with a few percent of false alarms. In addition to the first systematic use of visible cameras for disruption prevention in JET, the most relevant aspect of this work is related to the distribution of the alarms of the three predictors, which do not overlap and are sequential. Consequently, the three predictors are meant to work in parallel over running discharges and, depending on which one triggers the alarm, the cause can be determined and approximate remaining time to intervene can be estimated, potentially allowing for the optimisation of the remedial actions.

CVD diamond photodetectors for FTU plasma diagnostics

We report on the results achieved by installing two Chemical Vapor Deposition (CVD) single crystal diamond detectors in one of the equatorial ports of the Frascati Tokamak Upgrade (FTU) tokamak, during the last six months of operation of the machine. The devices were fabricated at the University of Rome “Tor Vergata” in a metal/instrinsic/p-type diamond layered structure, allowing them to be used as Schottky photodiodes for VUV and Soft-X ray detection. Both detectors were placed inside the machine and operated in current mode under high vacuum conditions.The fast response capabilities of diamond detectors allowed to observe several plasma events, like the so-called Anomalous Doppler instabilities, the pellet injection and ablation, and Multifaceted Asymmetric Radiation From the Edge (MARFE). Diamond detectors often but not always followed the Magnetohydrodynamics (MHD) activity, depending on their localization relative to the emitting region. Core temperature oscillations following Electron Cyclotron Heating system (ECH) modulation were also observed. In addition, the diamond signals were compared to selected channels of the FTU bolometry system with similar line-of-sight: the encouraging results have launched an R&D program for the development of diamond-based bolometers.

Thermal dynamic analyses of the primary heat transfer system for the WCCB blanket of CFETR

The Water Cooled Ceramic Breeder (WCCB) blanket is one of the blanket candidates of Chinese Fusion Engineering Testing Reactor (CFETR). The Primary Heat Transfer System (PHTS) of the WCCB blanket has been updated to fulfill the requirement of operating under 200 MW∼1.5 GW of fusion power based on the lasted CFETR design. The PHTS has two independent Cooling Systems (CSs). CS1 provides coolant for the First Wall (FW) and CS2 for Breeding Zones (BZs). In this paper, the PHTS design is optimized to obtain more uniform flow distribution among blanket sectors. Based on the optimized PHTS, plasma pulse operation and the Multifaceted Asymmetric Radiation From the Edge (MARFE) transient are simulated using RELAP5 to study the thermal dynamic behavior of the WCCB PHTS. The results show that CS1 and CS2 have different thermal dynamic behavior during plasma pulses due to different thermal inertia. Different schemes are considered for CS1 and CS2 to couple with the Power Conversion System (PCS). As for the MARFE, thermal hydraulic parameters of CS1 can return to steady state soon after the transient. The WCCB PHTS design is preliminarily evaluated as reliable according to thermal dynamic analyses.

Investigation of multifaceted asymmetric radiation from the edge (MARFE) with impurity injection from the upper divertor on the Experimental Advanced Superconducting Tokamak

The phenomenon of multifaceted asymmetric radiation from the edge (MARFE) is investigated with impurity gas puff from the upper divertor on the Experimental Advanced Superconducting Tokamak (EAST). A typical process in which dense radiation region in the vicinity of the X point (X-point MARFE) further evolves into MARFE on the high field side (wall MARFE) after the plasma makes a transition from H-mode to L-mode confinement is observed in discharges with the impurity gas seeding. The electron density distribution and evolution in the divertor volume are measured by means of spectroscopy. It is observed in the experiments that the final position of MARFE is related to the ion ∇B drift direction. After the phase difference of n = 1 resonant magnetic perturbation (RMP) change from 90° to 270°, MARFE begins to appear on the high field side, which may be caused by the density pump-out induced by RMP. At the same heating power, the density threshold for MARFE formation is somewhat higher in the Ne seeding discharge than in the Ar seeding discharge. This may be attributed to the fact that the divertor radiation fraction, Prad,div/Prad,main, in the Ne seeding discharge is lower than that in the Ar seeding discharge. In addition, MARFE is successfully suppressed by total radiated energy feedback control in radiative divertor experiments. Therefore, radiation feedback control may play a vital role in avoiding major disruption induced by impurity radiation in radiative divertor experiments.

