resistive wall mode stabilization of high- b nstx plasmas

NSTX ACS APS 2004

Resistive Wall Mode Stabilization of High- NSTX Plasmas

Supported by

Columbia UComp-X

General AtomicsINEL

Johns Hopkins ULANLLLNL

LodestarMIT

Nova PhotonicsNYU

ORNLPPPL

PSISNL

UC DavisUC Irvine

UCLAUCSD

U MarylandU New Mexico

U RochesterU Washington

U WisconsinCulham Sci Ctr

Hiroshima UHIST

Kyushu Tokai UNiigata U

Tsukuba UU Tokyo

JAERIIoffe Inst

TRINITIKBSI

KAISTENEA, Frascati

CEA, CadaracheIPP, Jülich

IPP, GarchingU Quebec

46th Annual Meeting of Division of Plasma PhysicsAmerican Physical Society

November 15-19, 2004Savannah, GA

A.C. Sontag1, S. A. Sabbagh1, J.M. Bialek1, D.A. Gates2, A. H. Glasser3, B.P. LeBlanc2, J.E. Menard2, W. Zhu1, M.G. Bell2, R. E. Bell2, A. Bondeson4, J.D. Callen5, M.S. Chu6, C.C. Hegna5, S. M. Kaye2, L. L. Lao6, Y. Liu4, R. Maingi7, D. Mueller2, K.C. Shaing5, D. Stutman8, K. Tritz8

1Department of Applied Physics, Columbia University2Plasma Physics Laboratory, Princeton University3Los Alamos National Laboratory4Institute for Electromagnetic Field Theory, Chalmers U., Goteborg, Sweden5University of Wisconsin - Madison6General Atomics7Oak Ridge National Laboratory8Johns Hopkins University

NSTX ACS APS 2004

Understanding of -Limiting Instabilities Necessary for Sustained High- Operation

• Motivation Recent machine upgrades have extended performance further into

the high- long-pulse regime Several instabilities can limit performance in this regime Resistive wall mode (RWM) can limit high- operations at low

plasma rotation and low-q

• Outline RWM identification at high- and low aspect ratio RWM rotation damping and critical rotation frequency Beta limiting mechanisms in wall stabilized regime Resonant field amplification using initial RWM stabilization coil

NSTX ACS APS 2004

NSTX Facility Equipped to Study RWM Physics at Low Aspect Ratio

NSTX ParametersIp

BT

R0

A

PNBI

1.5 MA 0.6 T0.86 m>1.27

2.67 MW

• New Capabilities: RWM sensors Initial stabilization coils

• Theory Ideal MHD stability – DCON (Glasser) Drift kinetic theory (Bondeson – Chu) RWM growth rate – VALEN (Bialek –

Boozer )

Initial RWM stabilization coilsSensors

Passive plates

NSTX ACS APS 2004

Long-Pulse High- Correlated with High Toroidal Rotation

• NSTX extending performance to higher-IN

t = 39% N = 6.8 achieved lower-q more susceptible

to RWM

Tor

oida

l (

%)

IN Ip/aBT (MA/m-T)

0 2 4 6 80

10

20

30

40

50N = 6

5

4

3

2

Shot 112546

t

N

DCONn = 1

I p (

MA

)

0.4

0.8

PN

BI (M

W)2

46

time (s)0 0.2 0.4 0.6 0.8 1.0 1.2

51015

t (

%)

Ip

PNBI

246

N

Ft (

kHz)

5

15

25 R = 1.3 mW

(ar

b.)

0

-20

-40

• Long-pulse, high-N requires wall stabilization

no-wallideal-wall

NSTX ACS APS 2004

RWM Can Limit at Low-q

• RWM is external kink modified by presence of resistive wall

• RWM Characteristics: slow growth: ~ 1/wall

slow rotation: fRWM ~ 1/wall

strong rotation damping stabilized by rotation & dissipation

• High toroidal rotation passively stabilizes RWM at higher-q

• Other modes can saturate before RWM is destabilized

Shot 114147

0.2

0.4

0.6

0.8

1.0

1.2

0

4

-4

I p (

MA

)

no-wallideal-wall

Ip

N

0

2

4

6

1

3

5

N

0.00 0.05 0.10 0.15 0.20 0.25 0.30time (s)

0.25 0.26 0.27time (s)

Bp

(Gau

ss)

0

2040

60locked n = 1 frominternal sensors

wallstabilized

DCON n = 1

W (

arb.

