Integrated simulations of saturated neoclassical tearing modes in DIII-D, Joint European Torus, and ITER plasmas

Abstract

tokamak discharges in order to investigate the adverse effects of multiple saturated magnetic islands driven by neoclassical tearing modes ͑NTMs͒. Simulations are carried out with a predictive model for the temperature and density pedestal at the edge of the high confinement mode ͑H-mode͒ plasma and with core transport described using the Multi-Mode model. The ISLAND module, which is used to compute magnetic island widths, includes the effects of an arbitrary aspect ratio and plasma cross sectional shape, the effect of the neoclassical bootstrap current, and the effect of the distortion in the shape of each magnetic island caused by the radial variation of the perturbed magnetic field. Radial transport is enhanced across the width of each magnetic island within the BALDUR integrated modeling simulations in order to produce a self-consistent local flattening of the plasma profiles. It is found that the main consequence of the NTM magnetic islands is a decrease in the central plasma temperature and total energy. For the DIII-D and JET discharges, it is found that inclusion of the NTMs typically results in a decrease in total energy of the order of 15%. In simulations of ITER, it is found that the saturated magnetic island widths normalized by the plasma minor radius, for the lowest order individual tearing modes, are approximately 24% for the 2 / 1 mode and 12% for the 3 / 2 mode. As a result, the ratio of ITER fusion power to heating power ͑fusion Q͒ is reduced from Q = 10.6 in simulations with no NTM islands to Q = 2.6 in simulations with fully saturated NTM islands.

Extended magneto-hydro-dynamic model for neoclassical tearing mode computations

“…Magnetic islands can grow in the plasma of magnetic confinement devices and cause a degradation of the energy confinement time Sauter et al (1997) that could impact the fusion gain of a fusion reactor Halpern et al (2006). As pressure is increased, their linear stability is improved Glasser et al (1975); Lütjens et al (2001), but a metastable branch emerges due to the nonlinear drive provided by neoclassical friction forces Carrera et al (1986).…”

Microtearing instabilities and electron thermal transport in low and high collisionality NSTX discharges

“…In earlier studies, it was found that the MM transport model version, which did not have the effect of microtearing modes, provided an appropriate description of the electron temperature profiles in high-collisionality standard tokamak discharges. 40,41 When the MM model, which involves transport associated with microtearing modes, is installed in the TRANSP code and is used in the study of electron thermal transport in NSTX discharges, it is found, as shown in Fig. 14(a), that compliance with the experimental electron temperature profile is substantially improved in the high collision NSTX discharge 129016.…”

Control-oriented modelling for neoclassical tearing mode stabilization via minimum-seeking techniques

“…j cd = j cd0 j cd (ζ, ψ, w/w cd , R/w cd ) and j cd0 is the peak value. The contour integral denotes the region −π < ζ < π for ψ > 0 and the interval where ψ + cos 2…”

“…The onset of neoclassical tearing modes (NTMs) has been shown to limit the achievable plasma performance in tokamaks by enhancing heat transport, reducing energy confinement time and reducing the achievable β (plasma kinetic pressure/magnetic pressure) [1]. If the magnetic islands driven by NTMs were allowed to grow to their maximum saturated widths in ITER, recent simulations [2] indicate that they would cover about a third of the plasma and would reduce the potential fusion power production by about a factor of four. Therefore, stabilization of NTMs, which are expected to occur in reactor-grade tokamaks such as ITER, is one of the most critical issues in tokamak reactors since these modes seriously limit the high-pressure operation in long-pulse discharges.…”