Resistive Wall Mode Stabilization of High- b NSTX Plasmas
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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)
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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
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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
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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
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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
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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
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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)
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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
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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