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Nonaxisymmetric experimental modal analysis and control of resistive wall MHD in RFPs: System identification and feedback control for the reversed-field pinch
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The reversed-field pinch (RFP) is a device for magnetic confinement of fusion plasmas. The main objective of fusion plasma research is to realise cost-effective thermonuclear fusion power plants. The RFP is highly unstable as can be explained by the theory of magnetohydrodynamics (MHD). Feed-back control technology appears to enable a robustly stable RFP operation.  Experimental control and identification of nonaxisymmetric multimode MHD is pursued in this thesis. It is shown that nonparametric multivariate identification methods can be utilised to estimate MHD spectral characteristics from plant-friendly closed-loop operational input-output data. It is also shown that accurate tracking of the radial magnetic field boundary condition is experimentally possible in the RFP. These results appear generically useful as tools in both control and physics research in magnetic confinement fusion.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , ix, 101 p.
Series
Trita-EES, 2012:20
Keyword [en]
Magnetic confinement fusion, Reversed-field pinch, System identification, Magnetohydrodynamics, Modal analysis, Automatic control, Resistive wall modes
National Category
Fusion, Plasma and Space Physics Control Engineering Signal Processing
Identifiers
URN: urn:nbn:se:kth:diva-94096ISBN: 978-91-7501-359-6 (print)OAI: oai:DiVA.org:kth-94096DiVA: diva2:525301
Public defence
2012-06-01, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20120508

Available from: 2012-05-08 Created: 2012-05-07 Last updated: 2013-12-04Bibliographically approved
List of papers
1. Controlled magnetohydrodynamic mode sustainment in the reversed-field pinch: Theory, design and experiments
Open this publication in new window or tab >>Controlled magnetohydrodynamic mode sustainment in the reversed-field pinch: Theory, design and experiments
2009 (English)In: Fusion engineering and design, ISSN 0920-3796, Vol. 84, no 7-11, 1455-1459 p.Article in journal (Refereed) Published
Abstract [en]

A novel control system design for magnetohydrodynamic (MHD) resistive-wall mode (RWM) stabilization is developed from the viewpoint of process control. The engineering approach assumed consists of system identification, selection of feedback interconnections, and subsequently, associated feedback gain tuning. A design for general output tracking is devised, implemented and experimentally verified to be capable of sustaining MHD modes in the reversed-field pinch (RFP) machine EXTRAP-T2R. In principle, by active feedback. the plasma column boundary is forced to 'user-specified' helicities of prescribed amplitudes and phases. Experimental success is mainly attributed to careful identification of local magnetic field diffusion time-constants, and individual actuator channel peripheral dynamics. Addition of functionality and key features of this new MHD feedback system software might provide a versatile tool for experimental plasma dynamics and innovative MHD stability research.

Keyword
Fusion plasma physics, Magnetohydrodynamics, Resistive-wall modes, Automatic control, System identification, Process control, INTELLIGENT SHELL, FEEDBACK, STABILIZATION
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-12891 (URN)10.1016/j.fusengdes.2008.11.052 (DOI)000268012700078 ()2-s2.0-67349189891 (Scopus ID)
Note
QC 20100518Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2012-05-08Bibliographically approved
2. Vector dither experiment design and direct parametric identification of reversed-field pinch normal modes
Open this publication in new window or tab >>Vector dither experiment design and direct parametric identification of reversed-field pinch normal modes
Show others...
2009 (English)In: Proceedings of the IEEE Conference on Decision and Control, 2009, 1348-1353 p.Conference paper, Published paper (Refereed)
Abstract [en]

Magnetic confinement fusion (MCF) research ambitiously endeavours to develop a major future energy source. MCF power plant designs, typically some variation on the tokamak, unfortunately suffer from magnetohydrodynamic (MHD) instabilities. One unstable mode is known as the resistive-wall mode (RWM) which is a macroscopically global type of perturbation that can degrade or even terminate the plasma in the reactor if not stabilized. In this work the topic of RWMs is studied for the reversed-field pinch (RFP), another toroidal MCF concept, similar to the tokamak. The problem of identifying RWM dynamics during closed-loop operation is tackled by letting physics-based parametric modeling join forces with convex programming experiment design. An established MHD normal modes description is assessed for the RFP by synthesizing a multivariable dither signal where spatial fourier modes are spectrally shaped, with regard to real experiment constraints, to yield minimum variance parameter estimates in the prediction-error framework. The dithering is applied to the real RFP plant EXTRAP-T2R, and experimental MHD spectra are obtained by an automated procedure.

Keyword
Automated procedures, Closed-loop operation, Convex programming, Dither signals, Energy source, Experiment design, Fourier modes
National Category
Control Engineering
Research subject
SRA - ICT
Identifiers
urn:nbn:se:kth:diva-12889 (URN)10.1109/CDC.2009.5400016 (DOI)000336893601138 ()2-s2.0-77950843926 (Scopus ID)978-142443871-6 (ISBN)
Conference
48th IEEE Conference on Decision and Control held jointly with 2009 28th Chinese Control Conference, CDC/CCC 2009, Shanghai, 15-18 December, 2009
Funder
Swedish Research Council, 621-2005-4345
Note

