A dual, high speed, real-time visible light camera setup was installed on the TCV tokamak to reconstruct optically and in real-time the plasma boundary shape. Localized light emission from the plasma boundary in tangential view, broadband visible images results in clearly resolved boundary edge-features. These projected features are detected in real-time and transformed to the poloidal plane to obtain a measurement of the plasma boundary. Plasma boundary reconstructions of diverted plasma discharges are presented, showing agreement of within 1 cm compared with magnetic equilibrium reconstruction. The resulting real-time plasma shape measurement is applied in a feedback control loop for the plasma position, demonstrating effective stabilization and tracking of the plasma vertical position.

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U5 - 6dc862485b6b1045595d0c48e382998f ER - TY - JOUR T1 - Demonstration of sawtooth period locking with power modulation in TCV plasmas JF - Nuclear Fusion Y1 - 2012 A1 - Lauret, M. A1 - Felici, F. A1 - Witvoet, G. A1 - Goodman, T. P. A1 - Vandersteen, G. A1 - Sauter, O. A1 - M.R. de Baar AB -Corroborating evidence is presented that the sawtooth period can follow the modulation frequency of an externally applied high power electron cyclotron wave source. Precise, fast and robust open loop control of the sawtooth period with a continuously changing reference period has been achieved. This period locking is not associated with the crash, but with the phase evolution of the inter-crash dynamics. This opens new possibilities of open loop control for physics studies and maybe for reactor performance control.

VL - 52 SN - 0029-5515 IS - 6 U1 - FP U2 - TP U5 - 5c85f5f5a81dc7fd9a55b30ff009f820 ER - TY - JOUR T1 - Robust sawtooth period control based on adaptive online optimization JF - Nuclear Fusion Y1 - 2012 A1 - Bolder, J. J. A1 - Witvoet, G. A1 - M.R. de Baar A1 - van de Wouw, N. A1 - Haring, M. A. M. A1 - Westerhof, E. A1 - Doelman, N. J. A1 - Steinbuch, M. KW - CONFINEMENT KW - DISCHARGES KW - EXTREMUM SEEKING CONTROL KW - PERFORMANCE IMPROVEMENT KW - STABILITY KW - SYSTEMS AB - The systematic design of a robust adaptive control strategy for the sawtooth period using electron cyclotron current drive (ECCD) is presented. Recent developments in extremum seeking control (ESC) are employed to derive an optimized controller structure and offer practical tuning guidelines for its parameters. In this technique a cost function in terms of the desired sawtooth period is optimized online by changing the ECCD deposition location based on online estimations of the gradient of the cost function. The controller design does not require a detailed model of the sawtooth instability. Therefore, the proposed ESC is widely applicable to any sawtoothing plasma or plasma simulation and is inherently robust against uncertainties or plasma variations. Moreover, it can handle a broad class of disturbances. This is demonstrated by time-domain simulations, which show successful tracking of time-varying sawtooth period references throughout the whole operating space, even in the presence of variations in plasma parameters, disturbances and slow launcher mirror dynamics. Due to its simplicity and robustness the proposed ESC is a valuable sawtooth control candidate for any experimental tokamak plasma, and may even be applicable to other fusion-related control problems. VL - 52 SN - 0029-5515 N1 - ISI Document Delivery No.: 970BXTimes Cited: 1Cited Reference Count: 35SI U1 - FP U2 - TP U5 - d29f870374501e267cb8501b5c4e6202 ER - TY - JOUR T1 - Sawtooth period control strategies and designs for improved performance JF - Nuclear Fusion Y1 - 2012 A1 - Witvoet, G. A1 - Steinbuch, M. A1 - M.R. de Baar A1 - Doelman, N. J. A1 - Westerhof, E. KW - CONFINEMENT KW - CURRENT DRIVE KW - DISCHARGES KW - JET AB - The sawtooth instability is associated with the triggering of neo-classical tearing modes, core fuelling, alpha-confinement and the exhaust of thermal helium. Sawtooth control is therefore important for optimal reactor performance in ELMy H-modes. Control