EX/5-2EX/5-2Dynamics and Stability of Resistive Wall Mode Dynamics and Stability of Resistive Wall Mode

in the JT-60U High-in the JT-60U High-ββ Plasmas Plasmas

Go MatsunagaGo Matsunaga, Y. Sakamoto, N. Aiba, K. Shinohara,, Y. Sakamoto, N. Aiba, K. Shinohara,M. Takechi, T. Suzuki, T. Fujita, A. Isayama,M. Takechi, T. Suzuki, T. Fujita, A. Isayama,N. Oyama, N. Asakura, Y. Kamada, T. OzekiN. Oyama, N. Asakura, Y. Kamada, T. Ozeki

and the JT-60 teamand the JT-60 team

22nd IAEA Fusion Energy Conference22nd IAEA Fusion Energy Conference13-18 October 200813-18 October 2008

Geneva, SwitzerlandGeneva, Switzerland

Japan Atomic Energy AgencyJapan Atomic Energy AgencyNaka 311-0193, JAPANNaka 311-0193, JAPAN

JT-60U 2/19

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

Introduction

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

that limits achievable βN.(RWM is characterized

by wall diffusion time, τw)

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

No-wall No-wall β

N

-limit -limit ((βN==βNno-wall no-wall ->C->Cββ=0)=0)

Ideal-wall Ideal-wall β

N

-limit-limit ((βN==βNideal-wall ideal-wall ->C->Cββ=1)=1)

Therefore, RWM stabilization is a key

issue for high-β

N

operation in ITER and a fusion reactor.

JT-60U 3/19

Introduction

Previous 21st IAEA FEC,JT-60U and DIII-D reported good news for RWM stabilization:

RWM can be stabilized

by plasma rotation slower than 1% of V

A

.

Based on the results, Demonstration of high-βN plasma sustainment with Vt>VtcriFurther investigation of rotation stabilization for RWM

are required.

On JT-60U, we have carried out the high-βN operation in the wall-stabilized high-βN region

with focusing on “the RWM dynamics and stability”.

JT-60U 4/19

Sustainment of high-Sustainment of high-ββNN plasma above plasma above ββNNno-wallno-wall

Observation of new MHD instabilities at Observation of new MHD instabilities at ββNN > > ββNNno-wallno-wall

Energetic particle driven Wall Mode (EWM)Energetic particle driven Wall Mode (EWM)

RWM PrecursorRWM Precursor

SummarySummary

Outline of this talk

Sustainment of high-βN plasma > βNno-wall

JT-60U 6/19High-High-ββNN plasmas have been sustained based on previously plasmas have been sustained based on previously

obtained critical rotation for RWM stabilizationobtained critical rotation for RWM stabilization

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

no-wallno-wall were transiently obtained.were transiently obtained.

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

no-wall no-wall with plasma with plasma rotation larger than Vrotation larger than Vtt

cricri..

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

JT-60U 7/19

On the best discharge, the high-On the best discharge, the high-ββNN was sustained for ~5s was sustained for ~5s

On the best discharge,

βN~3.0 (Cβ~0.4) was sustained by plasma rotation > Vt

cri.

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

Time duration is determined by the increase of βN

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

According to ACCOME, fCD≥80% and fBS~50% were also achieved.

~5s (~3~5s (~3ττRR))

JT-60U 8/19

Two instabilities have been newly observed just before Two instabilities have been newly observed just before the RWMthe RWM

However, the sustainment of high-However, the sustainment of high-ββN N is not is not straightforward.straightforward.Because almost all discharges were limited Because almost all discharges were limited byby

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

Furthermore,Furthermore,many discharges have been lost bymany discharges have been lost bynew instabilities:new instabilities:

Energetic particle drivenEnergetic particle driven Wall Mode (EWM)Wall Mode (EWM)

directly induces RWM despite Vdirectly induces RWM despite Vtt>V>Vttcricri

RWM PrecursorRWM Precursorstrongly affects Vt-profile at q=2,strongly affects Vt-profile at q=2,finally, induces RWM onset finally, induces RWM onset

Observation of MHD instabilities at βN > βNno-wall

Energetic Particle driven Wall Mode (EWM)

JT-60U 10/19

EWM can directly induce RWM, even though, rotation is enoughEWM can directly induce RWM, even though, rotation is enough

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

JT-60U 11/19

EWM has globally-spread structureEWM has globally-spread structure

Toroidal mode numberToroidal mode numbern=1n=1

Poloidal mode numberPoloidal mode numberm~3 (Kink Ballooning-like)m~3 (Kink Ballooning-like)

Radial mode structureRadial mode structureglobally-spreadglobally-spread

Growth timeGrowth time

1~2ms ~ 1~2ms ~ ττ

ww

JT-60U 12/19

EWM is only observed in the wall-stabilized region CEWM is only observed in the wall-stabilized region Cββ>0>0

The EWM were observed in high-The EWM were observed in high-ββNN plasmas. plasmas.

