Effect of Rotational Transform and Magnetic Shear on Confinement of Stellarators

Ascasíbar, E.; López‐Bruna, D.; Castejón, F.; Vargas, V. I.; Tribaldos, V.; Maaßberg, H.; Beidler, C. D.; Brakel, R.; Dinklage, A.; Geiger, J.; Harris, J. H.; Kus, A.; Mizuuchi, T.; Murakami, S.; Okamura, S.; Preuss, R.; Sano, F.; Stroth, U.; Suzuki, Yasuhiro; Talmadge, J. N.; Turkin, Y.; Watanabe, K. Y.; Yamada, H.; Yokoyama, M.

doi:10.1585/pfr.3.s1004

Cited by 17 publications

(21 citation statements)

References 42 publications

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“…In the first one the spatial position of the toroidally and poloidally circulating electron beam is detected electrically by measuring directly the electric current by means of moveable (electrostatic) probes or wire frames [8][9][10][11]. The second group comprises indirect optical detection methods of the electron beam, either by interaction with a background gas in the plasma vacuum vessel or by the excitation of a fluorescent detector rod [12][13][14][15][16][17][18] intersecting the magnetic field lines and hence generating a Poincaré plot. In addition, if the electron beam is exposed to a highly diluted background gas such as argon, nitrogen or hydrogen the whole three-dimensional structure of a field line is visible due to the inelastic collisional excitation.…”

Section: Methodsmentioning

confidence: 99%

Setup and initial results from the magnetic flux surface diagnostics at Wendelstein 7-X

Otte

Assmus

Biedermann

et al. 2016

Plasma Phys. Control. Fusion

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Wendelstein 7-X is an optimized stellarator with superconducting magnetic field coils that just started plasma operation at Max-Planck-Institut für Plasmaphysik (IPP) Greifswald. Utilizing the electron beam technique the first vacuum flux surface measurements were performed during the commissioning of the magnet system. For the magnetic configurations investigated so far the existence of closed and nested flux surfaces has been validated. All features of the configuration designed for the initial plasma operation phase, including a predicted island chain, were confirmed. No evidence on significant magnetic field errors was found. Furthermore, the effect of the elastic deformation of the non-planar coils was confirmed by the measurements.

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Section: Methodsmentioning

confidence: 99%

Setup and initial results from the magnetic flux surface diagnostics at Wendelstein 7-X

Otte

Assmus

Biedermann

et al. 2016

Plasma Phys. Control. Fusion

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“…The lowest values for χ e are found in regions around the positions of lowest order magnetic resonances, showing the influence of magnetic topology on confinement and discovering the evidence of correlation between local values of χ e and rotational transform. Remarkably, this happens in a low shear, although high rotational transform, environment [24,25].…”

Section: Transitions In Ecrh Plasmasmentioning

confidence: 95%

Overview of TJ-II experiments

Sánchez¹,

Acedo²,

Alegre³

et al. 2011

Nucl. Fusion

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This paper presents the last results on confinement studies in the TJ-II stellarator. The research of the dependence of spatially resolved transport coefficients on plasma parameters for ECH plasmas with Boronised wall shows that the heat confinement increases linearly with density while particle confinement increases sharply a factor four above a certain density threshold associated with the positive electric field. Remarkably, lowest order magnetic resonances, even in a low shear environment, reduce locally the effective diffusivities. The inherently strong plasma wall interaction of TJ-II has been successfully reduced after Lithium coating by vacuum evaporation. Besides H-retention and low Z, Li was chosen because there exists a reactor-oriented interest in this element, thus giving especial interest to the investigation of its properties. The Li-coating has led to important changes in plasma performance. Particularly, the effective density limit in NBI plasmas has been extended reaching central values of 8 x 10 19 m-3 and Te≈250-300 eV, with peaked density, rather flat Te profiles and increased ion temperatures. Alfvén modes are destabilised and their influence on fast ion confinement is studied in NBI discharges. Due to the achieved density control, a second type of transitions has been added to the low density ones previously observed in boronised wall. The high density transitions, under NBI with Li-coated walls are characterised by the fall of Hα emission, the onset of steep density gradient, and the reduction of the turbulence, which are characteristics of transition to H mode. TJ-II is therefore a unique device where first and second order phase transitions can be investigated.

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“…Actually, under low enough magnetic shear, magnetic resonances have been found to deteriorate dramatically the confinement in essentially every experimental device; but the amount of shear necessary to avoid such effects remains unquantified and the dependencies of such shear threshold on other parameters, e.g. the value of the rotational transform itself, is currently a matter of research [2]. Moreover, it is another experimental fact -also in tokamaks-that the magnetic resonances can be linked to the location of transport barriers.…”

Section: Motivationmentioning

confidence: 99%

Magnetic Resonances in ECR‐Heated Plasmas of the TJ‐II Heliac

López‐Bruna

Romero

Löpez‐Fraguas

et al. 2010

Contrib. Plasma Phys.

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Low order rational values of the rotational transform -magnetic resonances, for brevity-can be present inside ECR-heated plasmas of the TJ-II Heliac, in low magnetic shear conditions, without causing damage to confinement. Moreover, in agreement with previous experience in the TJ-II, the resonances seem to benefit confinement and are associated to changes in the radial electric field, at least in the density gradient region. These results encourage considering magnetic resonances as possible external actuators on confinement in stellarator/heliotron devices.

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Effect of Rotational Transform and Magnetic Shear on Confinement of Stellarators

Cited by 17 publications

References 42 publications

Setup and initial results from the magnetic flux surface diagnostics at Wendelstein 7-X

Setup and initial results from the magnetic flux surface diagnostics at Wendelstein 7-X

Overview of TJ-II experiments

Magnetic Resonances in ECR‐Heated Plasmas of the TJ‐II Heliac

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