Interaction of bootstrap-current-driven magnetic islands

Abstract

The formation and interaction of fluctuating neoclassical pressure gradient driven magnetic islands is examined. The interaction of magnetic islands produces a stochastic region around the separatrices of the islands. This interaction causes the island pressure profile to be broadened, reducing the island bootstrap current and drive for the magnetic island. A model is presented that describes the magnetic topology as a bath of interacting magnetic islands with low to medium poloidal mode number (m = 3-30). The islands grow by the bootstrap current effect and damp due to the flattening of the pressure profile near the island separatrix caused by the interaction of the magnetic islands. The effect of this sporadic growth and decay of the islands ("magnetic bubbling") is not normally addressed in theories of plasma transport due to magnetic fluctuations. The nature of the transport differs from statistical approaches to magnetic turbulence since the radial step size "of the plasma transport is now given by the characteristic island width. This model suggests that tokamak experiments have relatively short-lived, coherent, long wavelength magneticoscillationspresentin the steeppressure-gradient regionsof theplasma.

On the stabilization of neoclassical magnetohydrodynamic tearing modes using localized current drive or heating

“…[1][2][3][4] In particular, the self-consistent effects of perturbed bootstrap currents allow pressure-induced magnetic island formation in the positive shear regions of tokamaks. This instability process can, in principle, give rise to magnetic islands at every low-order rational surface despite the fact that resistive MHD theories predict stability.…”

Pressure profiles, resonant Pfirsch-Schlueter currents, thermal instabilities and magnetic island formation

“…As pointed out in ref. (13), island growth, interaction and decay rnay play an important role in the transpotX properties of tokamak plasmas.…”

“…[11][12][13] Clearly, it is desirable to understand how the island formation affects the plasma pressure profile so that a self-consistent construction of the plasma equilibrium including the effects on the plasma current and magnetic fields can be understood.…”

Stabilization of Neoclassical Tearing Modes by rf Current

“…The neoclassical tearing mode (NTM) is expected to form a magnetic island structure which locally flattens the plasma pressure, hence losing the bootstrap current [1] [2] [3] [4]. Low mode number NTMs, especially the / 3 / 2 m n = (m and n are the poloidal and toroidal mode numbers)and 2/1 modes, are observed to cause the confinement degradation and/or disruptions, limiting the achievable plasma β value well below the predictions of ideal magnetohydrodynamic (MHD) calculations for positive magnetic shear [4]- [12].…”