)

wall ~ 5 ms)

RWM

NSTX ACS APS 2004

Visible Image & Theory Show Mode Structure

• Toroidal asymmetry observed on visible camera images during RWM

• DCON computed B structure EFIT reconstruction of experimental

equilibrium includes sum of n=1-3 components scaled to measured amplitudes and

phases

Before RWM activity

RWM with Bp = 92 G Theoretical B (x10) with n=1-3 (DCON)

(interior view)(exterior view)

114147t = 0.268s

114147t = 0.250s

NSTX ACS APS 2004

Internal Sensor Array Used for RWM Detection• Internal toroidal arrays of Bz and Br

sensors installed 5 kHz digitization rate instrumented to detect n = 1 - 3

• Measured coil currents used for background subtraction ~2 Gauss noise level achieved allows real time mode detection

• SVD mode detection is robust

CL

Br

Shot 114150

BzI p

(M

A)

0.40.81.2

10

20

10

20

10

20

B (

Gau

ss)

lower Bpupper Bp

external Br

0.25 0.26 0.27 0.28 0.29 0.30time (s)

2

4

6

NIp

N

n=1

n=2

n=3

Passive Stabilizing Plate•See poster JP1.005 (S.A. Sabbagh - Wed. Afternoon)

NSTX ACS APS 2004

SXR Measurement Shows RWM Toroidal Asymmetry

• Experiment / theory show RWM not edge localized Supported by

measured Te

Bay G array (0 deg)Bay J array (90 deg)

114024, t=0.273sN = 5

0.27 0.28 0.29

8

4

0

time (s)

Inte

nsity

(ar

b)

0.0 0.5 1.0 1.5 2.0R(m)

2

1

0

-1

-2

Z(m

)

USXR separated by 90 degrees toroidally

6

2

2

1

00.0 0.2 0.4 0.6 0.8 1.0

DCON n = 1 modedecomposition

n

(arb

)

m=34

56

m=2

m=1

Te During RWM

1139240.26s - 0.277s

T

e (k

eV)

0.0 0.4 0.8 1.2

R (m)

0.0

0.1

0.2

0.3

RWM growth

n = 0.4

NSTX ACS APS 2004

RWM Rotation Damping Differs from Other Modes

• RWM damping is global 1/1 mode not present

• Edge rotation does not go to zero consistent with neoclassical

toroidal viscosity (NTV)

• 1/1 causes core damping leads to ‘rigid rotor’ plasma

core

• momentum transfer across rational surface at R = 1.3 m

RWM Core 1/1

€

Torque NTV ~ δB2Ti0.5

Ft (

kHz)

Radius (cm)90 100 110 120 130 140

10

0

20

30

40

Ft (

kHz)

10

0

20

30

40

150

50

Radius (cm)90 100 110 120 130 140 150

Time (s)0.2250.2350.255

Time (s)0.4150.4250.4350.445

NSTX ACS APS 2004

Neoclassical Toroidal Viscosity Theory Matches Measured Damping Profile

• Damping due to NTV only

• B magnitude and profile from measured Te

• No low frequency islands during this period

• Damping due to NTV and JB torques

• B magnitude and profile from measured SXR emission

core 1/1 modeRWM

*See talk CO3.008 by W. Zhu for more detail- Monday Afternoon, Room 204/205 SCC

113924

Td

(N m

-2)

0.40.30.20.10.0

-0.1-0.2

t = 0.276s

TNTV

theory3.0

2.0

1.0

0.0

1.61.41.21.00.8

TNTV+JxB

theory

measuredTd

(N m

-2)

R (m)

-R2(d/dt)

-R2(d/dt)measured

0.8 1.0 1.2 1.4 1.6R (m)

NSTX ACS APS 2004

Experiment Supports 1/q2 Scaling of Critical Rotation Frequency

• Scaling from two stability classes: stabilized no RWM for t >>

wall when N > N no-wall

not stabilized collapse after a few wall when N > N no-wall

• Bondeson-Chu drift-kinetic model* agrees with observed scaling trapped particle effects weaken

stabilizing ion Landau damping toroidal inertia enhancement

becomes more important Alfven continuum damping

gives:

€

crit ≡ωt

ωA

=14q2 q

t(q

)/

A

0.0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5

*Phys. Plasmas 3 (1996) 3013

not stabilized1/4q2stabilized

t/A(q,t) profiles

NSTX ACS APS 2004

High- Discharges Below crit(q) Can Suffer Collapse

• RWM growth when: rotation below crit inside q = 2

when N exceeds no-wall limit example - shot 114147:

