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Choi, M.J.,Park, H.K.,Yun, G.S.,Lee, W.,Luhmann Jr., N.C.,Lee, K.D.,Ko, W.-H.,Park, Y.-S.,Park, B.H.,In, Y. IOP 2016 Nuclear fusion Vol.56 No.6
<P>Minor and major disruptions by explosive MHD instabilities were observed with the novel quasi 3D electron cyclotron emission imaging (ECEI) system in the KSTAR plasma. The fine electron temperature (<I>T</I> <SUB>e</SUB>) fluctuation images revealed two types of minor disruptions: a small minor disruption is a <img ALIGN='MIDDLE' ALT='$q\sim 2$ ' SRC='http://ej.iop.org/images/0029-5515/56/6/066013/nfaa223aieqn001.gif'/> localized fast transport event due to a single <I>m</I>/<I>n</I> = 2/1 magnetic island growth, while a large minor disruption is partial collapse of the <img ALIGN='MIDDLE' ALT='$q\leqslant 2$ ' SRC='http://ej.iop.org/images/0029-5515/56/6/066013/nfaa223aieqn002.gif'/> region with two successive fast heat transport events by the correlated <I>m</I>/<I>n</I> = 2/1 and <I>m</I>/<I>n</I> = 1/1 instabilities. The <I>m</I>/<I>n</I> = 2/1 magnetic island growth during the minor disruption is normally limited below the saturation width. However, as the additional interchange-like perturbation grows near the inner separatrix of the 2/1 island, the 2/1 island can expand beyond the limit through coupling with the cold bubble formed by the interchange-like perturbation.</P>
Classen, I G J,Lauber, Ph,Curran, D,Boom, J E,Tobias, B J,Domier, C W,Luhmann Jr, N C,Park, H K,Garcia Munoz, M,Geiger, B,Maraschek, M,Van Zeeland, M A,da Graç,a, S Published jointly by The Institute of Physics and 2011 Plasma physics and controlled fusion Vol.53 No.12
<P>Detailed measurements of the 2D mode structure of Alfvén instabilities in the current ramp-up phase of neutral beam heated discharges were performed on ASDEX Upgrade, using the electron cyclotron emission imaging (ECEI) diagnostic. This paper focuses on the observation of reversed shear Alfvén eigenmodes (RSAEs) and bursting modes that, with the use of the information from ECEI, have been identified as beta-induced Alfvén eigenmodes (BAEs). Both RSAEs with first and second radial harmonic mode structures were observed. Calculations with the linear gyro-kinetic code LIGKA revealed that the ratio of the damping rates and the frequency difference between the first and second harmonic modes strongly depended on the shape of the <I>q</I>-profile. The bursting character of the BAE type modes, which were radially localized to rational <I>q</I> surfaces, was observed to sensitively depend on the plasma parameters, ranging from strongly bursting to almost steady state.</P>
Study of ion-gyroscale fluctuations in low-density L-mode plasmas heated by NBI on KSTAR
Lee, W.,Ko, S.H.,Leem, J.,Yun, G.S.,Park, H.K.,Wang, W.X.,Budny, R.V.,Kim, K.W.,Luhmann Jr, N.C. International Atomic Energy Agency 2018 Nuclear fusion Vol.58 No.4
<P>Broadband density fluctuations with peak frequency ranging from 150 to 400 kHz were measured using a multichannel microwave imaging reflectometer in core region of the low-density L-mode plasmas heated by neutral beam injection on KSTAR. These fluctuations have been studied by comparing the dominant mode scales estimated from the measurement with those predicted from linear gyrokinetic simulation. The measured poloidal wavenumbers are qualitatively comparable to those of the ‘fastest growing modes’ from simulations, whereas they are larger than those of the ‘transport-dominant modes’ by about a factor of three. The agreement on wavenumbers between the measurement and linear simulation (for the fastest growing modes) is probably due to sufficiently weak <img ALIGN='MIDDLE' ALT='$E \times B$ ' SRC='http://ej.iop.org/images/0029-5515/58/4/046009/nfaaac4bieqn001.gif'/> flow shear compared to the maximum linear growth rate. Meanwhile, the transport-dominant modes seem to be related to the fluctuations in lower frequencies (∼80–150 kHz) observed in some of the measurement.</P>
Multiscale interaction between a large scale magnetic island and small scale turbulence
Choi, M.J.,Kim, J.,Kwon, J.-M.,Park, H.K.,In, Y.,Lee, W.,Lee, K.D.,Yun, G.S.,Lee, J.,Kim, M.,Ko, W.-H.,Lee, J.H.,Park, Y.S.,Na, Y.-S.,Luhmann Jr, N.C.,Park, B.H. IOP 2017 Nuclear fusion. Fusion nucléaire. &n.Illiga Vol.57 No.12
<P>Multiscale interaction between the magnetic island and turbulence has been demonstrated through simultaneous two-dimensional measurements of turbulence and temperature and flow profiles. The magnetic island and turbulence can mutually interact via coupling between the electron temperature (<I>T</I> <SUB> <I>e</I> </SUB>) gradient, the <I>T</I> <SUB> <I>e</I> </SUB> turbulence, and the poloidal flow. The <I>T</I> <SUB> <I>e</I> </SUB> gradient altered by the magnetic island steepens outside and flattens inside the island. The <I>T</I> <SUB> <I>e</I> </SUB> turbulence can appear in increased <I>T</I> <SUB> <I>e</I> </SUB> gradient regions. The combined effects of the <I>T</I> <SUB> <I>e</I> </SUB> gradient and the poloidal flow shear determines the two-dimensional distribution of the <I>T</I> <SUB> <I>e</I> </SUB> turbulence. When the poloidal vortex flow forms, it can maintain the steepest <I>T</I> <SUB> <I>e</I> </SUB> gradient and the magnetic island acts more like an electron heat transport barrier. Interestingly, when the <I>T</I> <SUB> <I>e</I> </SUB> gradient, the <I>T</I> <SUB> <I>e</I> </SUB> turbulence, and the vortex flow shear increase beyond critical levels, the magnetic island turns into a fast electron heat transport channel, which directly leads to the minor disruption.</P>