schemes for the sawtooth period have been published in the literature, but the systematic design of high-performance controllers (yielding accurate and fast convergent responses) has not been addressed. In this work, three control strategies for high-performance sawtooth control are presented using electron cyclotron current drive (ECCD). Both degrees of freedom of the ECCD actuator will be explored and combined with advanced controller designs. First, the ECCD deposition location is used as a control variable, for which a gain-scheduled feedback controller and static feedforward control is derived. Second, the use of the driven current as a control variable is explored, and a simple controller is designed based on the identified dynamics. In the third approach both control variables are joined in an overall controller design, which enables the combination of high-performance control of the sawtooth period and control of the gyrotron power. Time-domain simulations with a combined Kadomtsev-Porcelli sawtooth model show that each strategy obtains a better closed-loop performance than standard linear feedback techniques on merely the deposition location. VL - 52 SN - 0029-5515 N1 - ISI Document Delivery No.: 970BXTimes Cited: 1Cited Reference Count: 33SI U1 - FP U2 - TP U5 - dd62a63f707b66112f21d38c350af1c4 ER - TY - Generic T1 - Real-time period determination for relaxation oscillators using wavelets T2 - Proceedings of the American Control Conference 2012 Y1 - 2012 A1 - van Berkel, M. A1 - Witvoet, G. A1 - M.R. de Baar A1 - Nuij, Pwjm A1 - ter Morsche, H. G. A1 - Steinbuch, M. JF - Proceedings of the American Control Conference 2012 PB - IEEE CY - Montreal, Canada UR - http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=6297579 N1 - 2012/06/29 U1 - FP U2 - TP U5 - 479e45428fcb26726316e1ecdd2d3cad ER - TY - CONF T1 - A new mechanism for sawtooth period control T2 - EPJ Web of Conferences Y1 - 2012 A1 - Lauret, M. A1 - Felici, F. A1 - Witvoet, G. A1 - Goodman, T. A1 - Vandersteen, G. A1 - Westerhof, E. A1 - Steinbuch, M. A1 - Sauter, O. A1 - M.R. de Baar JF - EPJ Web of Conferences VL - 32 U1 - FP U2 - TP U5 - c2f90f8f4849b7908e59a4a75cf576e4 ER - TY - JOUR T1 - Numerical demonstration of injection locking of the sawtooth period by means of modulated EC current drive JF - Nuclear Fusion Y1 - 2011 A1 - Witvoet, G. A1 - Lauret, M. A1 - M.R. de Baar A1 - Westerhof, E. A1 - Steinbuch, M. KW - ASDEX UPGRADE KW - DISCHARGES KW - FREQUENCY CONTROL KW - JET KW - OSCILLATIONS KW - SAWTEETH KW - STABILIZATION KW - TOKAMAK AB - In this paper the sawtooth period behaviour under periodic forcing by electron cyclotron waves is investigated. The deposition location is kept constant while the gyrotron power is modulated with a certain period and duty cycle. Extensive simulations on a representative dynamic sawtooth model show that when this modulation is properly chosen, the sawtooth period quickly synchronizes to the same period and remains locked at this value. It is shown that the range of modulation periods and duty cycles over which sawtooth period locking occurs, depends on the deposition location, but is particularly large for depositions near the q = 1 surface. The simulation results reveal a novel approach to control the sawtooth period in open loop, based on injection locking, which is a well-known technique to control limit cycles of non-linear dynamic oscillators. The locking and convergence results are therefore used in a simple open-loop locking controller design, with which accurate sawtooth period tracking to any desired value is indeed demonstrated. Injection locking appears to let the sawtooth period converge to the modulation period quickly, partly because it does not suffer from slow EC mirror launcher dynamics. Moreover, simulations show that the method has a relatively large robustness against general uncertainties and disturbances. Hence, injection locking is expected to outperform conventional sawtooth control methods using a variable deposition location and constant gyrotron power. Finally, the recent result with sawtooth pacing is shown to be a