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2. Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

Author

Abdullaev, S. S., Finken, K. H., Wongrach, K., Tokar, M., Koslowski, H. R., Willi, O., Zeng, L., and team, the TEXTOR

Subjects
Physics - Plasma Physics
Abstract

A new physical mechanism of formation of runaway electron (RE) beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between $q=1$ and the closest low--order rational magnetic surfaces [$q=5/4$ or $q=4/3$, \dots]. It results in that runaway electron beam current has a helical nature with a predominant $m/n=1/1$ component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high--energy electrons with the $m/n=1/1$ MHD mode are discussed., Comment: 5 pages, 4 figures, one table. arXiv admin note: text overlap with arXiv:1501.01404

Published
2015
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3. Mechanisms of plasma disruption and runaway electron losses in tokamaks

Author

Abdullaev, S. S., Finken, K. H., Wongrach, K., Tokar, M., Koslowski, H. R., Willi, O., Zeng, L., and team, the TEXTOR

Subjects
Physics - Plasma Physics
Abstract

Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron beams and their losses are proposed. The plasma disruption is caused by strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is hypothesized that the runaway electron beam is formed in the central plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of plasmas with the safety factor $q(0)<1$ the most stable runaway electron beams are formed by the intact magnetic surface located between the magnetic surface $q=1$ and the closest low--order rational surface $q=m/n>1$ ($q=5/4$, $q=4/3$, ...). The thermal quench time the current quench time are estimated. The runaway electron beam current is modeled as a sum of toroidally symmetric part and a small amplitude helical current with a predominant $m/n=1/1$ component. The runaway electrons are lost due to two effects: ($i$) by outward drift of electrons in a toroidal electric field until they touch wall and ($ii$) by the formation of stochastic layer of runaway electrons at the beam edge. Such a stochastic layer for high--energy runaway electrons is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to wall. The effect of external resonant magnetic perturbations on runaway electron loss is discussed. A possible cause of the sudden MHD signals accompanied by runaway electron bursts is explained by the redistribution of runaway current during the resonant interaction of high--energetic electron orbits with the $m/n=1/1$ MHD mode., Comment: 34 pages, 18 figures, 2 tables

Published
2015

4. Modelling of magnetic field structure of the Ergodic Divertor of Tore Supra and comparison to the Dynamic Ergodic Divertor of TEXTOR

Author

Abdullaev, S. S., Van Rompuy, T., Finken, K. H., Lehnen, M., and Jakubowski, M.

Subjects
Physics - Plasma Physics
Abstract

An analytical model previously developed to study the structure of the magnetic field for the TEXTOR-DED [S.S. Abdullaev et al. Phys. Plasmas, 6, 153 (1999)] is applied to the similar study of the Ergodic Divertor of Tore Supra tokamak [Ph. Ghendrih, Plasma Phys. Control. Fusion, 38, 1653 (1996)]. The coil configuration of ED Tore Supra consists of six modules equidistantly located along the toroidal direction on the low-field-side of the torus with given toroidal and poloidal extensions. The Hamiltonian formulation of field line equations in straight-field-line coordinates (Boozer coordinates) and the computationally efficient mapping method for integration of the Hamiltonian field line equations are used to study the magnetic field structure in the ED. Asymptotical formulas for the perturbation magnetic field created by the ED coils are obtained and the spectrum of magnetic perturbations is analyzed and compared with the one of the TEXTOR-DED. The structure of ergodic and laminar zones are studied by plotting Poincare sections, so-called laminar plots (contour plots of wall to wall connection lengths) and magnetic footprints. The radial profiles of field line diffusion coefficients are calculated for different perturbation currents and it is found that for the Tore Supra case in the ergodic zone the numerical field line diffusion coefficients perfectly follow the quasilinear formula for smaller perturbation currents although the situation is different for the maximum perturbation current., Comment: 32 pages, 27 figures

Published
2008

7. Runaway generation during disruptions in JET and TEXTOR

Author

Lehnen, M., Abdullaev, S. S., Arnoux, G., Bozhenkov, S. A., Jakubowski, M. W., Jaspers, R., Plyusnin, V. V., Riccardo, V., Samm, U., JET EFDA Contributors, TEXTOR Team, JET EFDA Contributors, and TEXTOR Team

Subjects
Nuclear and High Energy Physics, Jet (fluid), Tokamak, Chemistry, Nuclear engineering, Plasma, Nuclear reactor, Fusion power, Resonant magnetic perturbations, law.invention, Nuclear physics, Nuclear Energy and Engineering, Runaway electrons, law, General Materials Science, Heat load, Astrophysics::Galaxy Astrophysics
Abstract

Runaway electrons generated during ITER disruptions are of concern for the integrity of the plasma facing components. It is expected that a power of up to 8 GW is exposed to ITER PFCs. We present ill this article observations from JET and TEXTOR on the generation of runaways and the heat load deposition. Suppression techniques like massive gas injection and resonant magnetic perturbations are discussed. (C) 2009 Elsevier B.V. All rights reserved.

