The collisionality dependence of tokamak -limits

An interpretation of the collisionality dependence of , which has been observed in many tokamaks, is presented in terms of neoclassical tearing mode theory. Magnetic islands are predicted to be driven to large saturated widths by the perturbed bootstrap current, provided their width exceeds a certain threshold value. This width is of the order of the ion banana width for low collisionality plasmas, but is substantially larger in collisional plasmas (though still within the banana regime); below this threshold value the stabilizing ion polarization current causes an island to decay. Assuming that `background' MHD events such as ELMs or sawteeth can provide an initial `seed' magnetic island of width (through toroidal coupling, for example) one can develop a model equation which retains the essential properties of neoclassical tearing mode theory and is able to reproduce many of the experimental trends associated with the reduced -limit.

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

[2]  J. Connor,et al.  Theory of isolated, small-scale magnetic islands in a high temperature tokamak plasma , 1995 .

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

[4]  G. Janeschitz,et al.  Status of ITER , 1995 .

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

[6]  E. Lazzaro,et al.  Rotating nonlinear magnetic islands in a tokamak plasma , 1995 .

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

[8]  Crpp.  Papers presented at the 22nd EPS Conference on Controlled Fusion and Plasma Physics, Bournemouth, Great Britain, July 3 - 7, 1995 , 1995 .

[9]  X. Garbet,et al.  Kinetic theory of magnetic island stability in tokamaks , 1994 .

[10]  David P. Schissel,et al.  ITER H mode confinement database update , 1994 .

[11]  P. Haynes,et al.  Field error instabilities in JET , 1994 .

[12]  Andrei Smolyakov,et al.  Nonlinear evolution of tearing modes in inhomogeneous plasmas , 1992 .

[13]  P. Rebut,et al.  Magnetic turbulence self-sustainment by finite Larmor radius effect , 1991 .

[14]  H. Zohm,et al.  MHD mode structure and propagation in the ASDEX tokamak , 1991 .

[15]  R. White,et al.  Nonlinear self‐sustainment of magnetic islands , 1989 .

[16]  R. Carrera,et al.  Island bootstrap current modification of the nonlinear dynamics of the tearing mode , 1986 .

[17]  A. Samain.  Diamagnetic destabilization of magnetic islands in the non-linear regime , 1984 .

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