JT-60U JT-60U

Resistive Wall Mode (RWM) Study on JT-60U

Resistive Wall Mode (RWM) Study on JT-60U

Go Matsunaga松永　剛

Japan Atomic Energy Agency, Naka, Japan

JSPS-CAS Core University Program 2008 in ASIPP Plasma and Nuclear FusionFeb. 16-21, 2009 in ASIPP

Feb. 16-21, 2009 G. Matsunaga JAEA, CUP in ASIPP 2

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Outline

Introduction Current driven RWM in OH plasmas RWM in high- plasmas Recent RWM topics Summery & Suggestion for EAST experiments

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Introduction

Toward fusion reactors, the high-N operation is very attractive and advantageous, because high bootstrap current (fBS) and high fusion output (Pfus) are expected.

Finite wall resistivity makes another mode, Resistive Wall Mode (RWM)

that limits achievable N.(RWM is characterized

by wall diffusion time, w)

However, achievable N is limited by low-n MHD instability.

No-wall No-wall N-limit -limit ((N==Nno-wallno-wall ->C->C=0)=0)

Ideal-wall Ideal-wall N-limit-limit ((N==Nideal-wallideal-wall ->C->C=1)=1)

Therefore, RWM stabilization is a key issue for high-N operation in ITER and a fusion reactor.

DeviceSize

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What is key for RWM study?

RWM behaviors Wall location effect Rotation stabilization effect

→Stabilization Mechanisms Feedback control

→Establishment, Mode controllability Interaction with other instabilities

→ELMs, Energetic particle driven modes Error field effect

→Resonant field amplification (RFA), Active sensing

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Useful tools for RWM study onJT-60U

Plasam-Wall clearance feedback control

Plasam-Wall clearance feedback control

Positive ion based NBs (PNB) 4 tangential

CO ~ 4MW CTR ~ 4MW

7 perpendicular Negative ion based NBs (NNB)

2 tangential CO ~ 4MW

Positive ion based NBs (PNB) 4 tangential

CO ~ 4MW CTR ~ 4MW

7 perpendicular Negative ion based NBs (NNB)

2 tangential CO ~ 4MW

Various NB injectionsVarious NB injections

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Current driven RWM in OH plasmas

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Current driven RWM experiments

In order to investigate wall location effect on MHD instability, plasma-wall gap scan has been performed in OH plasma.

→ since only q-profile can determine the stability, wall effect can be clearly measured.

To destabilize current driven external kink mode, surface q was decreasing by plasma current ramping up.

Feb. 16-21, 2009 G. Matsunaga JAEA, CUP in ASIPP 8

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m/n=3/1 Current driven RWM is observed

qeff was just below 3, m/n=3/1 instability appeared and thermal collapse occurred.

The growth time of this mode is about 10ms.

→ On JT-60U, w is several milliseconds.

Current driven RWM↑

external kink mode +

wall stabilizing effect

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Wall location effect for RWM

G. Matsunaga, PPCF, Vol. 49, p.95(2007)

Wall stabilizing of current-driven kink mode on OH plasma

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RWM growth rates vs. wall location

G. Matsunaga et al., PPCF, Vol. 49, pp.95-103 (2007)　

G. Matsunaga et al., PPCF, Vol. 49, pp.95-103 (2007)　

Increasing d/a, RWM growth rate increased.

According to AEOLUS-FT with taking into account a resistive wall, m/n=3/1kink and m/n=2/1 tearing modes are unstable.

The dependence qualitatively agrees with RWM dispersion relation without plasma rotation.

m/n=2/1tearing modes

m/n=3/1kink mode

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RWM in high- plasmas

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Identification of critical rotation for RWM stabilizing

To identify critical plasma rotation for RWM stabilization, we only changed plasma rotation.

At 5.9s : Stored energy FB was started

→ keeping N constant

At 6.0s : Tang NBs were switched from CTR-NB to CO-NB

→ slowly reducing Plasma rotation

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High- RWM was observed by reducing plasma rotation

Just before collapse, n=1 radial magnetic field was growing with ~10ms growth time.

→ RWM

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Plasma rotation profiles

Since N was kept constant, deceleration of plasma rotation was thought to make the RWM unstable.

Focusing on the plasma rotation at the q=2, critical plasma rotation is less than 1kHz.

This value is corresponding to 0.3% of Alfvén velocity.

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Dependence of critical rotation on C

Target value of stored energy FB was changed to get the dependence of the critical plasma rotation.

The dependence of the critical rotation on C is weak.

This means that we can sustain the high-βup to the ideal wall limit.

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Recent RWM topics

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Challenge of sustainment of high- discharge

Previously, on JT-60U, the high-N plasmas > N

no-wall were transiently obtained.

In this campaign, we have tried to sustain the high-N plasma > N

no-wall with plasma rotation larger than Vt

cri.

We have successfully obtained the high-N plasma for several seconds.

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Best discharge; N~3.0, ~5sec

On the best discharge,

N~3.0 (C~0.4) was sustained by plasma rotation > Vt

cri.

Sustained duration is ~5s, which is ~3 time longer than R.

Time duration is determined by the increase of N

no-wall due to gradual j(r) penetration.

According to ACCOME, fCD80% and fBS~50% were also achieved.

