Sawtooth-like phenomena in LHD

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• Sawtooth-like phenomena in LHD
• S. Ohdachi, S.Yamamoto, K. Toi, K. Y.Watanabe,
  S.Sakakibara, H. Yamada, LHD Exp.Group

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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.

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Large Helical Device(LHD)
L 2, m10 Heliotron type device R 3.5 -
3.9m, a 0.6m Bt 0.5 2.8 T NBI 9MW,
ICH2.5MW, ECH1.9MW Te lt 10keV, Ti lt 5keV
Best performance of plasma can be produced with
Rax 3.6m configuration where anomalous/neo-class
ical transports are improved.
Superconducting coil system
Poloidal Coil
Vacuum Vessel
Helical Coil
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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, i n/m (1/2), 2/3,
  1/1, 4/3, 3/2, in the hill region at b
  1(3.6m).
• PP0(1-r8)2 is assumed.

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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.

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Operation Regime
Pellet
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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.
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Inversion radius and i profile

• Difference between before and after are shown.
• Inversion radius is around i 1/2.

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Pressure gradient and Magnetic shear
Small change in iota profile(10kA). Shear at the
rational surface is affected.
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Electron temperature profile with/without
sawtooth activitiy

• With ST activities, the electron temperature
  profile becomes flat.

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Fluctuation with ST

• Fluctuations are analyzed by SVD.
• Slightly inside the inversion radius, m2
  oscillations are seen. Its amplitude decreases
  with ST crash.
• Heat pulse triggered by ST crash excite edge mode
  with m/n 1/1.

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Sawtooth observed by a fast framing tangentially
viewing SX camera
Framing rate 4.5kHz(256x256) 13.5kHz(128x128)
Response of phosphor is fast enough ( ms order).
Sample Image
Two dimensional image with a framing rate of
4.5kHz can be measured with this system without
complicated tomographic reconstruction.
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Energy flow with ST crash

• Energy flow seems to be isotropic.

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Core Fluctuations with Rax 3.53m

• In Rax3.53 another collapse are found with lower
  beta plasma. In this case, energy flow is m2
  nature.

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SVD component

• 4-5 component represent represent m2 rotation
  mode. (r0.5 Dr0.1)
• Exhaust from the core region can be seen in .3
  component, which synchronizes with the m2
  rotation.
• We are trying to find out the detailed structure
  like this event, at ST-crash with faster camera.

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i 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.

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Summary 1

• Sawtooth-like events observed in LHD is similar
  to the sawtooth in tokamks.
• It is localized on i 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.

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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.

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Sawtooth and pre/post Cursor

• m2 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.