Quasi-Coherent Fluctuations Limiting the Pedestal Growth on Alcator - - PowerPoint PPT Presentation

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Quasi-Coherent Fluctuations Limiting the Pedestal Growth on Alcator - - PowerPoint PPT Presentation

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Quasi-Coherent Fluctuations Limiting the Pedestal Growth on Alcator CMod: Experiment and Modeling Ahmed Diallo, PPPL Presented by J.W. Hughes, MIT PSFC M. Greenwald, J. Walk, C. Theiler, J. Canik a ,P. Snyder b , R. Churchill, B.

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SLIDE 1

Ahmed Diallo, PPPL

  • Presented by J.W. Hughes, MIT PSFC
  • M. Greenwald, J. Walk, C. Theiler, J. Canika,P. Snyderb, R. Churchill,
  • B. LaBombard, M.L. Reinke,T. Golfinopoulos, E. Davis, S-G. Baek,
  • I. Cziegler, L. Delgado-Aparicio*, A. Hubbard, J. Terry, A.White,

and the Alcator C-Mod team.

  • Plasma Science and Fusion Center, MIT, Cambridge, MA, USA.

* Princeton Plasma Physics Laboratory, Princeton, NJ, USA.

aOak Ridge National Laboratory, Oak Ridge, TN, USA. bGeneral Atomics, San Diego, CA, USA.

October 15, 2014 IAEA-FEC 2014 St Petersburg, Russia EX/3-2

1

Quasi-Coherent Fluctuations Limiting the Pedestal Growth on Alcator C–Mod: Experiment and Modeling

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SLIDE 2

Objective: Understanding the pedestal structure is crucial for performance prediction of fusion devices

  • Substantial pedestal heights are critical for achieving high fusion power in

ITER

  • Link between pedestal height and global confinement well established by

current experiments, transport modeling

2

1.0 2.0 3.0 4.0 5.0 6.0

Tρ=0.95 (keV)

800 700 600 500 400 300 200 100 0.0 Pfus (MW)

ITER H-mode Paux=30 MW nped=9.0e19 ne(0)/nped=1.1 vφ=0

GLF23 TGLF (s-α) TGLF-APS07 TGLF-09

Q=10

Predicted fusion power vs pedestal temperature at fixed pedestal density

Kinsey Nucl Fus (2011)

Pfusion ∼ P 2

ped

Pped, nped, Tped

0.80 0.85 0.90 Normalized radial coordinate 0.95 1.00

Pedestal

20 40 60 80

L-mode H-mode

slide-3
SLIDE 3

EPED predictive model provides a candidate mechanism for pedestal formation

  • EPED: pedestal structure set by two key limiting instabilities:
  • non-local peeling–ballooning modes (PBM) — trigger for edge-localized mode (ELM)
  • nearly local kinetic ballooning modes (KBM) — regulates transport between ELMs

– Combining these two constraints allows prediction of two unknowns, the pedestal height and width.

3

1.0 0.8 0.6 0.4 0.2 1.0 0.5 Normalized edge current density Normalized pressure gradient

Kink/Peeling Unstable

Ballooning Unstable

Paths to ELM

0.02 0.03 0.04 0.05 0.06 0.07 0.08 5 10 15 20 Pedestal Width (ΔψN) Pedestal Height (pped, kPa) PB unstable KBM unstable Predicted S t a b l e bl

I n t e r

  • E

L M e v

  • l

u t i

  • n

Peeling-Ballooning (PB) diagram EPED model

Connor, PoP (1998); Wilson, PoP (2002); Snyder, PoP (2001); Snyder, NF (2011)

Inter-ELM evolution?

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SLIDE 4

EPED predictive model provides a candidate mechanism for pedestal formation

4

0.02 0.03 0.04 0.05 0.06 0.07 0.08 5 10 15 20 Pedestal Width (ΔψN) Pedestal Height (pped, kPa) PB unstable KBM unstable Predicted S t a b l e bl

I n t e r

  • E

L M e v

  • l

u t i

  • n

EPED predictions compared to experiment EPED model

Can we find signatures of pedestal-limiting mechanisms between ELMs?

Connor, PoP (1998); Wilson, PoP (2002); Snyder, PoP (2001); Snyder, NF (2011)

Hughes, Nuclear Fusion (2013)

Inter-ELM evolution?

  • EPED: pedestal structure set by two key limiting instabilities:
  • non-local peeling–ballooning modes (PBM) — trigger for edge-localized mode (ELM)
  • nearly local kinetic ballooning modes (KBM) — regulates transport between ELMs

– Combining these two constraints allows prediction of two unknowns, the pedestal height and width.

slide-5
SLIDE 5

Theory predicts a sensitivity of KBM growth rate to β — observable between ELMs?