~5s (~3~5s (~3RR))

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What limits for high-N long discharges

However, the sustainment of high-N is not straightforward.

Because almost all discharges were limited by

Resistive Wall Mode (RWM) Neoclassical Tearing Mode (NTM)

Furthermore, many discharges have been lost by new instabilities:

Energetic particle driven Wall Mode (EWM)

directly induces RWM despite Vt > Vtcri

RWM Precursor

strongly affects Vt-profile at q=2,

finally, induces RWM onset

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EWM can directly induce RWM

In the wall-stabilized high-bN region, Energetic particle driven Wall Mode (EWM) is newly observed.

At RWM onset, rotation was At RWM onset, rotation was enough for stabilization.enough for stabilization.

The EWM is dangerousThe EWM is dangerous for RWM

n=1

n=1

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Features of EWM

Toroidal mode number : n=1Poloidal mode number : m~3 (Kink Ballooning-like)Radial mode structure : globally-spreadGrowth time : 1~2ms

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Trapped energetic particle by PERP-NBs (85keV)

Mode frequency is chirping down as mode amplitude is increasing.

Initial mode frequency agrees with the precession frequency of the energetic particles from the PERP-NB.

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Hot pressure of PERP-NB seems to drive

hh//total total ~ -10%~ -10%

EWM is stabilized by reducing PERP-NB injection power while keeping N constant.

→　 Driving source is trapped energetic particle pressure

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N>Nno-wall (C>0) is required to drive EWM

The EWM were observed in high-N plasmas.

However, the EWM requires C>0, NOT only high-N.

CC>0>0,, NN<3.0<3.0CC>0>0,, NN<3.0<3.0

EWMEWM

CC>0>0,, NN~3.0~3.0CC>0>0,, NN~3.0~3.0

EWMEWM

CC~0~0,, NN~3.0~3.0CC~0~0,, NN~3.0~3.0

No EWMNo EWM

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EWM stability domains

If the no-wall limit is changed by j(r), EWM is always destabilized above the no-wall limit.

Increasing plasma rotation, EWM boundary seems to follow it.→ EWM has a similar stability to RWM

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Summery

RWM is a key issue in an economical aspect for future fusion reactors.

On JT-60U, RWM has been well studied; Current driven RWM → Wall location effect, High- RWM → Plasma rotation stabilizing, Instability related to RWM → Coupling to energetic particles.

JT-60U has been shut down in last August. We must wait for JT-60SA for further RWM study.

Our corroborations become important!

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m/n=1/1 Internal-kinkm/n=1/1 Internal-kink

FishboneFishbone

Possible interpretation for EWM

EWM is a coupling mode between energetic particles

andmarginally stable RWM.

Kinetic contribution Kinetic contribution of fast particlesof fast particles

MHD MHD

marginally stablemarginally stable marginally stablemarginally stable unstableunstable unstableunstable

•RWMRWM

Energetic particle driven Wall mode (EWM)Energetic particle driven Wall mode (EWM)

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Suggestion for EAST experiment

Current driven RWM by Ip ramping Wall location effect Stabilizing by fast ion tail by ICRF

External coils Feedback control Active sensing (RFA) Rotation control (Error field effect)

Neutral Beam High- RWM Energetic particle effect ELM interaction

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RWM dispersion relation

KineticKineticEnergyEnergyIntegralIntegral

PlasmaPlasmaPotentialPotentialEnergyEnergy

VacuumVacuumEnergyEnergy

withwithNo WallNo Wall

M. S. Chu et al., Phys. Plasma, Vol. 11, p.2497(2004) M. S. Chu et al., Phys. Plasma, Vol. 2, p.2236(1995) M. S. Chu et al., Phys. Plasma, Vol. 11, p.2497(2004) M. S. Chu et al., Phys. Plasma, Vol. 2, p.2236(1995)

WallWallSkinSkinTimeTime

KineticKineticEnergyEnergyIntegralIntegral

PlasmaPlasmaPotentialPotentialEnergyEnergy

Vacuum Energy with Ideal WallVacuum Energy with Ideal Wall

VacuumVacuumEnergyEnergy

withwithResistiveResistive

WallWall

DissipationDissipationEnergyEnergyIntegralIntegral

Vacuum EnergyVacuum Energywithout Wallwithout Wall

ComplexComplexGrowthGrowth

RateRate

PlasmaRotation

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Plasma rotation stabilizing effect on RWM

Some models predict that the critical rotation is several % of Alfven speed at the rational surface.

→ Dissipation and rotation are required for RWM stabilization.

How much is the critical rotation for RWM stabilization?

Future devices will have low plasma rotation.

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m/n=1/1 Internal-kinkm/n=1/1 Internal-kink

FishboneFishbone

RWMRWM

Energetic particle driven Wall mode (EWM)Energetic particle driven Wall mode (EWM)

Possible interpretation for EWM

EWM is originated from energetic particles and marginally stable RWM.

Kinetic contribution Kinetic contribution of fast particlesof fast particles

MHD MHD

marginally stablemarginally stable

marginally stablemarginally stable

unstableunstable

unstableunstable

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Ideal MHD analysis by MARG2D

This mode is unstable w/o wall, however, stable with ideal wall.

The mode structure is localized in the LFS

→ Kink-Ballooning mode structure