Multifaceted Asymmetric Radiation Research Articles

High-Speed Image Processing Algorithms for Real-Time Detection of MARFEs on JET

This paper presents a high-speed image processing algorithm applied to multifaceted asymmetric radiation from the edge detection on Joint European Torus. The algorithm has been developed in C/C++ language using the OpenCV library for image processing and the LibSVM library for pattern classification and has been tested on a dedicated Intel 64-bit Linux computing platform. The code implemented achieves at the same time high accuracy and good run-time performance. The final version presents a correct detection rate of 93.3% and an average image processing rate of 650 frames per second. A complete analysis of each image processing module is presented to illustrate the overall characteristics and performance of the algorithm.

Phase Congruency Image Classification for MARFE Detection on JET with a Carbon Wall

Multifaceted asymmetric radiation from the edge (MARFE) instabilities may reduce confinement leading to harmful disruptions. They cause a significant increase in impurity radiation, and therefore, they leave a clear signature in the video data. This information can be exploited for automatic identification and tracking. A MARFE classifier, based on the phase congruency theory, has been developed and adjusted to extract the structural information in the images of Joint European Torus (JET) cameras. This approach has the advantage of using a dimensionless quantity and providing information that is invariant to image illumination, contrast, and magnification. The method was tested on JET experimental data and has proved to provide a good prediction rate.

Formation of a natural X-point multifaceted asymmetric radiation from the edge in numerical simulations of divertor plasmas

Multifaceted asymmetric radiation from the edges (MARFEs) are regions of high-density and low-temperature plasma observed under certain conditions at the tokamak edge. Such structures are found to form in a natural way on the closed flux surfaces near the X-point in 2D numerical simulations (B2-EIRENE) of the divertor plasmas at high discharge densities. Stable steady-state solutions can be obtained by the conventional edge code if the hydrogen line-radiation opacity is taken into account in the model, both with and without impurity radiation. In addition to extra power losses from the MARFE the detached divertor solution is affected due to (i) reduction of the separatrix temperature and thus plasma pressure; (ii) additional momentum losses due to friction with atoms produced by the recombining plasma of the MARFE.

Investigation on the relation between edge radial electric field asymmetries in RFX-mod and density limit

In all major confinement devices (tokamaks, stellarators, spheromaks and reversed-field pinches—RFPs), a density limit has been found. Results summarized in a recent work by Puiatti et al (2009 Nucl. Fusion 49 045012) show that in the RFP high density does not cause a disruption, but a sequence of increasingly critical phenomena. First, at intermediate density there is the disappearance of the high-confinement quasisingle helicity/single helical axis regimes. Then, at densities close to the Greenwald limit, toroidally and radially localized density accumulation and radiation condensation are observed, together with a fast resistive decay of the plasma current, which constitutes the real operative limit of the device. In this paper we discuss the effect of the magnetic ripple on test particle motion, showing that the accumulation of electrons in the X-points of the edge m = 0 islands is responsible for a modulation of the radial electric field Er which is at the core of the density limit mechanism. These results can be also relevant for the explanation of X-point multifaceted asymmetric radiation from the edge formation, observed in L-mode density limit discharges of JET.

Advanced tokamak research with integrated modeling in JT-60 Upgrade

Researches on advanced tokamak (AT) have progressed with integrated modeling in JT-60 Upgrade [N. Oyama et al., Nucl. Fusion 49, 104007 (2009)]. Based on JT-60U experimental analyses and first principle simulations, new models were developed and integrated into core, rotation, edge/pedestal, and scrape-off-layer (SOL)/divertor codes. The integrated models clarified complex and autonomous features in AT. An integrated core model was implemented to take account of an anomalous radial transport of alpha particles caused by Alfven eigenmodes. It showed the reduction in the fusion gain by the anomalous radial transport and further escape of alpha particles. Integrated rotation model showed mechanisms of rotation driven by the magnetic-field-ripple loss of fast ions and the charge separation due to fast-ion drift. An inward pinch model of high-Z impurity due to the atomic process was developed and indicated that the pinch velocity increases with the toroidal rotation. Integrated edge/pedestal model clarified causes of collisionality dependence of energy loss due to the edge localized mode and the enhancement of energy loss by steepening a core pressure gradient just inside the pedestal top. An ideal magnetohydrodynamics stability code was developed to take account of toroidal rotation and clarified a destabilizing effect of rotation on the pedestal. Integrated SOL/divertor model clarified a mechanism of X-point multifaceted asymmetric radiation from edge. A model of the SOL flow driven by core particle orbits which partially enter the SOL was developed by introducing the ion-orbit-induced flow to fluid equations.