  • Experimental goal: Identify

and characterize turbulent fluctuations during the ELM cycle

  • Expected measurable

characteristics

– Pedestal localized – Intermediate-n and electromagnetic mode – Sudden change in growth rate – Ion spatial scale (kρs< 1) – Propagates in ion diamagnetic direction.

5

Belli and Candy Phys. Plasmas 17, 112314 (2010)

Variations can be captured between ELM cycle

Cyclone base case

  • Norm. growth rate
  • Norm. real freq.

Electron

GYRO

slide-6
SLIDE 6

Experimental collisionality scans are used to access Type I ELMy H-mode

6

Time [s] Line−averaged ne [1020 m-3] 0.5 1 1.5 1 2 3 4 Time [s] 0.5 1 1.5 2 4 6 8 10

Line-av. Density Pedestal collisionality

EDA 1120815008 ELMy 1120815026 ELMy 1120815026 vis Dalpha [W/m2/sterad] 2 4 6 8 10 12 4

ELMy 1120815026 EDA 1120815008 Dalpha

Transition from enhanced Dα (EDA) H-mode to ELMy H-mode

  • ccurs around 𝝃*~1
slide-7
SLIDE 7

Radially resolved profiles may be either averaged over ELMs or binned by phase of ELM cycle

ELMy H-mode

7

R[cm]

80 82 84 86 88 90

Temperature [keV]

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Te Thomson 1120815026 800−1128 ms 67−100 % ELM cycle Ti (2 mm radial shift) CXRS 1120815025 time = 1.1054 s Ti XCS equiv. 1120815026 time = 1.0785 s Edge Te from ECE

Temperature [keV]

  • ELM crash induces fast drop in Te and measurable rebuild time
  • ELM perturbation to density is weaker
  • Pressure evolution is a test bed for KBM onset

Hughes, Nuclear Fusion (2013)

Ti = Te

slide-8
SLIDE 8

Various poloidally separated diagnostics provide edge fluctuation measurements between ELMs

8

LSN - 2MW ICRF heated ELMy discharges

B magnetic probe ~

O-mode Reflectometer Gas-Puff Imaging (GPI)

Phase-Contrast Imaging (PCI)

Local electron density fmuctuations Proxy local density fmuctuations Line-averaged electrons density fmuctuations magnetic fmuctuations

Line-integrated electron density fluctuations

slide-9
SLIDE 9

ELMy H-mode

100 200 300 400 500 600

Frequency[kHz]

0.01 1 10 intensity[(1016 m-2)2/kHz] 1.12 1.13 1.14 1.15 1.16 1.17

time[s]

0.3 0.4 0.5 0.6 0.7 Dα (AU)

1120815026, ch = 15 (R[m] = 0.687)

D (AU)

Quasi-coherent fluctuations (QCF) are observed

  • n phase contrast imaging (PCI) spectrogram

PCI provides an estimate the radial component wavevector kR ➠ kθ when mode is edge localized

9

New mode

  • 10
  • 5

5 10 200 400 600 800 1000 Frequency[kHz] 11208156026 time[s] = 1.145

R

slide-10
SLIDE 10

Signatures of the QCF have been observed on gas puff imaging (GPI) between ELMs

  • QCF is coherent in frequency and wavenumber
  • Propagates in the electron direction in the lab frame

10

  • 6
  • 4
  • 2

2 4 6 100 200 300 400 500 Frequncy[kHz]

  • 6
  • 4
  • 2

2 4 6

  • 6
  • 4
  • 2

2 4 6 GPI wave number spectra between ELM

1207 - 1211 ms 1211 - 1215 ms 1215 - 1219 ms

1140826025

QCF

ELM cycle

km /s rad/cm

slide-11
SLIDE 11

GPI indicates strong radial localization of QCF

11

88 89 90 91 92 R (cm)

  • 0.5

0.0 0.5 1.0 1.5 2.0 δI/I0 (%) nominal LCFS

QCF radial width ~ 0.5 cm

Uncertainty in LCFS position

Fluctuations in measured intensity (proportional to δn/n) vs. radius

slide-12
SLIDE 12

O-mode reflectometry localizes the QCFs to the sub- centimeter scale density pedestal

12

1.2

Time [s] Pedestal top 112 GHz

(b)

Steep gradient region 88 GHz

(c)

Steep gradient region 75 GHz Steep gradient region 60 GHz

(d) (e)

ψn Electron Density [1020 m−3] 0.2 0.4 0.6 0.8 1 1.2 0.5 1 1.5 2 2.5 3 ne Thomson 1120815026 time = 0.89998 − 1.12 s 112 GHz 88 GHz 75 GHz 60 GHz

(a)

Inter-ELM Electron density

Typical pedestal width = 0.5 cm

Frequency [kHz] Frequency [kHz] Frequency [kHz]

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SLIDE 13

Inter-ELM magnetic fluctuations track the edge electron temperature

  • ECE shows prompt drop in Te.
  • Each ELM event is followed by period of the

pedestal-Te increase and then saturation

  • Similar Te dependence with washboard

modes on JET

  • Mode turn on is correlated with the pedestal

saturation

  • β-limit is consistent with the expected KBM
  • r microtearing growth rate dependencies