However, the EWM requires CHowever, the EWM requires C

ββ

>0, NOT only high->0, NOT only high-ββ

NN

..CC

ββ

>0>0, , ββNN<3.0<3.0

EWMEWM

CC

ββ

>0>0, , ββ

NN

~3.0~3.0

EWMEWM

CCββ~0~0, , ββ

NN

~3.0~3.0

No EWMNo EWM

JT-60U 13/19

EWM is destabilized by trapped energetic particlesEWM is destabilized by trapped energetic particles

EWM frequency was chirping down as EWM frequency was chirping down as amplitude was increasing.amplitude was increasing.Initial frequency corresponds to Initial frequency corresponds to precession freq. of PERP-NB (~90keV)precession freq. of PERP-NB (~90keV)

EWM is destabilized by trapped energetic particleEWM is destabilized by trapped energetic particle

PERP-NB power was turned off PERP-NB power was turned off with keeping high-with keeping high-ββNN..

-> EWM can be stabilized.-> EWM can be stabilized.

JT-60U 14/19

EWM is thought to be the coupling between trapped energetic EWM is thought to be the coupling between trapped energetic particle and marginal resistive wall modeparticle and marginal resistive wall mode

It is concluded that EWM isIt is concluded that EWM is

ffmodemode~f~fprepre(90keV) with chirping down(90keV) with chirping down

Destabilized by PERP-NBsDestabilized by PERP-NBs

Only observed at COnly observed at Cββ>0, NOT high->0, NOT high-ββNN

Growth time ~Growth time ~ττWW (a few ms) (a few ms)

Globally-spread mode structureGlobally-spread mode structureEWM is the coupling between energetic particle

and marginal RWM.

Energetic Particle driven Wall Mode (EWM)Possible interpretation,

Fishbone branch of RWM with taking into account δWkfast

This mode should be paid attention because it can induce RWM

Driven by Energetic ParticlesDriven by Energetic Particles(trapped energetic particles) (trapped energetic particles)

Behave like Resistive Wall ModeBehave like Resistive Wall Mode

Observation of MHD instabilities at βN > βNno-wall

RWM Precursor

JT-60U 16/19

In the wall-stabilized high-In the wall-stabilized high-ββNN region, a region, a slowly growing precursor is observed slowly growing precursor is observed just before RWM onset.just before RWM onset.

RWM PrecursorRWM Precursor

Main features:Main features:Growth time ~50ms ~ Growth time ~50ms ~ ττTMTM,,Toroidal mode number n=1,Toroidal mode number n=1,Confinement is not much degraded,Confinement is not much degraded,It is triggered by ELM or EWM,It is triggered by ELM or EWM,Finally, rapidly growing (RWM),Finally, rapidly growing (RWM),Vt-profile at q=2 is strongly affected,Vt-profile at q=2 is strongly affected,Mode frequency does not Mode frequency does not correspond to Vt at q=2.correspond to Vt at q=2.

RWM precursor can strongly affect Vt-profile at q=2 RWM precursor can strongly affect Vt-profile at q=2

JT-60U 17/19

RWM precursor is quite different from NTMRWM precursor is quite different from NTM

RWM precursor looks like NTM (growth time ~50ms, affected location).RWM precursor looks like NTM (growth time ~50ms, affected location).However, according to mode structure comparison,However, according to mode structure comparison,

RWM precursor does not have any clear islands at q=2.RWM precursor does not have any clear islands at q=2.Rather, poloidal mode structure is m~3 at the wall.Rather, poloidal mode structure is m~3 at the wall.

RWM precursor is different from m/n=2/1 NTMRWM precursor is different from m/n=2/1 NTM

JT-60U 18/19

RWM precursor :RWM precursor :Different from NTM, (No clear island)Different from NTM, (No clear island)Mode struct. : n=1, m~3 (KB-like),Mode struct. : n=1, m~3 (KB-like),ffmodemode : NOT correspond to Vt at q=2, : NOT correspond to Vt at q=2,

These are RWM-like behaviors.These are RWM-like behaviors.RWM precursor is thought to be RWM precursor is thought to be fundamentally the same as RWMfundamentally the same as RWM(Growth time is further slowdown)(Growth time is further slowdown)

RWM precursor can strongly reduce RWM precursor can strongly reduce VVtt and/or and/or dVdVtt/dr/dr at q=2, at q=2,

When When VVtt->V->Vttcricri or or dVdVtt/dr->0/dr->0,,Finally, RWM onset.Finally, RWM onset.

Rotational shear was lost and finally RWM became unstable Rotational shear was lost and finally RWM became unstable

Rotational shear is important for RWM stabilityRotational shear is important for RWM stabilityas well as rotation speedas well as rotation speed

JT-60U 19/19

SummarySummaryThe high-βN~3 plasma above βN

no-wall has been successfully sustained by plasma rotation for ~5s (~3τR).

In the wall-stabilized high-βN region, interesting instabilities related to RWM were newly observed:

Energetic Particle driven Wall Mode (EWM) can directly induce the RWM, is destabilized by trapped energetic particles, is though to be coupling between energetic particles and RWM.

RWM precursor has slowly growth time ~50ms, strongly affects Vt-profile at q=2, can decrease rotational shear at q=2 and RWM became unstable.

These results suggest that the energetic particles and rotational shear play important roles for the RWM stability.

On JT-60SA, we can find answers for high-β

N

MHD physics.