APS DPP 2006 October 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 th Annual Meeting of the Division of Plasma Physics American Physical Society 30 October – 3 November 2006 Philadelphia, Pennsylvania

APS DPP 2006 October 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].

APS DPP 2006 October 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

APS DPP 2006 October References 1.ISLAND module from NTCC module library: Background, finding saturated magnetic island widths, ISLAND: –G. Bateman and R. Morris, Phys. Fluids 29 (3) (1986) –F. Halpern, Physics of Plasmas 13 (2006) Similar work expressing current drive in Hamada coordinates: –Giruzzi et al., Nuclear Fusion 39 (1999) –C. Hegna and J. Callen, Physics of Plasmas 4 (1997) Computing elliptic integrals: 5.Dependence of NTM Stabilization on Location of Current Drive Relative to Island –J. Woodby, APS 2006 Philadelphia, Poster Session JP

APS DPP 2006 October 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 NTM=neoclassical tearing mode, magnetic “islands” result from tearing and reconnection of ideally nested magnetic flux surfaces Starting from force-balance equations Background

APS DPP 2006 October Current drive model Start by assuming that the current drive has the following form

APS DPP 2006 October Current applied in u -coordinates gets spread over magnetic flux surfaces Current drive model Please see Ref. #5 for detailed deviation J0J0 u α

APS DPP 2006 October Averaged driving current distribution: Taking the derivative where Current drive model K is the complete elliptic integral of the first kind; E is the complete elliptic integral of the second kind.

APS DPP 2006 October The superposed current density derivative Current drive model A current drive is determined by three parameters –location ( a, in u coordinate) –width ( b, in u coordinate) –strength ( J 0 ) J EC is positive. A FORTRAN module is developed for ISLAND to handle current drive

APS DPP 2006 October 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.

APS DPP 2006 October 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)

APS DPP 2006 October 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)

APS DPP 2006 October Dependence of island width on the 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)

APS DPP 2006 October EXTREMUM SEEKING – HOW DOES IT WORK? function to be minimized minimum of the static map second derivative (if positive J(  ) has a minimum) estimate of  adaptation gain amplitude of the probing signal frequency of probing signal cutoff frequency of high-pass filter unknown parameter that minimizes Jmodulation/demodulation Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October EXTREMUM SEEKING  Any C 2 function J(  ) can be approximated locally in this way. The assumption is made without loss of generality. If J” 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 α cos (  k ) helps to get a measure of gradient information of the static map J(  ). Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October EXTREMUM SEEKING Let Thus Which gives Estimation Error Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October EXTREMUM SEEKING Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October EXTREMUM SEEKING Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October EXTREMUM SEEKING Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October EXTREMUM SEEKING Stable System Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October EXTREMUM SEEKING In our case θ is the position (a) J is the half island width Iteration relations Plant Low-Pass Filter High-Pass Filter

APS DPP 2006 October 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

APS DPP 2006 October Extremum seeking results Cost function J (half island width) progression ( b=1.5, J0=1.0 )

APS DPP 2006 October 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

APS DPP 2006 October 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