Operational limits during high power long pulses with radiofrequency heating in Tore Supra

Issues related to the limitations and optimization of long pulse operation at high radiofrequency (RF) power levels in the Tore Supra tokamak are presented. An increasing operational limitation was encountered during the experimental campaigns in 2006–2007, affecting the high power and long pulse performance. This limitation was characterized by the sudden appearance of a multifaceted asymmetric radiation from the edge (MARFE), often followed by a disruption. The analyses revealed that the limitation could be linked to over-heating and flaking of the carbon re-deposition layers on the main plasma facing components (PFCs). The carbon deposits on all PFCs were therefore completely removed during the winter shutdown 2007–2008. Following this, a remarkable improvement in the injected power capability was observed, resulting in almost 12 MW of injected power during 10 s, without any of the previous signs of limitation (MARFE, disruption).Furthermore, the RF antennas are subject to localized heat loads due to RF sheath effects and interaction by fast particles, effects which need to be minimized in particular for long pulse operation. Experimental results concerning the heat load on the antennas, caused by fast ion losses in the presence of magnetic ripple, are presented in this paper.

Effect of poloidal inhomogeneity in plasma parameters on edge anomalous transport

It is demonstrated that anomalous transport at the plasma edge in tokamaks is essentially affected by poloidal inhomogeneities in the plasma temperature and density arising, e.g., by the formation of multifaceted asymmetric radiation from the edge at the density limit.

Analysis of radiative disruptions in RF-heated Tore Supra plasmas using infrared imaging

The precursors and following sequential events leading to radiative disruptions in Tore Supra have been analysed using infrared imaging, together with visible and ultraviolet spectroscopy of impurity species. A common feature observed prior to the disruptions is the appearance of a small (∼cm 2) hot spot on the main plasma facing component, the Toroidal Pumped Limiter (TPL), clearly localised in a zone of thick carbon re-deposition (>100 μm). A MARFE (Multifaceted Asymmetric Radiation From the Edge) is often triggered, followed by disruption. Such hot spots have been observed in ∼24% of the analysed disruptions, which is consistent with the fact that only 4/18 (22%) of the total area of the TPL is monitored with infrared cameras. These results suggest that over-heating of thick carbon re-deposition layers may play a role in the operational limits (MARFE, disruption) encountered.

MARFE stability and movement in an ELMy H-mode NSTX discharge

The results of a comparison of Multifaceted Asymmetric Radiation From the Edge (MARFE) theory with experiment in the National Spherical Torus Experiment (NSTX) are presented. A variety of MARFE behavior was observed using a fast-framing camera. A basic MARFE theory was applied to NSTX Multi-Pulse Thomson Scattering (MPTS) and Charge-Exchange Recombination Spectroscopy (CHERS) data. MARFE theory showed some limited agreement with experiment, but uncertainty in the separatrix location constrained the analysis. A method based on shifting iso- T e flux surfaces was used to estimate the separatrix location. The movements of MARFEs in NSTX are interpreted to result from diamagnetic heat flux driven drifts relative to the E × B drift and imply slowing edge poloidal rotation and/or changing edge profiles before a large ELM.

Development of integrated SOL/divertor code and simulation study of the JT-60U/JT-60SA tokamaks

To predict the heat and particle controllability in the divertor of tokamak reactors and to optimize the divertor design, comprehensive simulations by integrated modeling allowing for various physical processes are indispensable. SOL/divertor codes have been developed in the Japan Atomic Energy Agency for the interpretation and the prediction of behaviour of SOL/divertor plasmas, neutrals and impurities. The code system consists of the two-dimensional fluid code SOLDOR, the neutral Monte-Carlo (MC) code NEUT2D and the impurity MC code IMPMC. Their integration code ‘SONIC’ is almost completed and examined to simulate self-consistently the SOL/divertor plasmas in JT-60U. In order to establish the physics modelling used in fluid simulations, the particle simulation code PARASOL has also been developed.Simulation studies using those codes have progressed with the analysis of JT-60U experiments and the divertor designing of JT-60SA (modification program of JT-60U). The X-point multifaceted asymmetric radiation from the edge in the JT-60U experiment is simulated. It is found that the deep penetration of chemically sputtered carbon at the dome causes the large radiation peaking near the X-point. The pumping capability of JT-60SA is evaluated through the simulation. A guideline to enhance the pumping efficiency is obtained in terms of the exhaust slot width and the strike point distance. Transient behaviour of SOL/divertor plasmas after an ELM crash is characterized by the PARASOL simulation; the fast-time-scale heat transport is affected by collisions while the slow-time-scale behaviour is affected by the recycling.

