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Adaptive Extremum Seeking Control of ECCD for NTM Stabilization

Adaptive Extremum Seeking Control of ECCD for NTM Stabilization. L. Luo 1 , J. Woodby 1 , E. Schuster 1 F. D. Halpern 2 , G. Bateman 2 , A. H. Kritz 2 1 Department of Mechanical Engineering 2 Department of Physics Lehigh University, Bethlehem, PA 18015

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Adaptive Extremum Seeking Control of ECCD for NTM Stabilization

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  1. Adaptive Extremum Seeking Control of ECCDfor NTM Stabilization L. Luo1, J. Woodby1, E. Schuster1F. D. Halpern2, G. Bateman2, A. H. Kritz2 1Department of Mechanical Engineering 2Department of Physics Lehigh University, Bethlehem, PA 18015 48th Annual Meeting of the Division of Plasma Physics American Physical Society 30 October – 3 November 2006 Philadelphia, Pennsylvania

  2. Abstract Neoclassical Tearing Modes (NTMs) drive magnetic islands to grow to their saturated widths, at which they can persist stably in the plasma. The presence of magnetic islands leads to a local flattening of the current density and pressure profiles, which degrade plasma confinement. Since the bootstrap current density is proportional to the pressure gradient, this current is nearly absent within each island. One common method of stabilizing NTMs and therefore shrinking the island widths involves replacing the lost current via Electron Cyclotron Current Drive (ECCD). In order for ECCD to be successful at shrinking the island widths, the current must be driven at the flux surfaces that contain the islands. Moreover, in order to shrink each island with minimal ECCD power, the current must be deposited as close to the center of the island as possible. The difficulty lies in determining the locations of both the island flux surface and the ECCD deposition in real time. The Extremum Seeking feedback method is considered in this work for non-model based optimization of ECCD suppression of NTMs in tokamaks. ECCD steering change will be considered as mechanisms to maximize in real-time the alignment between the island flux surface and the current deposition location, and thus to minimize the ECCD power required for NTM stabilization. Theoretical analysis is done by Woodby [5].

  3. Objectives • Use BALDUR and ISLAND code to simulate NTM • Find a approximation model of the current drive. The shrinking effect is determined by the position, width and strength of the current drive. • Modify BALDUR and ISLAND to incorporate the current drive model • Introduce a feedback control on the current drive using extremum seeking scheme • Numerical simulations

  4. References • ISLAND module from NTCC module library: http://w3.pppl.gov/ntcc • Background, finding saturated magnetic island widths, ISLAND: • G. Bateman and R. Morris, Phys. Fluids 29 (3) (1986) • F. Halpern, Physics of Plasmas 13(2006)062510 • Similar work expressing current drive in Hamada coordinates: • Giruzzi et al., Nuclear Fusion 39 (1999) 107-125 • C. Hegna and J. Callen, Physics of Plasmas 4 (1997) 2940 • Computing elliptic integrals: www.netlib.org • Dependence of NTM Stabilization on Location of Current Drive Relative to Island • J. Woodby, APS 2006 Philadelphia, Poster Session JP1.00143

  5. Background • NTM=neoclassical tearing mode, magnetic “islands” result from tearing and reconnection of ideally nested magnetic flux surfaces • Starting from force-balance equations • Using Hamada-like coordinate system (V is any quantity which is constant over a flux surface, such as volume) • Get set of coupled ODEs which describe change in background current and pressure profiles due to presence of island • Implemented in ISLAND module, implemented in BALDUR, which computes saturated magnetic island widths

  6. Current drive model Start by assuming that the current drive has the following form

  7. Current applied in u-coordinates gets spread over magnetic flux surfaces Current drive model J0 α u Please see Ref. #5 for detailed deviation

  8. Averaged driving current distribution: Current drive model • Taking the derivative where K is the complete elliptic integral of the first kind; E is the complete elliptic integral of the second kind.

  9. Current drive model The superposed current density derivative • A current drive is determined by three parameters • location (a, in u coordinate) • width (b, in u coordinate) • strength (J0) • JEC is positive. • A FORTRAN module is developed for ISLAND to handle current drive

  10. Current density profile without current drive DIII-D 2/1 island test (no current drive) • without any island (left) • with island (right) x is the plasma minor radius (x=0 at the center of the plasma and x=1 at the edge of the plasma); j is the current density. Variables are non-dimensional.

  11. Current density profile with current drive The effect of current drive on the current density profile • b=0.8, J0=1, a=0 (left) • b=0.8, J0=1, a=2 (right)

  12. Dependence of island width on the location of the current drive Island half width as a function of location (a) • Narrow drive (b=0.8, left) • Wide drive (b=1.5, right)

  13. Dependence of island width onthe current drive strength Island half width as a function of current drive strength (J0) • Narrow drive (a=0, b=0.8, left) • Wide drive (a=0, b=1.5, right)

  14. Plant Low-Pass Filter High-Pass Filter function to be minimized adaptation gain amplitude of the probing signal minimum of the static map second derivative (if positive J() has a minimum) frequency of probing signal cutoff frequency of high-pass filter estimate of unknown parameter that minimizes J modulation/demodulation EXTREMUM SEEKING – HOW DOES IT WORK?

  15. Plant Low-Pass Filter High-Pass Filter EXTREMUM SEEKING • Any C2 function J() can be approximated locally in this way. The assumption is made without loss of generality. IfJ”<0 , we just replace ( > 0) in the figure with -. The purpose of the algorithm is to make -*as small as possible, so that the output J() is driven to its minimum J*. • The perturbation signal αcos(k) helps to get a measure of gradient information of the static map J().

  16. Plant Low-Pass Filter High-Pass Filter EXTREMUM SEEKING Let Estimation Error Thus Which gives

  17. Plant Low-Pass Filter High-Pass Filter EXTREMUM SEEKING

  18. Plant Low-Pass Filter High-Pass Filter EXTREMUM SEEKING

  19. Plant Low-Pass Filter High-Pass Filter EXTREMUM SEEKING

  20. Plant Low-Pass Filter High-Pass Filter EXTREMUM SEEKING Stable System

  21. Plant Low-Pass Filter High-Pass Filter EXTREMUM SEEKING Iteration relations In our case • θ is the position (a) • J is the half island width

  22. Extremum seeking results Position (a) progression (b=1.5, J0=1.0) • The position of current drive eventually converges at the center of the island • Oscillation is caused by the probing signal

  23. Extremum seeking results Cost function J (half island width) progression (b=1.5, J0=1.0)

  24. Conclusion • The island width is dependent on the location, width and strength of the proposed current drive • The modified ISLAND module gives estimation of island width and current density profile for different width and strength • Extremum seeking appears an effective method to steering the current drive and to maximize the island shrinking

  25. Future Research • A more accurate current drive model • Implementation of off-center current drive model in ISLAND/BALDUR • Extremum seeking feedback stabilization in time-dependent simulations • Code optimization for better performance

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