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Tearing mode dynamics in the presence of resonant magnetic perturbations
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.ORCID iD: 0000-0002-6554-9681
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetically confined fusion (MCF) plasmas are typically subject to several unstable modes. The growth of one mode can limit the plasma energy confinement and might cause a termination of the plasma. Externally applied resonant magnetic perturbations (RMPs) are used to control and to mitigate some of the unstable modes. Examples are, mitigation of edge localized modes and steering of neoclassical tearing mode position for stabilization by electron cyclotron current drive. Consequently, use of RMPs are considered necessary in planned future fusion machines. There are however negative consequences, the RMP interaction with a tearing mode (TM) of the same resonance can cause deceleration of the TM and possibly wall-locking. If a TM is non-rotating relative the machine-wall, it can grow and degrade fusion plasma performance and lead to a plasma disruption. Thus, all fusion confinement machines want to avoid wall-locked modes. Resonant magnetic fields can also be present in the form of machine-error-fields, which can produce the same effects. Clearly, it is of importance to understand the TM-RMP interaction. Typically, the modes with long wavelength are described by magnetohydrodynamic (MHD) theory. Considering the finite plasma resistivity, MHD predicts a mode that tears and reconnects magnetic field lines, called a tearing mode (TM). TMs occur at surfaces where the magnetic field lines close on themselves after a number of (m) toroidal and (n)poloidal turns. These surfaces are resonant in the sense that magnetic field and helical current perturbation has the same helicity, which minimize stabilizing effect of magnetic field line bending. In this thesis, the mechanisms of TM locking and unlocking due to external resonant magnetic perturbations (RMPs) are experimentally studied. The studies are conducted in two MCF machines of the type reversed-field pinch (RFP): EXTRAP T2R and Madison Symmetric Torus (MST). The studied machines exhibit multiple rotating TMs under normal operation. In EXTRAP T2R TM locking and unlocking are studied by application of a single harmonic RMP. Observations show that after the TM is locked, RMP amplitude has to be reduced significantly in order to unlock the TM. In similar studies in MST unlocking is not observed at all after turn-off of the RMP. Hence, in both machines, there is hysteresis in the locking and subsequent unlocking of a tearing mode. Results show qualitative agreement with a theoretical model of the TM evolution when subjected to an RMP. It is shown that the RMP cause a reduction of TM and plasma rotation at the resonant surface. The velocity reduction is opposed by a viscous torque from surrounding plasma. After TM locking, relaxation of the whole plasma rotation is observed, due to the transfer of velocity reduction via viscosity. This results in a reduced viscous resorting torque, which explains the observed hysteresis. The hysteresis is further deepened by the increase in amplitude of a locked mode.

Place, publisher, year, edition, pages
Stockholm: Universitetsservice US AB , 2016. , vii, 44 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:112
Keyword [en]
Tearing mode, plasma, fusion, reversed-field pinch, RFP, magnetic, confinement, resonant, perturbation, magnetohydrodynamics, MHD, EXTRAP T2R, Madison Symmetic Torus
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-180058ISBN: 978-91-7595-812-5 (print)OAI: oai:DiVA.org:kth-180058DiVA: diva2:891693
Presentation
2016-01-29, Seminarierummet, Teknikringen 31, KTH, Stcokholm, 13:15 (English)
Opponent
Supervisors
Note

QC 20160111

Available from: 2016-01-11 Created: 2016-01-07 Last updated: 2016-01-11Bibliographically approved
List of papers
1. Hysteresis in the tearing mode locking/unlocking due to resonant magnetic perturbations in EXTRAP T2R
Open this publication in new window or tab >>Hysteresis in the tearing mode locking/unlocking due to resonant magnetic perturbations in EXTRAP T2R
2015 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 57, no 10, 104008Article in journal (Refereed) Published
Abstract [en]

The physical mechanisms behind the hysteresis in the tearing mode locking and unlocking to a resonant magnetic perturbation (RMP) are experimentally studied in EXTRAP T2R reversed-field pinch. The experiments show that the electromagnetic and the viscous torque increase with increasing perturbation amplitude until the mode locks to the wall. At the wall-locking, the plasma velocity reduction profile is peaked at the radius where the RMP is resonant. Thereafter, the viscous torque drops due to the relaxation of the velocity in the central plasma. This is the main reason for the hysteresis in the RMP locking and unlocking amplitude. The increased amplitude of the locked tearing mode produces further deepening of the hysteresis. Both experimental results are in qualitative agreement with the model in Fitzpatrick et al (2001 Phys. Plasmas 8 4489)