Observation of instabilities during density limit experiments in the Hefei Tokamak-7

During a recent campaign at the Hefei Tokamak-7 (HT-7) [M. Asif et al., Phys. Plasmas 12, 082502 (2005)], experiments were performed with a moveable graphite limiter to investigate the influence of the horizontal plasma position on the density limit. It has been found that in HT-7 density limits differ by the radiative instability in the plasma boundary and various magnetohydrodynamic (MHD) instabilities, which precedes the disruption. Control of the impurity content, edge, and recycling properties prevents the growth of the multifaceted asymmetric radiation from the edge (MARFE) and makes it possible to exceed the Greenwald limit in HT-7. It has been observed that the m=2, MHD component dominates during almost the whole discharge duration, when the density limit is approached at values of the edge safety factor 3<q(a)<4. The observed sawtooth oscillation shows that oscillation with m=1 and n=1, where m and n are the poloidal and toroidal mode numbers, respectively, triggers m=2 and n=1 in sawtooth collapse, which finally causes disruption in the HT-7 tokamak.

High-beta steady-state research with integrated modeling in the JT-60 Upgrade

Improvement of high-beta performance and its long sustainment was obtained with ferritic steel tiles in the JT-60 Upgrade (JT-60U) [T. Fujita et al., Phys. Plasmas 50, 104 (2005)], which were installed inside the vacuum vessel to reduce fast ion loss by decreasing the toroidal field ripple. When a separation between the plasma surface and the wall was small, high-beta plasmas reached the ideal wall stability limit, i.e., the ideal magnetohydrodynamics stability limit with the wall stabilization. A small rotation velocity of 0.3% of the Alfvén velocity was found to be effective for suppressing the resistive wall mode. Sustainment of the high normalized beta value of βN=2.3 has been extended to 28.6s (∼15 times the current diffusion time) by improvement of the confinement and increase in the net heating power. Based on the research in JT-60U experiments and first-principle simulations, integrated models of core, edge-pedestal, and scrape-off-layer (SOL) divertors were developed, and they clarified complex features of reactor-relevant plasmas. The integrated core plasma model indicated that the small amount of electron cyclotron (EC) current density of about half the bootstrap current density could effectively stabilize the neoclassical tearing mode by the localized EC current accurately aligned to the magnetic island center. The integrated edge-pedestal model clarified that the collisionality dependence of energy loss due to the edge-localized mode was caused by the change in the width of the unstable mode and the SOL transport. The integrated SOL-divertor model clarified the effect of the exhaust slot on the pumping efficiency and the cause of enhanced radiation near the X-point multifaceted asymmetric radiation from edge. Success in these consistent analyses using the integrated code indicates that it is an effective means to investigate complex plasmas and to control the integrated performance.

The phenomenon of multifaceted asymmetric radiation from the edge with new graphite limiters in Hefei Tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters have been developed in Hefei Tokamak-7 (HT-7) for long-pulse operation. In this paper, the phenomenon of multifaceted asymmetric radiation from the edge (MARFE) has been studied in the ohmic discharges in HT-7. The best correlation has been found between the total input ohmic power and the product of the edge line-averaged density and Z eff. The critical density of the MARFE onset is observed in the region of , where f GW=n¯e/n GW (here n¯e is the maximum line-averaged electron density and n GW is the Greenwald density).

Multifaceted asymmetric radiation from the edge along improved confinement mode in LHCD plasmas with graphite limiters on HT-7 tokamak

Multifaceted asymmetric radiation from the edge (MARFE) phenomena during lower hybrid current drive (LHCD) Experiments on the HT-7 superconducting tokamak are summarized in this paper. The best correlation has been found between the total input (ohmic + LHCD) power and the product of the edge line average density and Z eff. Studies show that the critical density of MARFE onset is observed in the region of Z eff 1/2fGW = 0.6-0.9, where $f_{GW} ={\bar {n}_e }/{n_{GW}}$ , (here $\bar {n}_e$ is the maximum line average electron density and nGW is the Greenwald density). These MARFEs generally appear to have the same characteristics as high fGW MARFEs and are positionally stable throughout the LHCD pulse. Improved confinement mode induced by a MARFE is observed, and it is maintained for about 65 ms. MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked.

The study of MARFE during long pulse discharges in the HT-7 tokamak

Lower hybrid current drive (LHCD) experiments have been carried out to achieve high performance for long pulse operation in the HT-7 superconducting tokamak. Multifaceted asymmetric radiation from the edge (MARFE) phenomena is summarized and studied in the HT-7 tokamak. MARFE during LHCD discharge often occurs at the value of Zeff1/2fGW in range of 0.6–0.9. These MARFEs generally appear to have the same characteristics as high fGW density MARFEs and are locally stable throughout the LHCD pulse. The MARFE instability typically occurs at a fraction 33% of the nGW density during LHCD discharge, and it can often terminate the long pulse discharges in the HT-7 tokamak.