First experimental tests of a new small angle slot divertor on DIII-D

A new small angle slot (SAS) divertor concept has been developed to enhance neutral cooling across the divertor target by coupling a closed slot structure with appropriate target shaping. Initial tests on DIII-D find a strong interplay between such anticipated ‘SAS’ effects and cross-field drifts, favouring operation with the ion B × ∇B drift away from the X-point, as currently employed for advanced tokamaks. This offers the following key improvements relative to DIII-D’s open lower divertor or partially-closed upper divertor: (i) SAS allows for transition to low temperature moderately detached divertor conditions with Te ≲ 10 eV at very low main plasma densities, lower than are usually attainable at all in DIII-D high confinement (H-mode) plasmas as used in these tests; (ii) Pedestal performance and core confinement are significantly improved with SAS. The final confinement collapse associated with the onset of X-point MARFE (multifaceted asymmetric radiation from the edge) following deep detachment occurs at significantly higher pedestal densities, thus widening the window of H-mode operation compatible with a dissipative divertor. For operation with the ion B × ∇B drift toward the X-point, the divertor plasma transitions to a bifurcative detached state at much higher densities, similar to other divertor configurations in DIII-D. These results highlight the strong interplay between divertor closure and drifts, and point to an interesting divertor optimization path to explore that offers potential for future fusion reactors.

First time observation of local current shrinkage during the MARFE behavior on the J-TEXT tokamak

Multifaceted asymmetric radiation as well as strong poloidal asymmetry of the electron density from the edge, dubbed as ‘MARFE’, has been observed in high electron density Ohmically heated plasmas on J-TEXT tokamak. Equilibrium reconstruction based on the measured data from the 17-channel FIR polarimeter–interferometer indicates that an asymmetric plasma current density distribution forms at the edge region and the plasma current shrinkage locates at the MARFE affected region. Furthermore, associated with the localized plasma current shrinkage, a locked mode MHD activity is excited, which then terminate the discharge with a major disruption. Localized plasma current shrinkage at the MARFE region is considered to be the direct cause for the density limit disruptions, and the proposed interpretation is consistent with the experimental observations.

Progress of recent experimental research on the J-TEXT tokamak

The progress of experimental research over the last two years on the J-TEXT tokamak is reviewed and reported in this paper, including: investigations of resonant magnetic perturbations (RMPs) on the J-TEXT operation region show that moderate amplitude of applied RMPs either increases the density limit from less than 0.7nG to 0.85nG (nG is the Greenwald density, ) or lowers edge safety factor qa from 2.15 to nearly 2.0; observations of influence of RMPs with a large m/n = 3/1 dominant component (where m and n are the toroidal and poloidal mode numbers respectively) on electron density indicate electron density first increases (decreases) inside (around/outside) of the 3/1 rational surface, and it is increased globally later together with enhanced edge recycling; investigations of the effect of RMPs on the behavior of runaway electrons/current show that application of RMPs with m/n = 2/1 dominant component during disruptions can reduce runaway production. Furthermore, its application before the disruption can reduce both the amplitude and the length of runaway current; experimental results in the high-density disruption plasmas confirm that local current shrinkage during a multifaceted asymmetric radiation from the edge can directly terminate the discharge; measurements by a multi-channel Doppler reflectometer show that the quasi-coherent modes in the electron diamagnetic direction occur in the J-TEXT ohmic confinement regime in a large plasma region (r/a ~ 0.3–0.8) with frequency of 30–140 kHz.

Achieving temporary divertor plasma detachment with MARFE events by pellet injection in the EAST superconducting tokamak

A new pellet injection system has been equipped on the experimental advanced superconducting tokamak (EAST) in the 2012 campaign, with a pellet size of ϕ 2 mm × 2 mm, a frequency of 1 Hz–10 Hz and velocity of 150 m s−1–300 m s−1. The deuterium pellet is well-known for plasma fuelling as well as for triggering the edge localized mode (ELM). In the 2012 campaign, pellet injection experiments were successfully carried out on EAST. Temporary plasma detachment achieved by deuterium pellets has been observed in a double null (DN) divertor configuration, with multi-pellet injections at a repetition frequency of 2 Hz. The partial detachment of the outer divertors and complete detachment of the inner divertors was achieved after 35 ms of each pellet injection, which have a duration of 30–60 ms with the maximum degree of detachment (DOD) reaching 3.5 and 37, respectively. Meanwhile, the multifaceted asymmetric radiation from the edge (MARFE) phenomena was also observed at the high field side (HFS) near both the lower and upper X-points with radiation loss suddenly increased to about 15%–70%, which may be the main cause of divertor plasma detachment. The temporary detachment induced by pellet injection may act as a new way to study divertor detachment behaviors.