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2015
Keyword
EXTRAP T2R, hysteresis, locking unlocking, resonant magnetic perturbation, reversed-field pinch, tearing modes, Magnetic resonance, Magnetism, Pinch effect, Wave plasma interactions, Resonant magnetic perturbations, Reversed field pinch, Locks (fasteners)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-177743 (URN)10.1088/0741-3335/57/10/104008 (DOI)000366679100009 ()2-s2.0-84940838273 (Scopus ID)
Note

QC 20151130

Available from: 2015-11-30 Created: 2015-11-25 Last updated: 2017-12-01Bibliographically approved
2. Tearing mode dynamics and locking in the presence of external magnetic perturbations
Open this publication in new window or tab >>Tearing mode dynamics and locking in the presence of external magnetic perturbations
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-180320 (URN)10.1063/1.4953438 (DOI)
Note

QC 20160816

Available from: 2016-01-11 Created: 2016-01-11 Last updated: 2016-08-16Bibliographically approved
3. The tearing mode locking-unlocking mechanism to an external resonant field in EXTRAP T2R
Open this publication in new window or tab >>The tearing mode locking-unlocking mechanism to an external resonant field in EXTRAP T2R
2014 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 56, no 10, 104001- p.Article in journal (Refereed) Published
Abstract [en]

The tearing mode (TM) locking and unlocking process due to an external resonant magnetic perturbation (RMP) is experimentally studied in EXTRAP T2R. The RMP produces a reduction of the natural TM velocity and ultimately the TM locking if a threshold in the RMP amplitude is exceeded. During the braking process, the TM slows down via a mechanism composed of deceleration and acceleration phases. During the acceleration phases, the TM can reach velocities higher than the natural velocity. Once the TM locking occurs, the RMP must be reduced to a small amplitude to obtain the TM unlocking, showing that the unlocking threshold is significantly smaller than the locking threshold and that the process is characterized by hysteresis. Experimental results are in qualitative agreement with a model that describes the locking-unlocking process via the balance of the electromagnetic torque produced by the RMP that acts to brake the TM and the viscous torque that tends to re-establish the unperturbed velocity.

Keyword
RMP, locking, hysteresis, RFP, tearing mode
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-155144 (URN)10.1088/0741-3335/56/10/104001 (DOI)000342759900003 ()2-s2.0-84907246910 (Scopus ID)
Note

QC 20141105

Available from: 2014-11-05 Created: 2014-10-31 Last updated: 2017-12-05Bibliographically approved
4. A method for the estimate of the wall diffusion for non-axisymmetric fields using rotating external fields
Open this publication in new window or tab >>A method for the estimate of the wall diffusion for non-axisymmetric fields using rotating external fields
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2013 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 55, no 8, 084001- p.Article in journal (Refereed) Published
Abstract [en]

A new method for the estimate of the wall diffusion time of non-axisymmetric fields is developed. The method based on rotating external fields and on the measurement of the wall frequency response is developed and tested in EXTRAP T2R. The method allows the experimental estimate of the wall diffusion time for each Fourier harmonic and the estimate of the wall diffusion toroidal asymmetries. The method intrinsically considers the effects of three-dimensional structures and of the shell gaps. Far from the gaps, experimental results are in good agreement with the diffusion time estimated with a simple cylindrical model that assumes a homogeneous wall. The method is also applied with non-standard configurations of the coil array, in order to mimic tokamak-relevant settings with a partial wall coverage and active coils of large toroidal extent. The comparison with the full coverage results shows good agreement if the effects of the relevant sidebands are considered.

Keyword
Resistive-Wall, Feedback Stabilization, Active Control, DIII-D, Modes, Pinch, Device
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-127495 (URN)10.1088/0741-3335/55/8/084001 (DOI)000322702700002 ()2-s2.0-84881511440 (Scopus ID)
Conference
17th Annual Workshop on Magnetohydrodynamic (MHD) Stability Control - Addressing the Disruption Challenge for ITER, NOV 05-07, 2012, New York, NY
Note

QC 20130902

Available from: 2013-09-02 Created: 2013-08-30 Last updated: 2017-12-06Bibliographically approved

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