Formation of flux tubes by localized heating in KSTAR

Abstract

Slow buildup of energy followed by fast crash of a low n mode is a common MHD phenomenon in the core of tokamak plasma. The most notable example in the q = 1 region is the sawtooth crash, which may trigger other harmful MHD instabilities albeit its beneficial effect of expelling impurities. To control the crash time scale, localized electron cyclotron heating (ECH) is applied within and close to to q = 1 in KSTAR plasmas. For heating well inside q = 1, quasi-stable multiple flux tubes (MFTs) develop and evolve into a single flux tube mode (or dual modes), reminiscent of inverse cascade. Numerical modeling studies showed that a region of flat q = 1 is a necessary condition for MFT formation. Detailed 2D imaging diagnostics showed that heat transport increases in the presence of MFTs and the sawtooth crash does not occur until the merging of MFTs, which suggests that the flux tubes carry excess current and generate magnetic field lines, regulating thermal transport and MHD stability. As heating location comes close to q = 1, the number of flux tubes increases with smaller size. For heating very close to q = 1 (or the sawtooth inversion radius), flux tubes become indistinguishable and very slow sawtooth crashes are observed with the time scale of a few ms.