Dynamics of multiple flux tubes in sawtoothing KSTAR plasmas heated by electron cyclotron waves: II. Theoretical and numerical analysis

Abstract

The dynamics of multiple closed flux tubes in the core of a sawtoothing tokamak plasma are studied using nonlinear simulations. This is motivated by recent observations of long-lived hot spots in the electron cyclotron emission (ECE) images of KSTAR plasmas with electron cyclotron heating (ECH) (Yun et al 2012 Phys. Rev. Lett. 109 145003). Using an empirical source term in a reduced set of MHD equations, it is shown that flux tubes with helicity h = 1 are easily produced and survive for the observed time intervals only if the safety factor is close to unity (|q − 1| ≪ 0.5%) and the magnetic shear is small (|s| ≪ 1). This suggests that sawteeth in KSTAR leave behind wide regions where q ≈ 1. On the basis of the relevant time scales, we discuss how this magnetic geometry and the spatial localization of the EC resonance may allow ECH to actively induce the formation of flux tubes. Using simulations with q profiles that possess a wide q = 1 region inside the sawtooth inversion radius, we examine how the flux tubes merge and annihilate, and how their dynamics depend on the strength of the drive. The phenomena seen in the simulations and experiments lead us to conclude that, during the sawtooth ramp phase, there is a dynamic competition between sources and sinks of thermal and magnetic energy, where the flux tubes may play an important role; both as carriers of and channels for energy. The development of self-consistent simulation models is motivated and directions for future experiments are given.