magnetic islands with complex structure in nstx
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Magnetic islands with complex structure in NSTX
K. L. Wong
PPPL
and the NSTX Research Team
53rd APS-DPP annual meeting at Salt Lake City-UT
November 14-18, 2011
College W&MColorado Sch MinesColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin
Culham Sci CtrU St. Andrews
York UChubu UFukui U
Hiroshima UHyogo UKyoto U
Kyushu UKyushu Tokai U
NIFSNiigata UU Tokyo
JAEAHebrew UIoffe Inst
RRC Kurchatov InstTRINITI
KBSIKAIST
POSTECHASIPP
ENEA, FrascatiCEA, Cadarache
IPP, JülichIPP, Garching
ASCR, Czech RepU Quebec
NSTXNSTX Supported by
NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20** 2
NSTX plasmas are rich in MHD activities
• Alfvén Eigenmodes: 100 kHz – 2 MHz• Kink, tearing and ballooning modes: 1 – 100 kHz• Resistive wall modes < 1 kHz• Locked modes (due to tearing or RW modes) f 0• Typical Mirnov signal
NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Low frequency multiple harmonic oscillations (MHO)#138326: Ip=0.8MA H-mode, Pb=2MW, Prf=0.9MW turned on at 0.25s
Mirnov signal: fn ~ n Δf ~ n f1
Island identified by a flat region in fΦ is rotating with fΦ ~ 20 kHz ~ Δf
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Doppler shift: ω’ = ω + k.V = ω + kϕVϕ= ω + nVϕ/R
This relation was used to determine the
toroidal mode number n of TAEs in TFTR
Ref: Wong et al, PPCF(1994)
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For low-n tearing modes, ω << ω’ ~ nVϕ/R
Let B =m,n bmn exp[i(m+n)]
If an island localized at q=m/n has
multiple toroidal mode numbers,
then the island observed in the lab
should have multiple harmonic
frequencies with fn = n fϕ
Resonance condition: m/n=q
Multiple n multiple m
Large m => corrugated island surface/
irregular boundary in Poincaré plot
NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Most likely scenario in #138326: Prf turned on at 0.25s & form 1/1 island enhanced transport (or sawtooth crash?) between 0.2817-0.2983s during MHO
RF turned on at 0.25s, ELMy H-mode,
Wmhd & neutron rate level off after 0.3s
- enhanced transport due to MHO
(no broadband noise in Mirnov signal)
Te in core drops between
0.2817 – 0.2983s
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Stochastic magnetic field near separatrix of an island
Stochastic B during sawtooth crash
- due to the overlap for second order
island chains (action-angle variables)Ref: Lichtenberg et al., Nucl. Fusion (1992)
Express the perturbed B in Fourier series:
B =m,n bmn exp[i(m+n)]
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When the island has many m, n
harmonics, we can also expect a
larger region of multiple stochastic
layers and separatrix.
NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Island at q = 2 near r~0 (from MSE) after RF turned off#130608: 1MA, Pb=2MW, Prf~1.8MW tripped off by an ELM –TRANSP130608Z10
Δf~22kHz at 0.386s,
droops to 12kHz at 0.4s
RF tripped off by an ELM at 0.376s,
Then Wmhd & neutron rate drop
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Island fΦ approx. corresponds to Δf in Mirnov signal
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
MHOs enhance plasma energy transport
Grad.Te reduced in plasma core (r/a<0.6)
after MHOs appear
Grad.Ti reduced in plasma core (r/a<0.6)
after MHOs appear
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
MHO during RF flat-top#138413@0.41s, Ip=1MA, Pb~2MW, Prf~1.9MW,
There was an ELM just before 0.4s that may triggered the MHO
Prf & Pb remained constant in 0.4-0.44s
At t=0.406s, only 1 island @ q~1, 30kHz
At t=0.416s, another island @ q~2, 16 kHz
m/n = 2/1, 4/2, 6/3, 8/4 . . . .