13

0.26 . 2 2 0.18 0.14 1.11 1.12 1.13 1.14 1.15 Time [s] Integrated spectral power [200 -500] kHz Magnetic fuctuations spectrogram Magnetic fuctuations spectrogram 2.1 2.3 2.5

1120815027

(a) (b) (c) (d) (e) Diallo, PRL (2014)

Perez, PPCF 2004

slide-14
SLIDE 14

Quasi-coherent fluctuations are low kθ and propagate in electron diamagnetic direction (lab frame)

  • kθρs= 0.04 , n=10
  • Two-point correlation using a double-head magnetic

provides the wavenumber and propagation direction

14

  • 4
  • 2

2 4 200 400 600 800 1000

  • 3
  • 2
  • 1
  • 4
  • 2

2 4 200 400 600 800 1000

  • 2
  • 2

2 Poloidal wavenumber k [rad/cm]

S(f,k) spectum

1120815027

t=1.1259s

Frequency [kHz]

30 km/s

Wavenumber Spectrum vs. Time 1.11 1.12 1.13 1.14 Time [s] 1 2 3 4

Poloidal wavenumber [rad/cm]

1120815027

Power weighted spectra

slide-15
SLIDE 15

Wavenumbers from various diagnostics consistent with field-aligned perturbation

15

4 3 2 1

  • 0.4
  • 0.2

0.0 0.2

  • 0.4

Edge channel PCI at bottom Edge channel PCI at top GPI magnetic probe

slide-16
SLIDE 16

Pedestal-localized fluctuations are consistent with an ion mode, localized to Er well

  • Width of pedestal, width of well in radial electric field ~ millimeters
  • Uncertainty in flux surface mappings between poloidally separated diagnostics is of

similar scale!

  • Ongoing work to obtain accurate mapping of fluctuation radial location onto plasma

flow profile

  • Localization in the deepest part of the Er well would imply fluctuations propagating

in the ion direction

16

VE×B [km/s] r − rlcfs[cm]

E × B velocity

−2.5 −2 −1.5 −1 −0.5 0.5 −60 −40 −20 20

  • 4
  • 2

2 4 200 400 600 800 1000

  • 3
  • 2
  • 1
  • 4
  • 2

2 4 200 400 600 800 1000

  • 2
  • 2

2 Poloidal wavenumber k [rad/cm]

S(f,k) spectum

1120815027

t=1.1259s

Frequency [kHz]

30 km/s

Separatrix

Magnetic probe

slide-17
SLIDE 17

ELITE calculations indicate that the experimental point is near both the nominal PBM and KBM thresholds

17

Stability boundary for KBM stability

Stable Unstable

γ/(ω∗/2) and ∞−n ballooning contours

Ballooning

slide-18
SLIDE 18

GS2 linear stability predicts low kθρs < 0.2 mode propagating in the ion diamagnetic direction (plasma frame)

18

Magnetic shear 0.0 0.04 0.08 0.12 |β'| 10 12 14 16 18 20

0.02 0.97 1.92 2.87 3.82 4.77 5.73

0.00 0.04 0.08 0.12 |β'| 10 12 14 16 18 20

  • 3.86
  • 2.94
  • 2.01
  • 1.09
  • 0.17

0.76 1.68

Magnetic shear

γ [cs/a] Growth rate ωr [cs/a] real freq.

e

stable

0.0 0.5 1.0 1.5 2.0 γ (cs/a) 0.0 0.2 0.4 0.6 0.8 kθρs

  • 8
  • 6
  • 4
  • 2

ωr (cs/a)

consistent with experiment Electron mode Ion diamagnetic direction electron diamagnetic direction

Operatiing point

Pressure gradient Pressure gradient

slide-19
SLIDE 19

Experiments on C-Mod show evidence of QCF contributing to the pedestal dynamics between ELMs, suggestive of

  • Inter-ELM fluctuation measurements on C-Mod show onset of quasi-coherent

density and magnetic fluctuations, localized to pedestal – frequency of approximately 300 kHz and spatial poloidal scale kθρs ~ 0.04 – electron diamagnetic propagation in lab frame; possibly ion-directed in plasma frame

  • Results clearly show that the QCF is pedestal localized; its onset at a critical edge

pressure (or ∇p) is suggestive of the kinetic ballooning mode (KBM) – onset and saturation of this mode simultaneous with plateau in pedestal Te

  • Linear GS2 calculations indicate the most unstable mode is edge localized with kθρs

=0.03 and has KBM characteristics, consistent with experiment

  • Open questions and further investigations

– Why the relative coherence of the fluctuations? – Can we get at the transport driven by these fluctuations? – Can we improve our understanding with time-resolved profile evolution?

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