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Title: Experimental studies of high-confinement mode plasma response to non-axisymmetric magnetic perturbations in ASDEX Upgrade

Abstract

Here, the interaction of externally applied small non-axisymmetric magnetic perturbations (MP) with tokamak high-confinement mode (H-mode) plasmas is reviewed and illustrated by recent experiments in ASDEX Upgrade. The plasma response to the vacuum MP field is amplified by stable ideal kink modes with low toroidal mode number n driven by the H-mode edge pressure gradient (and associated bootstrap current) which is experimentally evidenced by an observable shift of the poloidal mode number m away from field alignment (m = qn, with q being the safety factor) at the response maximum. A torque scan experiment demonstrates the importance of the perpendicular electron flow for shielding of the resonant magnetic perturbation, as expected from a two-fluid MHD picture. Two significant effects of MP occur in H-mode plasmas at low pedestal collisionality, $$\nu _{\text{ped}}^{\ast}\leqslant 0.4$$ : (a) a reduction of the global plasma density by up to $$61 \% $$ and (b) a reduction of the energy loss associated with edge localised modes (ELMs) by a factor of up to 9. A comprehensive database of ELM mitigation pulses at low $${{\nu}^{\ast}}$$ in ASDEX Upgrade shows that the degree of ELM mitigation correlates with the reduction of pedestal pressure which in turn is limited and defined by the onset of ELMs, i. e. a modification of the ELM stability limit by the magnetic perturbation.

Authors:
 [1];  [2];  [3];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [2];  [1];  [1];  [4];  [1]
  1. Max-Planck-Institut fur Plasmaphysik, Garching (Germany)
  2. Culham Science Centre, Oxon (United Kingdom)
  3. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  4. Culham Science Centre, Oxon (United Kingdom); Univ. of York, York (United Kingdom)
Publication Date:
Research Org.:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1340288
Grant/Contract Number:
633053; AC02-09CH11466
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Plasma Physics and Controlled Fusion
Additional Journal Information:
Journal Volume: 59; Journal Issue: 1; Journal ID: ISSN 0741-3335
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; tokamak; ASDEX Upgrade; magnetic perturbation; high-confinement mode; edge-localised mode; ELM mitigation; ELM suppression; dynamic ergodic divertor; error field correction; resistive wall modes; high-beta; impurity transport; saddle coils; diii-d; discharges

Citation Formats

Suttrop, Wolfgang, Kirk, A., Nazikian, R., Leuthold, N., Strumberger, E., Willensdorfer, M., Cavedon, M., Dunne, M., Fischer, R., Fietz, S., Fuchs, J. C., Liu, Y. Q., McDermott, R. M., Orain, F., Ryan, D. A., and Viezzer, E.. Experimental studies of high-confinement mode plasma response to non-axisymmetric magnetic perturbations in ASDEX Upgrade. United States: N. p., 2016. Web. doi:10.1088/0741-3335/59/1/014049.
Suttrop, Wolfgang, Kirk, A., Nazikian, R., Leuthold, N., Strumberger, E., Willensdorfer, M., Cavedon, M., Dunne, M., Fischer, R., Fietz, S., Fuchs, J. C., Liu, Y. Q., McDermott, R. M., Orain, F., Ryan, D. A., & Viezzer, E.. Experimental studies of high-confinement mode plasma response to non-axisymmetric magnetic perturbations in ASDEX Upgrade. United States. doi:10.1088/0741-3335/59/1/014049.
Suttrop, Wolfgang, Kirk, A., Nazikian, R., Leuthold, N., Strumberger, E., Willensdorfer, M., Cavedon, M., Dunne, M., Fischer, R., Fietz, S., Fuchs, J. C., Liu, Y. Q., McDermott, R. M., Orain, F., Ryan, D. A., and Viezzer, E.. Tue . "Experimental studies of high-confinement mode plasma response to non-axisymmetric magnetic perturbations in ASDEX Upgrade". United States. doi:10.1088/0741-3335/59/1/014049. https://www.osti.gov/servlets/purl/1340288.
@article{osti_1340288,
title = {Experimental studies of high-confinement mode plasma response to non-axisymmetric magnetic perturbations in ASDEX Upgrade},
author = {Suttrop, Wolfgang and Kirk, A. and Nazikian, R. and Leuthold, N. and Strumberger, E. and Willensdorfer, M. and Cavedon, M. and Dunne, M. and Fischer, R. and Fietz, S. and Fuchs, J. C. and Liu, Y. Q. and McDermott, R. M. and Orain, F. and Ryan, D. A. and Viezzer, E.},
abstractNote = {Here, the interaction of externally applied small non-axisymmetric magnetic perturbations (MP) with tokamak high-confinement mode (H-mode) plasmas is reviewed and illustrated by recent experiments in ASDEX Upgrade. The plasma response to the vacuum MP field is amplified by stable ideal kink modes with low toroidal mode number n driven by the H-mode edge pressure gradient (and associated bootstrap current) which is experimentally evidenced by an observable shift of the poloidal mode number m away from field alignment (m = qn, with q being the safety factor) at the response maximum. A torque scan experiment demonstrates the importance of the perpendicular electron flow for shielding of the resonant magnetic perturbation, as expected from a two-fluid MHD picture. Two significant effects of MP occur in H-mode plasmas at low pedestal collisionality, $\nu _{\text{ped}}^{\ast}\leqslant 0.4$ : (a) a reduction of the global plasma density by up to $61 \% $ and (b) a reduction of the energy loss associated with edge localised modes (ELMs) by a factor of up to 9. A comprehensive database of ELM mitigation pulses at low ${{\nu}^{\ast}}$ in ASDEX Upgrade shows that the degree of ELM mitigation correlates with the reduction of pedestal pressure which in turn is limited and defined by the onset of ELMs, i. e. a modification of the ELM stability limit by the magnetic perturbation.},
doi = {10.1088/0741-3335/59/1/014049},
journal = {Plasma Physics and Controlled Fusion},
number = 1,
volume = 59,
place = {United States},
year = {Tue Nov 22 00:00:00 EST 2016},
month = {Tue Nov 22 00:00:00 EST 2016}
}

