Integrated modelling of island growth, stabilization and mode locking: consequences for NTM control on ITER

Full suppression of neoclassical tearing modes (NTMs) using electron cyclotron current drive (ECCD) should be reached before mode locking (stop of rotation) makes suppression impossible. For an ITER scenario 2 plasma, the similar time scales for locking and island growth necessitate the combined modelling of the growth of the mode and its slow down due to wall induced drag. Using such a model, the maximum allowed latency between the seeding of the mode and the start of ECCD deposition and maximum deviation in the radial position are determined. The maximum allowed latency is determined for two limiting models for island growth; the polarization model with W-marg = 2 cm, representing the worst case, and the transport model with W-marg = 6 cm, representing the best case. NTMs with seed island widths up to 9.5 cm and 12 cm for the 2/1 and the 3/2 NTM, respectively, are suppressible. The maximum allowed latency is 1.05 s and 2.95 s for the 2/1 and 3/2 NTM, respectively, for the worst case model. Radial misalignment should not exceed 7-10 mm for the 2/1 NTM and 5-16 mm for the 3/2 NTM depending on the model for island growth. As long as the alignment suffices, it does not reduce the maximum allowed latency. Mode locking has serious implications for any real-time NTM control system on ITER that aims to suppress NTMs by ECCD.

[1]  Laura Urso,et al.  Modelling and experiments on NTM stabilisation at ASDEX Upgrade , 2009 .

[2]  Paul H. Rutherford,et al.  Nonlinear growth of the tearing mode , 1973 .

[3]  E. Lazzaro,et al.  Feedback control of rotating resistive modes , 1988 .

[4]  H. Zohm,et al.  Cross–machine benchmarking for ITER of neoclassical tearing mode stabilization by electron cyclotron current drive , 2006 .

[5]  J. Callen,et al.  Global energy confinement degradation due to macroscopic phenomena in tokamaks , 1990 .

[6]  H R Wilson,et al.  Finite Larmor-radius theory of magnetic island evolution. , 2001, Physical review letters.

[7]  R. Prater,et al.  Heating and current drive by electron cyclotron waves , 2003 .

[8]  J. Weiland,et al.  Toroidal and poloidal momentum transport studies in tokamaks , 2007 .

[9]  E. Westerhof.  Tearing mode stabilization by local current density perturbations , 1990 .

[10]  H. R. Wilson,et al.  Threshold for neoclassical magnetic islands in a low collision frequency tokamak , 1996 .

[11]  Gerhard Raupp,et al.  Development of a feedback system to control MHD instabilities in ASDEX Upgrade , 2007 .

[12]  F. Leuterer,et al.  Enhancement of the stabilization efficiency of a neoclassical magnetic island by modulated electron cyclotron current drive in the ASDEX upgrade tokamak. , 2007, Physical review letters.

[13]  E. Westerhof,et al.  The role of asymmetries in the growth and suppression of neoclassical tearing modes , 2011 .

[14]  G. V. Pereverzev,et al.  TORBEAM, a beam tracing code for electron-cyclotron waves in tokamak plasmas , 2001 .

[15]  L. L. Lao,et al.  Beta limits in long-pulse tokamak discharges , 1997 .

[16]  M Maarten Steinbuch,et al.  Real-time control of tearing modes using a line-of-sight electron cyclotron emission diagnostic , 2010 .

[17]  R. Harvey,et al.  ITER ECE: Plans and Challenges , 2009 .

[18]  Resistive instabilities in a tokamak , 1976 .

[19]  S. Günter,et al.  Locking of neoclassical tearing modes by error fields and its stabilization by RF current , 2008 .

[20]  D. A. Humphreys,et al.  Requirements for alignment of electron cyclotron current drive for neoclassical tearing mode stabilization in ITER , 2008 .

[21]  D. A. Humphreys,et al.  Active control for stabilization of neoclassical tearing modes , 2005 .

[22]  Nicola Bertelli,et al.  Requirements on localized current drive for the suppression of neoclassical tearing modes , 2011 .

[23]  R. Fitzpatrick,et al.  Helical temperature perturbations associated with tearing modes in tokamak plasmas , 1995 .

[24]  Giovanni Ramponi,et al.  Overview of the ITER EC upper launcher , 2008 .

[25]  R. J. La Haye,et al.  Neoclassical tearing modes and their controla) , 2005 .

[26]  Olivier Sauter,et al.  On the requirements to control neoclassical tearing modes in burning plasmas , 2010 .

[27]  Russian Federation,et al.  ITER Gonfinement and Stability Modelling , 2002 .

[28]  J. W. Oosterbeek,et al.  Magnetic Island Localization for NTM Control by ECE Viewed Along the Same Optical Path of the ECCD Beam , 2009 .

[29]  M Maarten Steinbuch,et al.  Systematic design and simulation of a tearing mode suppression feedback control system for the TEXTOR tokamak , 2012 .

[30]  A. Collazos,et al.  Progress on the ITER H&CD EC Upper Launcher Steering-Mirror Control System , 2010, IEEE Transactions on Plasma Science.

[31]  M. F. F. Nave,et al.  Mode locking in tokamaks , 1990 .

[32]  James D. Callen,et al.  On the stabilization of neoclassical magnetohydrodynamic tearing modes using localized current drive or heating , 1997 .

[33]  E. Westerhof,et al.  On the merits of heating and current drive for tearing mode stabilization , 2009 .

[34]  G. Ramponi,et al.  On the stabilization of neoclassical tearing modes by electron cyclotron waves , 1999 .

[35]  Olivier Sauter,et al.  Physics analysis of the ITER ECW system for optimized performance , 2008 .

[36]  A. Polevoi,et al.  Chapter 1: Overview and summary , 2007 .

[37]  H. Zohm,et al.  Neoclassical tearing mode control using electron cyclotron current drive and magnetic island evolution in JT-60U , 2009 .