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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