Higher order and asymmetry effects on saturation of magnetic islands

Abstract

Higher order asymptotic matching procedure is developed to derive the nonlinear equation for saturated magnetic island with the symmetric profile of the equilibrium current. The theory extends the previous results to include higher order effects such as nonlinear modification of the equilibrium current and asymmetry in the boundary conditions in the outer region. It is shown that due to a finite width of the nonlinear region, the magnitude of the magnetic flux at the rational surface is different from the asymptotic value found from the outer solution, resulting in the suppression of the island growth for higher values of D 0 parameter. V C 2013 AIP Publishing LLC.

Nonlinear dynamics of turbulence driven magnetic islands. I. Theoretical aspects

“…In this limit, 34 the flux function and its first derivatives can be approximated by w ¼ x 2 = 2a ð Þ þ w 1 t ð Þ cos k 1 y ð Þ. This is a valid hypothesis in our simulations.…”

Physics conditions for robust control of tearing modes in a rotating tokamak plasma

“…Since these effects are independent of the island phase stringent requirements on phase tracking appear less motivated. Furthermore, finite magnetic islands are actually asymmetric with respect to the rational q surface, and the asymmetry is equivalent to a current perturbation which can either have stabilizing effects [10,11] or destabilizing,when associated with thermal losses.A current perturbation due to variations of the local (Spitzer) resistivity, consequent to radiative cooling of the island interior, has been shown to be destabilizing [11] in combination with asymmetry. Replacing the radiative energy losses by EC heating within a band encompassing the reconnection layer seems therefore a reasonable way to counteract these instabilitiies, also by freezing the reconnection process and it combines favorably with the effect of axisymmetric J CD [11,12], both being phase independent.…”

“…Recently the physics understanding has been enriched by new findings on nonuniformity effects on finite magnetic islands, of pressure and temperature associated with energy input [ECRH] and loss (e.g. by radiation) [9][10][11][12][13][14][15], rotation and [16][17][18][19][20][21], as well as by findings on small scale topological effects of the reconnection [22,23].…”

Two-fluid model of rf current condensation in magnetic islands

“…These unstable islands, known as neoclassical tearing modes (NTMs), are a major cause of disruptions 1,2 and set a principal performance limit in tokamaks. 3,4 Stabilization via current drive by rf waves 5 has long been recognized as the leading solution, and has been the subject of much theoretical [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and experimental work. [21][22][23][24][25][26][27][28][29] Only recently, however, has the presence of the island and its effect on rf deposition been accounted for.…”