Published
2009
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8. Transport and divertor properties of the dynamic ergodic divertor

Author

Lehnen, M., Abdullaev, S. S., Jachmich, S., Jakubowski, M. W., Jaspers, R., Koslowski, H. R., Krämer-Flecken, A., Kikuchi, Y., Liang, Y., Loozen, X., Pospieszczyk, A., van Rumpuy, T., Biel, W., Reiter, D., Samm, U., Schmitz, O., Sergienko, G., Tokar, M., Unterberg, B., Wolf, R., Zimmermann, O., de Bock, M. F. M., Brezinsek, S., Busch, C., Classen, I., Finken, K. H., Harting, D., and von Hellermann, M.

Subjects
Physics, Safety factor, Magnetic structure, Divertor, Magnetic confinement fusion, Laminar flow, Plasma, Condensed Matter Physics, Computational physics, Nuclear Energy and Engineering, Physics::Plasma Physics, Electromagnetic coil, ddc:530, Atomic physics, Excitation
Abstract

The concept of the dynamic ergodic divertor (DED) is based on plasma edge ergodization by a resonant perturbation. Such a divertor concept is closely related to helical or island divertors in stellerators. The base mode of the DED perturbation field can be m/n = 12/4, 6/2 or 3/1. The 3/1 base mode with its deep penetration of the perturbation field provides the excitation of tearing modes. This topic was presented elsewhere. In this contribution we concentrate on the divertor properties of the DED. We report on the characterization of the topology, transport properties in ergodic fields, impurity transport and density limit behaviour.The 12/4 base where the perturbation is restricted to the plasma edge is suitable for divertor operation. With increasing perturbation field island chains are built up at the resonance layers. Overlapping islands lead to ergodization. The plasma is guided in the laminar region via open field lines of short connection length to the divertor target. The magnetic topology is not only controlled by the coil current but especially by the edge safety factor. For appropriate edge safety factor we observe a strong temperature drop in the plasma edge, indicating an expansion of the laminar region, which is necessary to decouple the divertor plasma from the core plasma. The modifications of the magnetic topology can be directly seen, for example, from carbon emission lines. The magnetic structure is calculated by the ATLAS code and shows good agreement with the experimental findings.

Published
2005
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16. Change of the magnetic-field topology by an ergodic divertor on the plasma structure and transport

Author

Jakubowski, M. W., Schmitz, O., Wolf, R., TEXTOR, Team., Abdullaev, S. S., Brezinsek, S., Finken, K. H., Krämer-Flecken, A., Lehnen, M., Samm, U., Spatschek, K. H., and Unterberg, B.

Subjects
Physics::Plasma Physics, ddc:550
Abstract

The magnetic-field perturbation produced by the dynamic ergodic divertor in TEXTOR changes the topology of the magnetic field in the plasma edge, creating an open chaotic system. The perturbation spectrum contains only a few dominant harmonics and therefore it can be described by an analytical model. The modeling is performed in the vacuum approximation without assuming a backreaction of the plasma and does not rely on any experimentally obtained parameters. It is shown that this vacuum approximation predicts in many details the experimentally observed plasma structure. Several experiments have been performed to prove that the plasma edge behavior is defined mostly by the magnetic topology of the perturbed volume. The change in the transport can be explained with the knowledge of only the magnetic structures; i.e., the ergodic pattern dominates the plasma properties.

Published
2006
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18. The structure of magnetic field in the TEXTOR-DED

Author

Finken, K. H., Abdullaev, S. S., Jakubowski, M. W., Lehnen, M., Nicolai, A., and Spatschek, K. H.

Subjects
Physics::Plasma Physics, ddc:620
Abstract

The main component of the Dynamic Ergodic Divertor (DED) consists of a set of coils installed in the TEXTOR tokamak which creates resonant magnetic perturbations, preferentially at the plasma edge. The main purpose of the DED is a study the effect of the magnetic perturbations on the tokamak plasma. In particular, on the transport of the heat and particles to wall, the plasma confinement and rotation. This report is devoted to the systematic theoretical study of magnetic field and its structure in the TEXTOR-DED. It contains the description of the DED coil system in different operational regimes, the magnetic field created by this coil system, the study of formation of chaotic magnetic field lines and the structure of stochastic (ergodic) zone of field lines at the plasma edge and on the divertor plates, determination of field line diffusion coefficients and the Kolmogorov lengths. The modern mapping method for integration of Hamiltonian field line equations is employed for these studies. A description of the numerical Gourdon code to study the ergodic zone of the DED is also given. The experimental observations of the structure magnetic field lines performed recently in the TEXTOR-DED and their comparison with the modelling are also briefly discussed.