special case of the general locking effect. VL - 51 SN - 0029-5515 IS - 10 U1 - FP U2 - TP U5 - a2c89923fff8f39d6584f847a1720ff4 ER - TY - JOUR T1 - Systematic design of a sawtooth period feedback controller using a Kadomtsev-Porcelli sawtooth model JF - Nuclear Fusion Y1 - 2011 A1 - Witvoet, G. A1 - M.R. de Baar A1 - Westerhof, E. A1 - Steinbuch, M. A1 - Doelman, N. J. KW - CONFINEMENT KW - CURRENT DRIVE KW - DISCHARGES KW - JET AB - A systematic methodology for structured design of feedback controllers for the sawtooth period is presented, based on dedicated identification of the sawtooth dynamics. Therefore, a combined Kadomtsev-Porcelli model of a sawtoothing plasma actuated by an electron cyclotron current drive system has been set-up. This is used to derive the linearized input-output relations (transfer functions) from the varying deposition location of the electron cyclotron waves (ECW) to the sawtooth period. These transfer functions are derived around a large collection of operating points. Assessment of these control-relevant transfer functions shows that a sawtooth period controller requires an integral (I) action to guarantee closed-loop stability with zero steady-state error. Additional proportional-integral (PI) action can be applied to further increase the closed-loop performance. The parameters of both the I and PII controllers have been optimized in terms of stability, performance and robustness. Moreover, the effect of the mechanical ECW launcher on the closed-loop performance is studied for realistic cases. It is shown that the launcher dynamics seriously affects the achievable closed-loop performance in present-day experiments. VL - 51 SN - 0029-5515 IS - 7 U1 - FP U2 - TP U5 - 4526a168564ccb214ff00f1e5d0f9326 ER - TY - JOUR T1 - Real-time wavelet detection of crashes in limit cycles of non-stationary fusion plasmas JF - Fusion Engineering and Design Y1 - 2011 A1 - van Berkel, M. A1 - Witvoet, G. A1 - M.R. de Baar A1 - Nuij, Pwjm A1 - ter Morschec, H. G. A1 - Steinbuch, M. KW - ASDEX UPGRADE KW - CONFINEMENT KW - EDGE KW - Edge-detection KW - Filter-bank KW - FLUCTUATIONS KW - FREQUENCY ANALYSIS KW - INTERMITTENCY KW - Localized Mode (ELM) KW - MODES KW - Real-time KW - REFLECTOMETRY KW - Sawtooth KW - SAWTOOTH PERIOD KW - SIGNALS KW - TOKAMAK KW - Wavelet AB - The high performance mode (H-mode) is one of the baseline plasma scenarios for the experimental fusion reactor ITER. This scenario features a periodic crash-like reorganization of the plasma pressure and the magnetic flux in the plasma core and plasma periphery. The core instability is often referred to as the sawtooth instability while the instability at the edge of the plasma is referred to as ELM. In this paper we present an algorithm for optimized (low latency, robust and high fidelity) real-time sensing of the crashes. The algorithm is based on time-scale wavelet theory and edge-detection. It is argued that detection of crashes has considerably less delay than the other methods. The realized accuracy of the detection algorithm is well below the uncertainty of the crash period for most crashes. Multiresolution analysis enables distinction between different sizes of sawtooth crashes due to the different sizes of wavelets (scales), resulting in an algorithm, which is robust and accurate. Although strictly speaking, the crash detection method is demonstrated for sawteeth measured with ECE only, it can be applied to any periodic crash, measured with any temporally resolved data. Note that the possibility of differentiating between crash like events of different nature depends on their individual time-scales and used measurement setup. (C) 2011 Elsevier B.V. All rights reserved. VL - 86 SN - 0920-3796 IS - 12 U1 - FP U2 - TP U5 - 9b690e4fb3033a368ba88f376ceeaff9 ER - TY - THES T1 - Feedback control and injection locking of the sawtooth oscillation in fusion plasmas Y1 - 2011 A1 - Witvoet, G. PB - Eindhoven University of Technology CY - Eindhoven, Netherlands VL - PhD SN - 9789038630304 UR - http://repository.tue.nl/719868 U1 - FP U2 - TP U5 - dba100eeaf862e7f5256d8d2177f0980 ER -