Abstract Details
Abstracts
Author: Linda E. Sugiyama
Requested Type: Poster
Submitted: 2019-02-22 13:34:48
Co-authors: L. Xu, M. Okabayashi
Contact Info:
MIT
Bldg. 26-557, 77 Massachusetts
Cambridge, MA 02139
USA
Abstract Text:
Complete sawtooth crashes in a toroidal magnetically confined plasma can be caused by ideal MHD quasi-interchange (QI) modes as well as resistive internal kinks. Many such sawteeth have been observed in plasma experiments[1,2], but so far have not been predicted theoretically or numerically. The first numerical simulation of a complete QI sawtooth crash is reported, using the M3D extended MHD code, and shown to reproduce the characteristic properties of experimental QI crashes. The central safety factor q_o can be less than one, e.g., 0.94. Unlike the internal kink, multiple toroidal harmonics n=1,2 and higher can be simultaneously unstable and form a coherent growing ``eigenmode.'' The poloidal magnetic flux has large poloidal m=n+1 harmonics at q=1 that extend over q<(n+1)/n. The initial helical displacement of the peak temperature from the magnetic axis appears similar to the kink, but the remainder of the crash proceeds through a sequence of steps driven by QI flows, not through magnetic reconnection. Temperature and plasma are expelled from inside to outside q=1 through a pair of poloidally separated channels. The current density flattens inside the inversion radius over a significantly longer time as the perturbed magnetic field decays. Small residual central perturbations with mixed n=1 and 2 remain. The results explain previous predictions of mode saturation. In part, the q=1 flux surface shaping (elliptical with low triangularity) helps destabilize the mode. The results have important implications for sawteeth in fusion burning plasmas. In addition, comparison to the actual plasma suggests that a small rotating n=1 nonaxisymmetric perturbation may be able to stabilize QI sawteeth.
Work partially supported by the U.S. DoE Office of Fusion Energy; simulations run at NERSC.
[1] E. A. Lazarus, et al., Physics of Plasmas 14, 055701 (2007).
[2] J. A. Wesson, et al., Plasma Phys. Controlled Fusion 28, 243 (1986).
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