Published
2005

36. Improved Confinement due to Open Ergodic Field Lines Imposed by the Dynamic Ergodic Divertor in TEXTOR

Author

Finken, K. H., primary, Abdullaev, S. S., additional, Jakubowski, M. W., additional, de Bock, M. F. M., additional, Bozhenkov, S., additional, Busch, C., additional, von Hellermann, M., additional, Jaspers, R., additional, Kikuchi, Y., additional, Krämer-Flecken, A., additional, Lehnen, M., additional, Schega, D., additional, Schmitz, O., additional, Spatschek, K. H., additional, Unterberg, B., additional, Wingen, A., additional, Wolf, R. C., additional, and Zimmermann, O., additional

Published
2007
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40. Background and Initial Experiments with the Dynamic Ergodic Divertor on TEXTOR

Author

Finken, K. H., primary, Abdullaev, S. S., additional, De Bock, M. F. M., additional, Giesen, B., additional, von Hellermann, M., additional, Hogeweij, G. M. D., additional, Jakubowski, M., additional, Jaspers, R., additional, Kobayashi, M., additional, Koslowski, H. R., additional, Lehnen, M., additional, Matsunaga, G., additional, Neubauer, O., additional, Pospieszczyk, A., additional, Samm, U., additional, Schweer, B., additional, and Wolf, R., additional

Published
2005
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41. Toroidal Plasma Rotation Induced by the Dynamic Ergodic Divertor in the TEXTOR Tokamak

Author

Finken, K. H., primary, Abdullaev, S. S., additional, de Bock, M. F. M., additional, von Hellermann, M., additional, Jakubowski, M., additional, Jaspers, R., additional, Koslowski, H. R., additional, Krämer-Flecken, A., additional, Lehnen, M., additional, Liang, Y., additional, Nicolai, A., additional, Wolf, R. C., additional, Zimmermann, O., additional, de Baar, M., additional, Bertschinger, G., additional, Biel, W., additional, Brezinsek, S., additional, Busch, C., additional, Donné, A. J. H., additional, Esser, H. G., additional, Farshi, E., additional, Gerhauser, H., additional, Giesen, B., additional, Harting, D., additional, Hoekzema, J. A., additional, Hogeweij, G. M. D., additional, Hüttemann, P. W., additional, Jachmich, S., additional, Jakubowska, K., additional, Kalupin, D., additional, Kelly, F., additional, Kikuchi, Y., additional, Kirschner, A., additional, Koch, R., additional, Korten, M., additional, Kreter, A., additional, Krom, J., additional, Kruezi, U., additional, Lazaros, A., additional, Litnovsky, A., additional, Loozen, X., additional, Lopes Cardozo, N. J., additional, Lyssoivan, A., additional, Marchuk, O., additional, Matsunaga, G., additional, Mertens, Ph., additional, Messiaen, A., additional, Neubauer, O., additional, Noda, N., additional, Philipps, V., additional, Pospieszczyk, A., additional, Reiser, D., additional, Reiter, D., additional, Rogister, A. L., additional, Sakamoto, M., additional, Savtchkov, A., additional, Samm, U., additional, Schmitz, O., additional, Schorn, R. P., additional, Schweer, B., additional, Schüller, F. C., additional, Sergienko, G., additional, Spatschek, K. H., additional, Telesca, G., additional, Tokar, M., additional, Uhlemann, R., additional, Unterberg, B., additional, Van Oost, G., additional, Van Rompuy, T., additional, Van Wassenhove, G., additional, Westerhof, E., additional, Weynants, R., additional, Wiesen, S., additional, and Xu, Y. H., additional

Published
2005
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42. The dynamic ergodic divertor in the TEXTOR tokamak: plasma response to dynamic helical magnetic field perturbations