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
MHO with rising freq after Prf switched off (#138143)
Prf turned off at 0.46s. An island
appears at R>130cm & grows in size
and rotation speed. Te(r) falls when
island gets large (after 0.5s)
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Combination of several sets of MHOs at different regions
At 0.37s, several sets of MHOs appear at the same time with different Δf: fϕ slowing down in plasma core & spinning up at plasma edge
Several islands at different regions overlap – avalanche : severe deterioration of global plasma confinement – big drop in stored energy & neutron emission
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
MHO near edge (q~4) developed into locked mode#129169: Ip=1MA, Pb=2MW, Prf=1.2MW, L-mode edge
Rapid MHD growth at 0.27s,
RF tripped off at 0.28s,
Locked mode ( f ~ 0 ) at 0.293s
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Change of fΦ(R) due to MHO (0.27s - 0.29s)plasma core has reversed magnetic shear before 0.27s
fΦ(R) from TRANSP: 0.2s< t <0.3s
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
fΦ pedestal outside region of reversed magnetic shearfΦ and q measured with 10ms resolution
ΩΦ(R) changes slowly from .27-.29s :
ΩΦ pedestal in region with positive magnetic shear reversed magnetic shear improves momentum confinement
Sudden change of q(R) at .27s :Appearance of MHOs coincides with q(0) collapse (from 3.0 to 1.2)
- double tearing modes at 0.27s?
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Enhanced momentum & edge electron energy transportdue to locked mode
Grad ΩΦ(r/a:.4-.9) at .27s, .28s, .29s, .3s : falls with time after locked mode appears
Grad Te(r/a:.6-1) at .27s, .28s, .29s, .3s :
falls with time at plasma edge (r/a>0.85) due to edge cooling at r/a>0.8
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Enhanced core impurity transport & edge cooling due to MHOs MHOs at edge have similar effect as EHOs in DIII-D QH-mode plasmas
Te drops at r/a>0.8 during MHO
During MHO :
Zeff increases at r/a>0.8 , and
decreases at r/a<0.8
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Relevance to conventional tokamak experiments
Quiescent H-mode in D3D
- edge harmonic oscillations
- are these MHOs at plasma edge
that work as ergodic divertor ?
Current ribbon at q=4 in JET- Can be constructed by choosing bmn
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Why no MHOs in the core of conventional tokamaks?
• δB from unstable tearing
modes resonate at q=m/n to
form magnetic islands.
• Microtearing modes are
unstable from core to edge in
NSTX beam heated plasmas
MHOs from core to edge.
• They are stable in core of
conventional tokamaks
No MHOs in core.
• Microtearing modes can exist in
edge region of conventional
tokamaks.
Conjecture :
• When they appear at edge of
DIIID at quasi steady state
EHOs which reduce Zeff
QH-mode.
• When they appear at edge of
JET current ribbon.
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Multiple harmonics in high-k scattering dataThese data are rare : It requires a big island, and the scattering volume must be at the right place
nth harmonic of island rotating at fϕ produces EM fields in plasma with fn = n fϕ and the plasma will
respond at the same freq fn = n fϕ : Oscillations need not be normal mode - D(ω,k)~0
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NSTXNSTX Meeting name – abbreviated presentation title (last name) Month day, 20**
Summary
MHOs are common in NSTX
• It can happen under many different
plasma conditions.
• It is associated with a rotating
magnetic island with complex
structure - multiple helicity.
• The island location varies from
core to edge.
• They can produce stochastic B and
enhance plasma transport.
• Can cause multiple peaks in high-k
scattering signal.
Is this a special feature of ST?
• The cause for this behavior in NSTX
may be due to microtearing modes.
• Microtearing modes in NSTX beam-
heated plasmas can be unstable
from core to edge, but only at the
edge in conventional tokamaks.
• EHOs in DIII-D QH-mode may be a
rotation island with multiple helicity
at the plasma edge. Appropriate
values of bmn at q=4 can lead to
current ribbon in JET.
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