Reconnection Process in Sawtooth Crash in the Core of Tokamak Plasmas Hyeon K. Park Ulsan National Institute of Science and Technology, Ulsan, Korea National Fusion Research Institute, Daejeon, Korea 1 August 17-22, 2015 UNIST/POSTECH Pohang, Korea Animation of Tokamak plasma field lines, and sawtooth crash induced by the m/n=1/1 kink instability

2D T e variation during the crash Sawtooth oscillation in tokamak plasma  Sawtooth oscillation: periodic growth and decay of the core pressure of the toroidal plasma (S. von Goeler, 74): X-ray measurement  Full reconnection model (B. Kadomtsev, ’76):  Flattened T e (n e ) in the core  Flattened current density in the core A. Sykes et al. PRL, 36, 140, 1976 Change of q profile in the core Full reconnection process simulation

Complete reconnection process in sawtooth crash  Spontaneous reconnection process Magnetic field line reconnection induced by the m/n=1/1 kink instability around the q~1 surface leads a fast removal of the excess core pressure and current Predicted reconnection time scale was the combination of Alfven and resistive time scale The observed time scale has been faster than the predicted value in many cases but there are examples of much slower reconnection process Field line reconnection Helical magnetic field in the core Reconnection due to kink instability

Experimental measurement of q 0 [F. M. Levinton, PRL (1989)] 4

Incomplete reconnection process Partial reconnection process Full loss and full recovery of the central T e central Small loss of the central current density and recovery Many theoretical models for partial reconnection have been developed Biskamp - secondary reconnection model Porcelli – two step q profile and crash trigger mechanism There are other models too and all models are remained as model 1.Current density builds up and kink instability develops 2.Kink induces reconnection. 3.The reconnection process halts before the excess current is removed but excess pressure has been removed 4.The kink relaxes back slowly 5.Flux surface slowly recovers symmetry. 6.Go to (1) Small changes in core q profile

Measurement of q profile in incomplete reconnection model  MSE and Faraday rotation are extremely sensitive to the flux surface  Symmetry and accuracy of the flux surface are the key for the measurement of q value  Before crash, there is a period of non-axisymmetric kink instability (precursor)  Discrepancy in recovery of the pressure and current if the T e is considered as a flux quantity, unless the kink is stabilized instantly.  Plasma rotation (poloidal and toroidal) makes time average difficult Small changes in core q profile  If direct measurement is challenging, is there an alternative method to address the evolution of q profile that can be understood in conjunction with the basic MHD physics ??

G. H. Choe, NF (2014) Study of the core higher order mode sensitive to the background equilibrium q profile in KSTAR 7 Vertical red line: EC resonance position White dashed circle: inversion radius Orange solid circle: EC deposition layer

1)2)3) Angioni et al., Nucl Fusion 43 (2003) TCV exp. result with two ECCD( swept:0.45MW, fix:0.95MW) 1)  2) Sawtooth period (τ saw ) decreases from 17ms to 11ms, as z dep +δ dep becomes close to z q=1 2)  3), τ saw slowly increases. Trend of τ saw change is similar to the TCV result. KSTAR #9214, ECCD power : 0.66MW 1)2)3) Sawtooth period changes as the ECCD deposition is swept

22.5° 1) 2) 3) Pattern A : Triple  dual  single  crash 1)In the early phase, triple tubes appear (axisymmetric) 2)3/3 merges into 2/2 (axisymmetric) 3)2/2 to 1/1 (not axisymmetric)  crash 22.5° G-port H-port z dep

1) 2) 3) Pattern B : donut  3/3  3/3+1/1 1)In the early phase, T e increases near the q=1, resulting in a donut-shaped structure (axisymmetric) 2)3/3 form near the q=1 (axisymmetric) 3)As the central core develops and forms a triangular shape  crash. 22.5° G-port H-port z dep

G. H. Choe, NF (2014) Temporal behavior of the multiple mode structures During the ramp-up period, higher m/n mode transforms into the lower m/n mode → Eventually merges into a single m/n = 1/1 mode prior to the crash 11 ~17kHz ~11.4kHz ~5.7kHz Z (cm) R (cm) (s) Pattern A

Linear stability calculation with M3D-C1 [S. Jardin, Comput. Sci. Disc. (2012)] 12 Current perturbation (r/a)

m/n=1/1 dominant m/n=2/2 dominant m/n 3/3 Higer modes 13 (r/a)

m/n=1/1 dominant m/n 2/2 m/n=1/1 dominant m/n=2/2 m/n=3/3 14 (r/a)

Linear growth rates (γ)

Initial state with the m/n = 3/3 mode Transition to the m/n = 2/2 mode Interpretation of the observed 3/3 mode evolution q 0 is maintained above ~1 after the crash event Current blip is located between 0.24 ~ 0.27, where the m/n=3/3 mode is most unstable q 0 is maintained above ~1 Core current build-up due to heating and diffusion of the current blip forms a radially extended blip slightly toward the center Final state with m/n = 2/2 mode Two possibilities: 1. The position of the current blip moved close to the center due to combination of the core current density and diffusion of the original blip 2. q 0 dropped well below ~1 due to current build-up in a fast time scale 12

17 Multiple mode structures in the KSRAR H-mode discharge (1)(2) ECCD at z =7.5 cm ECH/CD at z =15 cm Z (cm) R (cm) Z (cm) (1) (2) ECE image R (cm)