MHD limits to tokamak operation and their control

A review of magnetohydrodynamic limits to tokamak operation in terms of current, density and pressure is given. Although the current and density limits in a tokamak usually lead to disruptive termination of the discharge, it is argued that these can be avoided by staying away from the respective limits. This is especially true since operation close to these limits is not really desirable, due to the decreased confinement at very high density and the high disruptivity at low q. On the other hand, the limit to plasma pressure set by neoclassical tearing modes (NTMs) and resistive wall modes (RWMs) is too low to guarantee economic operation of future fusion reactors. Therefore, active control of these two instabilities is now being studied. Noticeable progress has been made by NTM stabilization with ECCD. Avoidance of NTMs and RWMs by tailoring sawteeth and spinning the plasma, shows promising results. Also, experiments on direct RWM stabilization by active coils are showing their first encouraging results.

[1]  T. A. Gianakon,et al.  Dynamical modelling of tearing mode stabilization by RF current drive , 1999 .

[2]  D. A. Humphreys,et al.  CONTROL OF NEOCLASSICAL TEARING MODES IN DIII-D , 2001 .

[3]  S. Wolfe,et al.  Marfe: an edge plasma phenomenon , 1984 .

[4]  R. Neu,et al.  Tearing mode formation and radiative edge cooling prior to density limit disruptions in ASDEX Upgrade , 1997 .

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

[6]  R. Wolf,et al.  MHD phenomena in reversed shear discharges on ASDEX Upgrade , 2000 .

[7]  K. Kajiwara,et al.  Complete stabilization of a tearing mode in steady state high-βp H-mode discharges by the first harmonic electron cyclotron heating/current drive on JT-60U , 2000 .

[8]  McGuire,et al.  Observation of nonlinear neoclassical pressure-gradient-driven tearing modes in TFTR. , 1995, Physical review letters.

[9]  R. Gruber,et al.  MHD-limits to plasma confinement , 1984 .

[10]  Bondeson,et al.  Stabilization of external modes in tokamaks by resistive walls and plasma rotation. , 1994, Physical review letters.

[11]  F C Schuller,et al.  Disruptions in tokamaks , 1995 .

[12]  G. Giruzzi,et al.  Modeling of the stabilization of neoclassical tearing modes by localized radio frequency current drive , 2000 .

[13]  Julien Fuchs,et al.  MHD stability and disruption physics in ASDEX upgrade , 1995 .

[14]  K. F. Mast,et al.  Use of Impurity Pellets to Control Energy Dissipation During Disruption , 1996 .

[15]  G. Gantenbein,et al.  Neoclassical tearing modes and their stabilization by electron cyclotron current drive in ASDEX Upgrade , 2001 .

[16]  S. Wolfe,et al.  A new look at density limits in tokamaks , 1988 .

[17]  L. L. Lao,et al.  High beta tokamak operation in DIII-D limited at low density/collisionality by resistive tearing modes , 1997 .

[18]  O. Sauter,et al.  Control of neoclassical tearing modes by sawtooth control. , 2002, Physical review letters.

[19]  H. Zohm,et al.  ß scaling for the onset of neoclassical tearing modes at ASDEX Upgrade , 1998 .

[20]  A. Boozer.  Error field amplification and rotation damping in tokamak plasmas. , 2001, Physical review letters.

[21]  A. Bondeson,et al.  MHD modelling of density limit disruptions in tokamaks , 1991 .

[22]  Gunter,et al.  Complete suppression of neoclassical tearing modes with current drive at the electron-cyclotron-resonance frequency in ASDEX upgrade tokamak , 2000, Physical review letters.

[23]  D G Whyte,et al.  Mitigation of tokamak disruptions using high-pressure gas injection. , 2002, Physical review letters.

[24]  L. L. Lao,et al.  Sustained rotational stabilization of DIII-D plasmas above the no-wall beta limit , 2002 .

[25]  G. A. Navratil,et al.  Resistive wall stabilization of high-beta plasmas in DIII-D , 2002 .

[26]  T. Fujita,et al.  Resistive instabilities in reversed shear discharges and wall stabilization on JT-60U , 2002 .

[27]  W. Schneider,et al.  Thermal instabilities and poloidal asymmetries in the tokamak edge plasma , 1986 .

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