Study of confinement improvement in presence of a MARFE in the HT-7 superconducting tokamak

A new set of actively cooled toroidal double-ring graphite limiters have been developed on the HT-7 tokamk for long pulse operation. In this paper, a Multifaceted asymmetric radiation from the edge (MARFE) phenomena and improved particle confinement induced by a MARFE, characterized by Ha line emissions drops and the line-averaged density increase have been studied in the HT-7 ohmic discharges (where the plasma current Ip=145 kA, loop voltage Vloop =2-3 V, toroidal field BT =1.7 T, and Zeff =2-8). It is found that the improved particle confinement phase exists for about 90 ms and the particle confinement time tP increased about 1.6 times.

Thermal instability caused by plasma-wall coupling

Both analytical study and numerical modeling show that the interactions of plasma with the first wall saturated with hydrogen can cause thermal instability resulting in massive desorption of gas from the wall. In a tokamak it can lead to the formation of multifaceted asymmetric radiation from the edge (MARFE), detachment, or even disruption.

Influence of the Wall Characteristics on the Development of MARFE in Tokamaks

Multifaceted asymmetric radiation from the edge (MARFE) normally develops in fusion devices close to the density limit. MARFE is considered a result of thermal instabilities excited under critical conditions through different mechanisms: impurity radiation, recycling of neutral particles, anomalous transport of charged particles and energy. Recent experiments on tokamaks TEXTOR and JET show that plasma-wall interaction, leading to release of recycling neutrals and impurities, plays a very important role for the formation of MARFE. In the present contribution we develop further the MARFE models based on the instability of particle recycling on the tokamak wall by including a simple description for the release of recycling neutrals from the wall surface into the plasma. This development takes into account the time delay between the out flow of charged particles from the plasma and in flux of neutrals. The linear stability analysis shows that this does not change the critical plasma density for the MARFE formation but modifies significantly the growth rate of unstable perturbations developing when the density exceeds the threshold. These findings are confirmed in a non-linear consideration by solving the equations for the particle, momentum and energy transfer in the plasma coupled with the wall particle balance equations. This is done in a one-dimensional approximation by taking into account the variation of the main plasma parameters in the poloidal direction and making averaging in the radial direction over the plasma edge width of the penetration depth of neutrals. The intrinsic poloidal asymmetry of the system, defining the MARFE localization, is introduced by the Shafranov shift of magnetic flux surfaces. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Detachment Phenomena in LHD Compared to W7-AS

Self-sustained detachment, named the Serpens mode (Self-regulated plasma edge ’neath the last closed flux surface), has been observed in the Large Helical Device (LHD), which is equipped with an intrinsic open helical divertor, in spite of the low divertor neutral pressure of order 10–3 Pa. Other states of detachment are also observed; one is transient partial detachment, observed only in the gas puff port during massive gas puffing, and another is complete detachment, which takes place when the hot plasma boundary shrinks below the last closed flux surface. The Serpens mode is categorized as a subset of complete detachment, but with large fluctuations in the divertor flux and Hα signal, which result from a rotating radiation belt called the serpent. In this paper, these are compared with experimental observations of partial detachment and multifaceted asymmetric radiation from the edge (Marfe) in a modular stellarator, Wendelstein 7-AS (W7-AS). There are many similarities between transient partial detachment in LHD and partial detachment in W7-AS and, especially, between the Serpens mode in LHD and Marfe in W7-AS.

Improved confinement mode induced by a MARFE during lower hybrid current drive in the HT-7 tokamak

Lower hybrid current drive (LHCD) experiments have been carried out to achieve high performance for long pulse operation in the Hefei tokamak-7 (HT-7) superconducting tokamak. Multifaceted asymmetric radiation from the edge (MARFE) phenomena and an improved confinement mode induced by a MARFE are observed in LHCD plasmas when PLH>160 kW and the edge safety factor q(a) is slightly less than 6.5. It is found that an improved confinement mode induced by a MARFE, characterized by Hα line emissions drops and the line-averaged density increase is triggered in the MARFE discharges. The MARFE event occurs at t=1120 ms following the L–H transition, and the improved confinement phase exists for about 65 ms from t=1140 ms after the L–H transition. The LHCD plasma results basically are typical with MARFEs and it is an improved confinement mode; they are unique in that they appear at very low values in the HT -7 tokamak.