Dynamics of multiple flux tubes in sawtoothing KSTAR plasmas heated by electron cyclotron waves, 2; Theoretical and numerical analysis

Bierwage, A.; Yun, G. S.*; Choe, G. H.*; Nam, Y.*; Lee, W.*; Park, H. K.*; Bae, Y.-S.*

The dynamics of multiple flux tubes in the core of a sawtoothing tokamak plasma are studied numerically. This is motivated by observations of long-lived hot spots in the ECE images of KSTAR plasmas with ECH. Using a 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 q is close to unity and the magnetic shear is small. It is shown that under these conditions the spatial localization of ECH may allow it to actively induce the formation of the tubes. Using simulations, we examine how the flux tubes merge and annihilate, and how their dynamics depend on the strength of the drive. We conclude that flux tubes play an important role for the dynamic competition between sources and sinks of thermal and magnetic energy during the sawtooth ramp phase. The development of self-consistent simulation models is motivated and directions for experiments are given.