Magnetic Island Formation Due to Error Field in the JFT-2 Tokamak

Abstract

cathodoluminescence (STRCL) spectroscopy is implemented to assess the local carrier dynamics in a 70-nm-thick, very low threading dislocation (TD) density, pseudomorphic m-plane In 0:05 Ga 0:95 N epilayer grown on a freestanding GaN substrate by metalorganic vapor phase epitaxy. Although TDs or stacking faults are absent, sub-micrometer-wide zonary patterns parallel to the c-axis and 2-m-long-axis figureof-8 patterns parallel to the a-axis are clearly visualized in the monochromatic cathodoluminescence intensity images. Because the STRCL measurement reveals very little spatial variation of low-temperature radiative lifetime, the considerable peak energy variation is interpreted to originate from nonidentical In-incorporation efficiency for the growing surfaces exhibiting various miscut angles. The figure-of-8 patterns are ascribed to originate from the anisotropic, severe m-plane tilt mosaic along the a-axis of the GaN substrate, and the zonary patterns may originate from the m-plane tilt mosaic along the c-axis. #

Theory and observations of magnetic islands

“…A goal shared by most magnetic confinement concepts is to realize a configuration consisting of simply nested magnetic surfaces enveloping a closed field line, the magnetic axis [1,2]. In practice, small departures from this ideal configuration cause the flux surfaces to break into chains of magnetic islands, where each island is a tube of flux with its own private magnetic axis [3][4][5]. The primary significance of the islands is that heat can flow rapidly across them by following the field lines [6][7][8].…”

Magnetic island widths due to field perturbations in toroidal stellarators

“…Sources for these perturbation (error) fields include coil misalignments during installation, imperfections in coil windings, fields from bus works and leads, and the presence of ferromagnetic materials in the vicinity [1,2]. Recently, the mechanism of magnetic surface breakup and island formation by magnetic field irregularities has received considerable attention, and significant advances have been reported by many authors [3][4][5][6][7][8][9][10][11][12][13].…”

“…( 2) is basically accurate in the approximation of an infiniteaspect-ratio toroidal magnetic field configuration, in which magnetic flux surfaces are assumed to be concentric circles and hence the toroidal coupling of the Fourier components arising from non-circularity and shift of the flux surfaces is entirely neglected. In one way or another, this type of assumption is usually made in carrying out analytical studies of the effect of field perturbations on the magnetic flux surfaces in toroidal stellarator configurations [3][4][5][6][7][8][9][10][11][12]. If, however, a perturbation B mn given by Eq.…”

Magnetic field line mappings for a tokamak with ergodic limiters

“…The width of a magnetic island obtained in the SPM case can be estimated by the theoretical method developed by Matsuda and Yoshikawa to deal with general error fields [32]. We assume a cylindrical geometry (E = 0), but allow the existence of an arbitrary number p of EML rings.…”

“…Furthermore, one can evaluate the island width &,,, (in tokamak coordinates) by integration of equation (33) over a complete turn of the phase N. Using (37) we obtain [32] In order to apply the above equation, it is necessary to know the appropriate Fourier coefficient of the radial error field from its double trigonometric series If the rings are assembled with an angular displacement of 2n/p around the torus, B)." is symmetric by the change q--+ -(%, so that there is only one non-vanishing Fourier coefficient in (40),…”