Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Sawtooth-like phenomena in LHD S. Ohdachi, S.Yamamoto, K. Toi, K. Y.Watanabe, S.Sakakibara,

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Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Sawtooth-like phenomena in LHD S. Ohdachi, S.Yamamoto, K. Toi, K. Y.Watanabe, S.Sakakibara, H. Yamada, LHD Exp.Group

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Outline MHD stability in LHD. Sawtooth activities in high density plasma with pellet injection. Observations with a tangentially viewing SX camera. Edge activities in inward shifted plasmas.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Large Helical Device(LHD) Vacuum Vessel Helical Coil Poloidal Coil L = 2, m=10 Heliotron type device R = m, a ~ 0.6m B t = 0.5 – 2.8 T NBI ~ 9MW, ICH~2.5MW, ECH~1.9MW T e < 10keV, T i < 5keV Best performance of plasma can be produced with Rax = 3.6m configuration where anomalous/neoclassical transports are improved. Superconducting coil system

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. MHD stability and Magnetic Axis Position In inward shifted plasma (Rax = 3.6m), magnetic hill region is wider. Magnetic well region extend as the beta increases. We have rational surfaces,  = n/m = (1/2), 2/3, 1/1, 4/3, 3/2, in the hill region at  ~ 1%(3.6m). P=P 0 (1-  8 ) 2 is assumed.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Coherent Modes in inward-shifted plasmas Operation with Mercier Unstable conditions. (Rax = 3.53m 3.6m) Many coherent modes are simultaneously observed. (broad density profile) However, they do not affect the confinement so much. There are several case where MHD instabilities lead to sawtooth-like collapse.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Operation Regime Sawtooth-like events are observed in the high-density with pellet injection. Pellet

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Sawtooth-like activity Satooth-like activities are observed in pellet injected high-density plasmas. While the electron density decreases after pellet injection the electron pressure profile become peaked gradually. Sawtooth activities around i = 1/2 surface can be seen.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Inversion radius and  profile Difference between before and after are shown. Inversion radius is around  = 1/2.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Pressure gradient and Magnetic shear Small change in iota profile(10kA). Shear at the rational surface is affected.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Electron temperature profile with/without sawtooth activitiy With ST activities, the electron temperature profile becomes flat.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Fluctuation with ST Fluctuations are analyzed by SVD. Slightly inside the inversion radius, m=2 oscillations are seen. Its amplitude decreases with ST crash. Heat pulse triggered by ST crash excite edge mode with m/n = 1/1.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Sawtooth observed by a fast framing tangentially viewing SX camera Two dimensional image with a framing rate of 4.5kHz can be measured with this system without complicated tomographic reconstruction. Sample Image Framing rate 4.5kHz(256x256) 13.5kHz(128x128) Response of phosphor is fast enough (  s order).

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Energy flow with ST crash Energy flow seems to be isotropic.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Core Fluctuations with Rax = 3.53m In Rax=3.53 another collapse are found with lower beta plasma. In this case, energy flow is m=2 nature.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. SVD component #4-5 component represent represent m=2 rotation mode. (r=0.5 Dr=0.1) Exhaust from the core region can be seen in.#3 component, which synchronizes with the m=2 rotation. We are trying to find out the detailed structure like this event, at ST-crash with faster camera.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al.  = 4/3 and 1/1 In further inward shifted plasma, Rax = 3.53 and 3.6m, collapse events in the peripheral region also can be seen. The effect on the plasma performance, e.g., the stored energy is larger.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Summary 1 Sawtooth-like events observed in LHD is similar to the sawtooth in tokamks. It is localized on  = 1/2 surface with revered shear (tokamaks). When pressure gradient exceed the limit, activities are exited. Critical pressure gradient are affected by the local magnetic shear.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Summary 2 One events takes ms order. Slower than tokamak sawtooth. Energy flow at the events seems to be isotropic. Events at various rational surface can be observed in inward-shifted plasma with Mercier unstable conditions.

Active Control of MHDinstabilitiy 2002/11/19 S.Ohdachi et.al. Sawtooth and pre/post Cursor m=2 pre/post cursor can be seen in SX fluctuations. In high beta discharges (Bt = 1.3, 1.5T), magnetic flucutuations with n/m = 1/2 syncronized with SX fluctuations are observed.