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Cited by: 5works
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  • Ideal plasma shielding and amplification of resonant magnetic perturbations in non-axisymmetric tokamak is presented by field line tracing simulation with full ideal plasma response, compared to measurements of divertor lobe structures. Magnetic field line tracing simulations in NSTX with toroidal non-axisymmetry indicate the ideal plasma response can significantly shield/amplify and phase shift the vacuum resonant magnetic perturbations. Ideal plasma shielding for n = 3 mode is found to prevent magnetic islands from opening as consistently shown in the field line connection length profile and magnetic footprints on the divertor target. It is also found that the ideal plasma shielding modifiesmore » the degree of stochasticity but does not change the overall helical lobe structures of the vacuum field for n = 3. Furthermore, amplification of vacuum fields by the ideal plasma response is predicted for low toroidal mode n = 1, better reproducing measurements of strong striation of the field lines on the divertor plate in NSTX.« less
  • Ideal plasma shielding and ampli cation of the resonant magnetic perturbations in non-axisymmetric tokamak is presented with full ideal plasma response calculations. Magnetic field line tracing simulations in NSTX with toroidal non-axisymmetry indicate the ideal plasma response can significantly shield/amplify and phase shift the vacuum resonant magnetic perturbations. Ideal plasma shielding for n = 3 mode is found to prevent magnetic islands from opening as consistently shown in the field line connection length pro le and magnetic footprints on the divertor target. It is also found that the ideal plasma shielding modifies the degree of stochasticity but does not changemore » the overall helical lobe structures of the vacuum field. Amplification of vacuum fields by the ideal plasma response is predicted for low toroidal mode n = 1, better reproducing measurements of strong striation of the field lines on the divertor plate in NSTX.« less
  • Recent observations on DIII-D have advanced the understanding of plasma response to applied resonant magnetic perturbations (RMPs) in both H-mode and L-mode plasmas. Three distinct 3D features localized in minor radius are imaged via filtered soft x-ray emission: (i) the formation of lobes extending from the unperturbed separatrix in the X-point region at the plasma boundary, (ii) helical kink-like perturbations in the steep-gradient region inside the separatrix, and (iii) amplified islands in the core of a low-rotation L-mode plasma. These measurements are used to test and to validate plasma response models, which are crucial for providing predictive capability of edge-localizedmore » mode control. In particular, vacuum and two-fluid resistive magnetohydrodynamic (MHD) responses are tested in the regions of these measurements. At the plasma boundary in H-mode discharges with n = 3 RMPs applied, measurements compare well to vacuum-field calculations that predict lobe structures. Yet in the steep-gradient region, measurements agree better with calculations from the linear resistive two-fluid MHD code, M3D-C1. Relative to the vacuum fields, the resistive two-fluid MHD calculations show a reduction in the pitch-resonant components of the normal magnetic field (screening), and amplification of non-resonant components associated with ideal kink modes. However, the calculations still over-predict the amplitude of the measured perturbation by a factor of 4. In a slowly rotating L-mode plasma with n = 1 RMPs, core islands are observed amplified from vacuum predictions. These results indicate that while the vacuum approach describes measurements in the edge region well, it is important to include effects of extended MHD in the pedestal and deeper in the plasma core.« less
  • Parameter scans show the strong dependence of the plasma response on the poloidal structure of the applied field highlighting the importance of being able to control this parameter using non-axisymmetric coil sets. An extensive examination of the linear single fluid plasma response to n = 3 magnetic perturbations in L-mode DIII-D lower single null plasmas is presented. The effects of plasma resistivity, toroidal rotation and applied field structure are calculated using the linear single fluid MHD code, MARS-F. Measures which separate the response into a pitch-resonant and resonant field amplification (RFA) component are used to demonstrate the extent to whichmore » resonant screening and RFA occurs. The ability to control the ratio of pitch-resonant fields to RFA by varying the phasing between upper and lower resonant magnetic perturbations coils sets is shown. The predicted magnetic probe outputs and displacement at the x-point are also calculated for comparison with experiments. Additionally, modelling of the linear plasma response using experimental toroidal rotation profiles and Spitzer like resistivity profiles are compared with results which provide experimental evidence of a direct link between the decay of the resonant screening response and the formation of a 3D boundary. As a result, good agreement is found during the initial application of the MP, however, later in the shot a sudden drop in the poloidal magnetic probe output occurs which is not captured in the linear single fluid modelling.« less