Overview of image processing tools to extract physical information from JET videos

In magnetic confinement nuclear fusion devices such as JET, the last few years have witnessed a significant increase in the use of digital imagery, not only for the surveying and control of experiments, but also for the physical interpretation of results. More than 25 cameras are routinely used for imaging on JET in the infrared (IR) and visible spectral regions. These cameras can produce up to tens of Gbytes per shot and their information content can be very different, depending on the experimental conditions. However, the relevant information about the underlying physical processes is generally of much reduced dimensionality compared to the recorded data. The extraction of this information, which allows full exploitation of these diagnostics, is a challenging task. The image analysis consists, in most cases, of inverse problems which are typically ill-posed mathematically. The typology of objects to be analysed is very wide, and usually the images are affected by noise, low levels of contrast, low grey-level in-depth resolution, reshaping of moving objects, etc. Moreover, the plasma events have time constants of ms or tens of ms, which imposes tough conditions for real-time applications. On JET, in the last few years new tools and methods have been developed for physical information retrieval. The methodology of optical flow has allowed, under certain assumptions, the derivation of information about the dynamics of video objects associated with different physical phenomena, such as instabilities, pellets and filaments. The approach has been extended in order to approximate the optical flow within the MPEG compressed domain, allowing the manipulation of the large JET video databases and, in specific cases, even real-time data processing. The fast visible camera may provide new information that is potentially useful for disruption prediction. A set of methods, based on the extraction of structural information from the visual scene, have been developed for the automatic detection of MARFE (multifaceted asymmetric radiation from the edge) occurrences, which precede disruptions in density limit discharges. An original spot detection method has been developed for large surveys of videos in JET, and for the assessment of the long term trends in their evolution. The analysis of JET IR videos, recorded during JET operation with the ITER-like wall, allows the retrieval of data and hence correlation of the evolution of spots properties with macroscopic events, in particular series of intentional disruptions.

Dependence of the density limit on the toroidal magnetic field on FTU

The capability of predicting the density limit of a magnetically confined burning plasma is of crucial importance to establish the ultimate performance of a fusion power plant. The Greenwald density limit, commonly used as an empirical scaling law, predicts that the maximum achievable central line-averaged density is given by the relation , where is the average plasma current density and k is the plasma elongation. However, several experiments have pointed out that such a limit can be overcome in the presence of peaked density profiles. This paper proposes a new empirical scaling law for a limiter tokamak operating in the low-energy confinement mode (L-mode) concerning the case of peaked density profiles associated with the presence of multifaceted asymmetric radiation from the edges. This result is based on dedicated experiments performed on the Frascati Tokamak Upgrade (FTU) under extremely clean machine conditions (Zeff = 1.0–1.5), in which the high-density domain is explored in a wide range of values of plasma current (Ip = 500–900 kA) and toroidal magnetic field (BT = 4–8 T). It is found that the maximum achievable central line-averaged density essentially depends on the toroidal magnetic field only and does not depend on the average plasma current density: the behaviour is explained in terms of density profile peaking in the high-density domain. As a confirmation that the limit is an edge limit, it is also shown that a Greenwald-like scaling (i.e. depending on the current density) actually holds for the edge line-averaged density (at r/a ≃ 4/5).

A 10 000-Image-per-Second Parallel Algorithm for Real-Time Detection of MARFEs on JET

This paper presents a very high-speed image processing algorithm applied to multi-faceted asymmetric radiation from the edge (MARFE) detection on the Joint European Torus. The algorithm was built in serial and parallel versions and written in C/C+ using OpenCV, cvBlob, and LibSVM libraries. The code implemented was characterized by its accuracy and run-time performance. The final result of the parallel version achieves a correct detection rate of 97.6% for MARFE identification and an image processing rate of more than 10 000 frame per second. The parallel version divides the image processing chain into two groups and seven tasks. One group is responsible for Background Image Estimation and Image Binarization modules, and the other is responsible for region Feature Extraction and Pattern Classification. At the same time and to maximize the workload distribution, the parallel code uses data parallelism and pipeline strategies for these two groups, respectively. A master thread is responsible for opening, signaling, and transferring images between both groups. The algorithm has been tested in a dedicated Intel symmetric-multiprocessing computer architecture with a Linux operating system.

Density fluctuations induced by MARFE on FTU

High density operations on FTU (Frascati Tokamak Upgade) are affected by strong density oscillations due to the so-called MARFE (Multifaceted Asymmetric Radiation From the Edge). An analysis of these oscillations is reported here. The MARFE on FTU occurs at the value of ρ = 0.48 (ρ is ratio between line-averaged density with averaged current density in the plasma); at the MARFE onset, the volume averaged density versus central density is almost constant, corresponding to the value of 0.8. The MARFE is observed to move up and down along the poloidal circumference on FTU. The MARFE movement causes continuous density fluctuations. Amplitude of density fluctuations decreases with edge safety factor and with electron density. The mechanism to cause MARFE movement is discussed. If amplitude exceeds a certain threshold, it may cause poloidal heat flux asymmetry between the edges of the MARFE, which will make the MARFE moving along the poloidal circumference.