active control of mhdinstabilitiy 2002/11/19 s.ohdachi et.al. sawtooth-like phenomena in lhd s....

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


Outline

• MHD stability in LHD.• Sawtooth activities in high density plas

ma with pellet injection.• Observations with a tangentially viewing

SX camera.• Edge activities in inward shifted plasma

s.


Large Helical Device(LHD)

VacuumVessel

Helical Coil

Poloidal Coil

L = 2, m=10 Heliotron type device R = 3.5 - 3.9m, a ~ 0.6m Bt = 0.5 – 2.8 TNBI ~ 9MW, ICH~2.5MW, ECH~1.9MW Te < 10keV, Ti < 5keV

Best performance of plasma can be produced with Rax = 3.6m configuration where anomalous/neo-classical transports are improved.

Superconducting coil system


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=P0(1-8)2 is assumed.


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.


Operation Regime

• Sawtooth-like events are observed in the high-density with pellet injection.

Pellet

0 2 4 6 8 10 12 140

250

500

750

1000

1250

Wp [kJ]

ne [ 1019 m-3 ]


Sawtooth-like activitySatooth-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.


Inversion radius and profile

• Difference between before and after are shown.

• Inversion radius is around = 1/2.


Pressure gradient and Magnetic shear

Small change in iota profile(10kA).

Shear at the rational surface is affected.


Electron temperature profile with/without sawtooth activitiy

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


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.


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 rate4.5kHz(256x256)13.5kHz(128x128)

Response of phosphor is fast enough ( s order).


Energy flow with ST crash

• Energy flow seems to be isotropic.


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.

31083.mov


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.


= 4/3 and 1/1• In further inward shifted plas

ma, 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.


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.


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.


Sawtooth and pre/post Cursor• m=2 pre/post curs

or 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.

active control of mhdinstabilitiy 2002/11/19 s.ohdachi et.al. sawtooth-like phenomena in lhd s....

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