Beta limits in long-pulse tokamak discharges

TLDR

Long‑pulse tokamak discharges at low collisionality achieve a normalized beta far below short‑pulse values and ideal MHD predictions, mainly because low‑m/n nonideal MHD modes reduce confinement and can trigger disruptions. The study aims to quantitatively compare conventional resistive and neoclassical tearing‑mode theories with experimental data from multiple machines to pinpoint the limitations on ITER‑size tokamak beta limits. By analyzing low‑m/n mode behavior across several devices, the authors isolate key factors affecting beta limits and evaluate plasma control strategies that could raise the soft beta limit, suppress seed perturbations, or mitigate confinement losses.

Abstract

The maximum normalized beta achieved in long-pulse tokamak discharges at low collisionality falls significantly below both that observed in short pulse discharges and that predicted by the ideal MHD theory. Recent long-pulse experiments, in particular those simulating the International Thermonuclear Experimental Reactor (ITER) [M. Rosenbluth et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1995), Vol. 2, p. 517] scenarios with low collisionality νe*, are often limited by low-m/n nonideal magnetohydrodynamic (MHD) modes. The effect of saturated MHD modes is a reduction of the confinement time by 10%–20%, depending on the island size and location, and can lead to a disruption. Recent theories on neoclassical destabilization of tearing modes, including the effects of a perturbed helical bootstrap current, are successful in explaining the qualitative behavior of the resistive modes and recent results are consistent with the size of the saturated islands. Also, a strong correlation is observed between the onset of these low-m/n modes with sawteeth, edge localized modes (ELM), or fishbone events, consistent with the seed island required by the theory. We will focus on a quantitative comparison between both the conventional resistive and neoclassical theories, and the experimental results of several machines, which have all observed these low-m/n nonideal modes. This enables us to single out the key issues in projecting the long-pulse beta limits of ITER-size tokamaks and also to discuss possible plasma control methods that can increase the soft β limit, decrease the seed perturbations, and/or diminish the effects on confinement.