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A method for the estimate of the wall diffusion for non-axisymmetric fields using rotating external fields
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.ORCID iD: 0000-0002-9546-4494
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.ORCID iD: 0000-0002-6554-9681
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
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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.

Place, publisher, year, edition, pages
2013. Vol. 55, no 8, 084001- p.
Keyword [en]
Resistive-Wall, Feedback Stabilization, Active Control, DIII-D, Modes, Pinch, Device
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-127495DOI: 10.1088/0741-3335/55/8/084001ISI: 000322702700002Scopus ID: 2-s2.0-84881511440OAI: oai:DiVA.org:kth-127495DiVA: diva2:644814
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
In thesis
1. Model predictive control of resistive wall modes in the reversed-field pinch
Open this publication in new window or tab >>Model predictive control of resistive wall modes in the reversed-field pinch
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The reversed-field pinch (RFP) is a magnetic confinement fusion (MCF) device. It exhibits a variety of unstable modes that can be explained by magnetohydrodynamic (MHD) theory. A particular unstable mode that is treated in this work is the resistive wall mode (RWM), which occurs when the shell of the device has finite conductivity. Application of control engineering tools appears to be important for the operation of the RFP. A model-based control approach is pursued to stabilize the RWM. The approach consists of experimental modeling of RWM using a class of system identification techniques. The obtained model is then used as a basis for Mode Predictive Control (MPC) design. The MPC employs the model to build predictions of the system and find a control input that optimizes the predicted behavior of the system. It is shown that the formulation of the MPC allows the user to incorporate several physics relevant phenomena aside from RWMs. The results are encouraging for MPC to be a useful tool for future MCF operation.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. v, 38 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:027
National Category
Fusion, Plasma and Space Physics Control Engineering
Identifiers
urn:nbn:se:kth:diva-168617 (URN)978-91-7595-596-4 (ISBN)
Presentation
2015-06-12, Alfvénlab seminarierum (1419), Teknikringen 31, KTH, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

QC 20150605

Available from: 2015-06-05 Created: 2015-06-05 Last updated: 2015-06-05Bibliographically approved
2. Tearing mode dynamics in the presence of resonant magnetic perturbations
Open this publication in new window or tab >>Tearing mode dynamics in the presence of resonant magnetic perturbations
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
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:nbn:se:kth:diva-180058 (URN)978-91-7595-812-5 (ISBN)
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
3. Model based approach to resistive wall magnetohydrodynamic instability control: Experimental modeling and optimal control for the reversed-field pinch
Open this publication in new window or tab >>Model based approach to resistive wall magnetohydrodynamic instability control: Experimental modeling and optimal control for the reversed-field pinch
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The primary objective of fusion research is to realize a thermonuclear fusion power plant. The main method to confine the hot plasma is by using a magnetic field. The reversed-field pinch is a type of magnetic confinement device which suffers from variety of magnetohy- drodynamic (MHD) instabilities. A particular unstable mode that is treated in this work is the resistive wall mode (RWM), which occurs due to the current gradient in the RFP and has growth rates of the order of the magnetic diffusion time of the wall. Application of control engineering tools appears to allow a robust and stable RFP operation.A model-based approach to stabilize the RWMs is pursued in this thesis. The approach consists of empirical modeling of RWMs using a class of subspace identification methodology. The obtained model is then used as a basis for a model based controller. In particular the first experimental results of using a predictive control for RWM stabilization are obtained. It is shown that the formulation of the model based controller allows the user to incorporate several physics relevant phenomena along with the stabilization of RWM. Another use of the model is shown to estimate and compensate the inherent error field. The results are encouraging, and the methods appear to be generically useful as research tools in controlled magnetic confinement fusion.

Abstract [sv]

Fusionsforskningens primära mål är att förverkliga en ny typ av kraftverk baserade på termonukleär fusion. Den viktigaste metoden för att innesluta det heta plasmat är användandet av  magnetfält. ”Reverserat-fält pinch” (RFP) är en typ av anläggning för magnetisk inneslutning av fusionsplasma som uppvisar ett flertal magneto-hydrodynamiska instabiliteter. En specifik instabil mod som behandlas i detta arbete är”resistiv-vägg” moden (RWM). Den orsakas av strömgradienten i RFPn och tillväxer med en tidskonstant som är av samma storleksordning som magnetfältets diffusionstid i det omgivande metallskalet.  Tillämpning av verktyg från reglerteknikområdet förefaller tillåta en robust och stabil RFP drift. I detta arbete används ett modell-baserat tillvägagångssätt för kompensering av RWM. Det innefattar empirisk modellering av RWM med användning av ”subspace” system-identifieringsmetoder. Den erhållna modellen används sedan som grund för en modell-baserad regulator. De första experimentella resultaten från modell-prediktiv kompensering av RWM har erhållits.  I detta arbete har också visats att formuleringen av den modellbaserade regulatorn tillåter användaren att integrera flera relevanta fysikaliska aspekter förutom RWM. Ytterligare en användning av modellen är för att göra uppskattning och kompensering av avvikelser i anläggningens magnetfält, så kallade fält-fel. Resultaten är uppmuntrande, och det förefaller som om de undersökta metoderna är allmänt användbara som verktyg för forskning om magnetisk inneslutning av fusionsplasma.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 59 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2016:192
Keyword
magnetohydrodynamic, model based control, reversed-field pinch
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-200817 (URN)978-91-7729-228-9 (ISBN)
Public defence
2017-02-09, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Supervisors
Note

QC 20170202

Available from: 2017-02-02 Created: 2017-02-02 Last updated: 2017-02-02Bibliographically approved

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