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Magnetic islands with complex structure in NSTX

NSTX. Supported by. Magnetic islands with complex structure in NSTX. K. L. Wong PPPL and the NSTX Research Team. College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL

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Magnetic islands with complex structure in NSTX

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  1. NSTX Supported by Magnetic islands with complex structure in NSTX K. L. Wong PPPL and the NSTX Research Team College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec 53rd APS-DPP annual meeting at Salt Lake City-UT November 14-18, 2011

  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

  3. 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

  4. 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) For low-n tearing modes, ω << ω’ ~ nVϕ/R Let B =m,nbmn 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

  5. 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

  6. 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,nbmn exp[i(m+n)] When the island has many m, n harmonics, we can also expect a larger region of multiple stochastic layers and separatrix.

  7. 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

  8. Island fΦ approx. corresponds to Δf in Mirnov signal

  9. 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

  10. 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 . . . .

  11. 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)

  12. 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

  13. 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

  14. 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

  15. 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?

  16. 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 atplasma edge (r/a>0.85) due to edge cooling at r/a>0.8

  17. 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

  18. 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

  19. Why no MHOs in the core of conventional tokamaks? 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. • 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.

  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 : Oscillationsneed not be normal mode - D(ω,k)~0

  21. Summary MHOs are common in NSTX 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. EHOsin 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. • 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.

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