Author

Finken, K H, primary, Abdullaev, S S, additional, Biel, W, additional, Bock, M F M de, additional, Busch, C, additional, Farshi, E, additional, Hellermann, M von, additional, Hogeweij, G M D, additional, Jakubowski, M, additional, Jaspers, R, additional, Koslowski, H R, additional, Kraemer-Flecken, A, additional, Lazaros, A, additional, Lehnen, M, additional, Liang, Y, additional, Nicolai, A, additional, Schmitz, O, additional, Unterberg, B, additional, Westerhof, E, additional, Wolf, R, additional, Zimmermann, O, additional, Baar, M de, additional, Bertschinger, G, additional, Brezinsek, S, additional, Classen, I G J, additional, Donné, A J H, additional, Esser, H G, additional, Gerhauser, H, additional, Giesen, B, additional, Harting, D, additional, Hoekzema, J A, additional, Huettemann, P W, additional, Jachmich, S, additional, Jakubowska, K, additional, Kalupin, D, additional, Kelly, F, additional, Kikuchi, Y, additional, Kirschner, A, additional, Koch, R, additional, Korten, M, additional, Kreter, A, additional, Krom, J, additional, Kruezi, U, additional, Litnovsky, A, additional, Loozen, X, additional, Cardozo, N J Lopes, additional, Lyssoivan, A, additional, Marchuk, O, additional, Mertens, Ph, additional, Messiaen, A, additional, Neubauer, O, additional, Philipps, V, additional, Pospieszczyk, A, additional, Reiser, D, additional, Reiter, D, additional, Rogister, A L, additional, Rompuy, T Van, additional, Savtchkov, A, additional, Samm, U, additional, Schorn, R P, additional, Schueller, F C, additional, Schweer, B, additional, Sergienko, G, additional, Telesca, K H G, additional, Tokar, M, additional, Oost, G Van, additional, Uhlemann, R, additional, Wassenhove, G Van, additional, Weynants, R, additional, Wiesen, S, additional, and Xu, Y, additional

Published
2004
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45. The Optimization of the Ergodic Structure of the Dynamic Ergodic Divertor In The TEXTOR Tokamak.

Author

Jakubowski, M. W., Abdullaev, S. S., Finken, K. H., Kikuchi, Y., Krämer-Flecken, A., Lehnen, M., Schmitz, O., Unterberg, B., and Wolf, R. C.

Subjects
*TOKAMAKS, *ERGODIC theory, *PLASMA gases, *MAGNETIC fields, *PLASMA dynamics, *HEAT transfer
Abstract

The dynamic ergodic divertor (DED) in TEXTOR is designed to provide a perturbed volume in the plasma edge in order to control heat and particle exhaust. The stochastic boundary layer is generated in the outermost region of the plasma, which can be divided into ergodic and laminar regions. The topology of the magnetic field is substantial for the transport properties and plasma parameters. It is expected that the formation of the proper laminar zone allows decoupling the plasma edge from the core. Due to enhancement of the radial electron heat transport in the ergodic region the electron temperature in the plasma boundary is reduced. Therefore one needs to find the proper ratio of the ergodic and laminar zone, which gives optimal performance of the divertor. The structure of the perturbed volume strongly depends on the safety factor profile and the plasma pressure. At the higher level of ergodization (i.e. at higher plasma current and lower beta poloidal) the laminar zone is dominant, while at the lower level of ergodization the ergodic region is more important. The topology of the perturbed volume is modeled with the ATLAS-code and basing on the results of modeling the experiments are performed and compared to the calculated structures. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Published
2006
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49. Edge ambipolar potential in toroidal fusion plasmas.

Author

Spizzo, G., Vianello, N., White, R. B., Abdullaev, S. S., Agostini, M., Cavazzana, R., Ciaccio, G., Puiatti, M. E., Scarin, P., Schmitz, O., Spolaore, M., and Terranova, D.

Subjects
TOROIDAL harmonics, TOKAMAKS, PLASMA devices, STATISTICAL bootstrapping, ELECTRIC fields, STELLARATORS
Abstract

A series of issues with toroidally confined fusion plasmas are related to the generation of 3D flow patterns by means of edge magnetic islands, embedded in a chaotic field and interacting with the wall. These issues include the Greenwald limit in Tokamaks and reversed-field pinches, the collisionality window for ELM mitigation with the resonant magnetic perturbations (RMPs) in Tokamaks, and edge islands interacting with the bootstrap current in stellarators. Measurements of the 2D map of the edge electric field Er(r = α, ɵ, ø) in the RFX reversed-field pinch show that Er has the same helicity of the magnetic islands generated by a m/n perturbation: in fact, defining the helical angle u = mɵ - nø +ɷt maps show a sinusoidal dependence as a function of u, Er=Er sin u. The associated E×B flow displays a huge convective cell with v(α)≢ 0 which, in RFX and near the Greenwald limit, determines a stagnation point for density and a reversal of the sign of Er. From a theoretical point of view, the question is how a perturbed toroidal flux of symmetry m/n gives rise to an ambipolar potential Φ =Φsin u. On the basis of a model developed with the guiding center code ORBIT and applied to RFX and the TEXTOR tokamak, we will show that the presence of an m/n perturbation in any kind of device breaks the toroidal symmetry with a drift proportional to the gyroradius q, thus larger for ions (ρi⪢ρe). Immediately, an ambipolar potential arises to balance the drifts, with the same symmetry as the original perturbation. [ABSTRACT FROM AUTHOR]

Published
2014
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