QC 20100518

Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2015-06-10Bibliographically approved
3. Predictor-based multivariable closed-loop system identification of the EXTRAP T2R reversed field pinch external plasma response
Open this publication in new window or tab >>Predictor-based multivariable closed-loop system identification of the EXTRAP T2R reversed field pinch external plasma response
Show others...
2011 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 53, no 8, 084003- p.Article in journal (Refereed) Published
Abstract [en]

The usage of computationally feasible overparametrized and nonregularized system identification signal processing methods is assessed for automated determination of the full reversed-field pinch external plasma response spectrum for the experiment EXTRAP T2R. No assumptions on the geometry of eigenmodes are imposed. The attempted approach consists of high-order autoregressive exogenous estimation followed by Markov block coefficient construction and Hankel matrix singular value decomposition. It is seen that the obtained 'black-box' state-space models indeed can be compared with the commonplace ideal magnetohydrodynamics (MHD) resistive thin-shell model in cylindrical geometry. It is possible to directly map the most unstable autodetected empirical system pole to the corresponding theoretical resistive shell MHD eigenmode.

Keyword
RESISTIVE-WALL MODES; SUBSPACE IDENTIFICATION; INTELLIGENT SHELL; TOKAMAK
National Category
Fusion, Plasma and Space Physics Control Engineering
Identifiers
urn:nbn:se:kth:diva-35113 (URN)10.1088/0741-3335/53/8/084003 (DOI)000291207300007 ()2-s2.0-79961047060 (Scopus ID)
Note
QC 20110630Available from: 2011-06-30 Created: 2011-06-20 Last updated: 2017-12-11Bibliographically approved
4. Cascade and multibatch subspace system identification for multivariate vacuum-plasma response characterisation
Open this publication in new window or tab >>Cascade and multibatch subspace system identification for multivariate vacuum-plasma response characterisation
Show others...
2011 (English)In: Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC11), 2011, 2614-2619 p.Conference paper, Published paper (Refereed)
Abstract [en]

A particular cascade structure system identification problem is formulated for the purpose of characterizing the vacuum-plasma response for a magnetic confinement fusion experiment. A predictor-form closed-loop subspace system identification approach is advocated due to (i) plant instability (ii) sizes of input-output vectors and (iii) inherent multivariate eigenmodes of the physical system. Since experiment data come in relatively short batches, specialised means for data merging for subspace identification are developed. A batchwise deletegroup jackknife procedure is utilised to estimate the standard error of the estimate of the dominant unstable empirical plasma response eigenvalue.

 

National Category
Control Engineering
Research subject
SRA - ICT
Identifiers
urn:nbn:se:kth:diva-72596 (URN)000303506203036 ()2-s2.0-84860650429 (Scopus ID)
Conference
50th IEEE Conference on Decision and Control and European Control Conference
Note

QC 20120508

Available from: 2012-02-01 Created: 2012-01-31 Last updated: 2013-04-29Bibliographically approved
5. Measurements of the vacuum-plasma response in EXTRAP T2R using generic closed-loop subspace system identification
Open this publication in new window or tab >>Measurements of the vacuum-plasma response in EXTRAP T2R using generic closed-loop subspace system identification
2012 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 87, no 12, 1926-1929 p.Article in journal (Refereed) Published
Abstract [en]

A multibatch formulation of a multi-input multi-output closed-loop subspace system identification method is employed for the purpose of obtaining control-relevant models of the vacuum-plasma response in the magnetic confinement fusion experiment EXTRAP T2R. The accuracy of the estimate of the plant dynamics is estimated by computing bootstrap replication statistics of the dataset. It is seen that the thus identified models exhibit both predictive capabilities and physical spectral properties.

Keyword
Computational statistics, Discrete-time linear dynamical systems, Magnetic confinement fusion, Plasma control, Plasma response, Signal processing, Subspace system identification, Supervised learning
National Category
Control Engineering Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-94094 (URN)10.1016/j.fusengdes.2012.04.001 (DOI)000314138900010 ()2-s2.0-84870751694 (Scopus ID)
Conference
8th IAEA Technical Meeting on Control, Data Acquisition, and Remote Participation for Fusion Research, San Francisco, CA, JUN 20-24, 2011
Note

QC 20130114

Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved
6. A first attempt at few coils and low-coverage resistive wall mode stabilization of EXTRAP T2R
Open this publication in new window or tab >>A first attempt at few coils and low-coverage resistive wall mode stabilization of EXTRAP T2R
2012 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 54, no 9, 094005- p.Article in journal (Refereed) Published
Abstract [en]

The reversed-field pinch features resistive-shell-type instabilities at any (vanishing and finite) plasma pressure. An attempt to stabilize the full spectrum of these modes using both (i) incomplete coverage and (ii) few coils is presented. Two empirically derived model-based control algorithms are compared with a baseline guaranteed suboptimal intelligent-shell-type (IS) feedback. Experimental stabilization could not be achieved for the coil array subset sizes considered by this first study. But the model-based controllers appear to significantly outperform the decentralized IS method.

Keyword
Coil arrays, Full spectrum, Model-based control, Model-based controller, Plasma pressures, Resistive wall modes, Reversed-field pinch, Algorithms, Plasma diagnostics, Stabilization
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-103670 (URN)10.1088/0741-3335/54/9/094005 (DOI)000308046300007 ()2-s2.0-84865176374 (Scopus ID)
Note

QC 20121017

Available from: 2012-10-17 Created: 2012-10-17 Last updated: 2017-12-07Bibliographically approved

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