• exceeds no-wall limit at ~ 0.2s

• RWM collapse at ~0.265 s

• Edge and low-order rational surfaces not stabilized

• Observed in all RWM shots

t(q

)/

A

0.0

0.1

0.2

0.3

1 2 3 4q

0.40.81.2

I p (

MA

)

Shot 114147

246

N

204060

Bp

(G)

0.20 0.22 0.24 0.26 0.28time (s)

0.21 s0.22 s0.23 s0.24 s0.25 s

n = 1

Ip

N

NSTX ACS APS 2004

Global Mode Limits at Highest N

• Collapse observed at highest N on many shots

• Mode involves both core and edge Coincident D spike and neutron collapse

• Rotation collapse is global but brief not associated with rotating MHD

Shot 112794

0.0 0.2 0.4 0.6 0.8time (s)

D

neu

tro

ns/1

012

0

2

4

6

200

400600

24

6

NF

t (kH

z)

5

15

25 R = 1.3mFt (

kHz)

neutronsD

0

10

20

30

40

50

1.0 1.2 1.4 1.6R (m)

Time (s)0.4850.4950.5050.515

NSTX ACS APS 2004

Growth Rate During Collapse Approaches Ideal

• early phase growth ~ 5 ms from RWM sensors

• later growth ~ 670 s from Mirnovs

• VALEN growth = 530 s

B (

Gauss

) Odd-n0.2-40 kHz

2468

Bp

(Gau

ss)

LowerArrayn = 1

0.45 0.46 0.47 0.48

-505

• Consistent with external to internal kink transition DCON shows kink becomes more

internal wall stabilization less effective

• VALEN shows mode growth rate approaching ideal

N

4

6

8Shot 112785

0.30 0.35 0.40 0.45 0.50time (s)

W

0-10

(s

-1)

-20

50010001500

ideal-wallno-wall

NSTX ACS APS 2004

Collapse Restores Stability, Triggers Rotating Mode

• collapse removes unstable drive

• Rotation profile moves toward stability N > N no-wall before and after

collapse

• Relatively low amplitude Bp < 10 Gauss Bp > 30 Gauss during typical

RWM

• n = 1 triggered by collapse pure n = 2, n = 3 modes

triggered in similar discharges

n = 1 2 3

F (

kHz)

20

40

60

80

time (s)0.35 0.450.30 0.500

0.40

Magnetic PickupMode Spectrum

NSTX ACS APS 2004

Resonant Field Amplification by Stable RWM Observed

• Single RWM coil pair available for CY 04 run 180 degree separation

• Resonant field amplification (RFA) when N > N no-wall

amplification of external perturbations by stable RWM

predicted by theory*

Shot 113955

0.20 0.24 0.28 0.32time (s)

0.2

0.6

1.0

2

4

6

N

I p (

MA

)

Ip

N

W (

arb.

)

0

4

-4

-8

5

10

15B

p (G

auss

)

ideal-wallno-wall

coil on

DCONn = 1

n = 1plasma

response

*Boozer, A.H., Phys. Rev. Lett., 86 5059 (2001)

NSTX ACS APS 2004

RWM Coil Used to Study RFA Characteristics

• RFA amplitude increases with N

similar to DIII-D

• RFA decay after coil pulse gives negative mode growth rate ~3 ms decay time observed ~ wall

• Full coil set will enable more detailed study of RWM rotating error field odd and even n harmonics

0.24 0.26 0.28 0.30

4 5 6

0.1

0.2

0.3

0.4

0.5

0.6

1

2

3

4

5

6

4.5 5.50.0

0

time (s)

N

RF

AB

p (G

auss

)

no-walllimit

decayperiod

Active Coil On

n = 1

NSTX ACS APS 2004

Understanding Key RWM Physics Important for Sustained High- Operation in NSTX

• Control system improvements allow access to high-IN & high- regime t = 39%, N = 6.8 early H-mode gives routine access to long-pulse

• n = 1 - 3 RWM measured

• Rotation damping consistent with neoclassical toroidal viscosity model

• Rotation data indicate crit ~ A/q2

• Global mode with growth rate >> 1/wall observed to limit highest N plasmas

• Initial resonant field amplification results consistent with DIII-D observations

• Preparation for RFA suppression and active feedback stabilization of RWM is underway