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Pedestals and Fluctuations inC-Mod Enhanced Dαααα H-modes

Presented by A.E.HubbardWith Contributions from

R.L. Boivin, B.A. Carreras1, S. Gangadhara, R. Granetz, M. Greenwald,J. Hughes, I. Hutchinson, J. Irby, V. Klein2, B. LaBombard, Y. Lin,E. S. Marmar, A. Mazurenko, D. Mossessian, T. Sunn Pedersen4,

M. Porkolab, J.A. Snipes, J. Terry, S. Wolfe, S. Wukitch, S. Zweben3

and the Alcator C-Mod GroupMIT Plasma Science and Fusion Center

1Oak Ridge National Laboratory2Univ. New Mexico

3Princeton Plasma Physics Laboratory4Columbia University

Research supported by US. Dept. of Energy

APS-DPP MeetingQuebec, Canada23 October 2000

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Pedestals and Fluctuations inC-Mod Enhanced Dαααα H-modes

• Description of EDA H-Mode• Pedestals

– Profiles– Stability– Time evolution

• Quasi-coherent edge fluctuations• Conditions for achieving EDA• Possible physical mechanisms

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C-ModGlobal Features of EDA H-Mode

• EDA H-modes have:– good energy

confinement– Low particle

confinement– Low radiation– No large ELMs– Steady State (>8ττττE)

• Obtained with bothOhmic and RF heating,PRF<5 MW, W<240 kJ.

• Highly attractive reactorregime(no ELM erosion).

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High Resolution Diagnostics ofEdge Pedestals and Fluctuations

Scanning Langmuir probe

Scanning Langmuir probe

Top edge xray array

Outboard edge xray array

(38 chords, dZ=1.1 mm)

(38 chords, dR=1.5 mm)

Edge Thomson scattering(22 points, dZ=2.5 mm)

Helium beam

Reflectometer

(14 pts, dR=3 mm)

( 5 channels)ECE ( 9 & 19 channels){Edge bolo array

Edge vis. brem. array

(20 ch, dR=2 mm)

(dR=1 mm)

Lyman α(20 channels, dR=2 mm)

Phase Contrast Imaging (12 ch, dR=3mm)

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Pedestals are few mm wide, showdifferences in structure, position

• Density width 2-5 mm.• Steepest Te region

(~60 keV/m) few mm wide,Wider region with ~25 keV/m(2-3X core gradient).

• X-ray (impurity) pedestal attop of ne gradient, consistentwith neoclassical pinch.

• Only the impurity width issignificantly wider in EDA vsELM-free; Te, ne pedestalsshow little difference.

• Only impurity width getsnarrower at high Ip.

Temperature

Density

Emissivity

J. W. Hughes UP1.095Thurs. am.

Alcator

C-ModEdge Gradients Challenge MHD Limit

• Edge electron profiles fromhigh resolution Thomsonscattering– assume Ti = Te

• Modeling shows gradients are~30% above the first stabilityballooning limit with onlyohmic current.– Edge bootstrap current

increases stability limit• No Type I ELMs

(PRF≤≤≤≤5 MW, P′≤′≤′≤′≤12 MPa/m)– Small ELMs when βN≥1.2

Major Radius (m)

Te

ne

Plasma Pressure (kPa)

dP/dR (MPa/m)

LCFS

n e (1

020

m-3

)

T e (k

eV)

Pres

sure

(kPa

)P'

(MPa

/m)

D. MossessianUP1.096 Thurs. am.

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Time evolution of Te, ne pedestalsstudied using power ramps

• RF input power continuouslyvariable, ramped slowly upand down.

• Te, ne measured with ms timeresolution by ECE,bremsstrahlung array.

• Strong hysteresis in net P.• H-mode threshold in Tedge is

found.• Te pedestal varies in height

and width with P• ne pedestal independent of P

(above LH threshold).

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Transients at LH transition consistentwith sudden drop of χ in pedestal

• Rate of increase is largestnear top of pedestal.dwped/dt requires ‘blocking’most of the heat flux Q.

• No time delays in pedestalregion, within ms resolution.

• Transients in core are slower,appear diffusive.

• Contributions of n, T arecomparable at moderate ne.

• wped(t)~ t1/2 , well fit by powerbalance analysis.

( ) ( ) ( )( )1/ 2 1/ 2

/0

χ χπ χ χ χ

−= +

+A b

ped pedb A A

Q Aw t w t

B. Carreras

R/a=0.86

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C-ModAt lower density, Te rise dominates

• Density rise is smaller.Measured neutral ionizationrate also ~10x lower.

• Te pedestal higher (900 eV)• Discharge is ELM-free• Pressure rise about the

same, still fit by the sametransport model, χ s.

• At even lower ne, no densitypedestal is formed and theH-mode is not sustained.This might be part of thereason for the ‘low densitylimit’

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Quasi-Coherent Mode seen in DensityFluctuations in EDA H-modes

• Quasi-coherent edgemode always associatedwith EDA H-Mode

• After brief ELM-free period(~20 msec), mode appears

• Frequency in lab framedecreases after onset (→~100 kHz in steady state)– change in poloidal

rotation• Reflectometer localizes

mode to density pedestal

990915007

Reflectometer

Phase Contrast Imaging

Dα

Time (s)

Freq

uenc

y (k

Hz)

Freq

uenc

y (k

Hz)

Inte

nsity

(W

/sr/

m2 )

Y. Lin UP1.094Thurs. am.

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Phase Contrast Imaging measuresk-poloidal ~ 6 cm-1 (λλλλ~1 cm)

kR, cm-1Fr

eque

ncy

(kHz

)-10 -5 0 5 10

200

150

100

50

0

•PCI measures k radially at topand bottom of plasma.

•mainly poloidal component.

•

• Frequency range 60-250 kHz

• Width ∆F/F ~ 0.05-0.2

0.13θ ρ �skA. Mazurenko UP1.105Thurs. am.

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Probe Measurements Confirm ModeDrives Particle Transport

• Langmuir probes see modewhen inserted into pedestal(only possible in low power,ohmic, H-modes)

• Amplitude up to ~50% in n, E• Multiple probes on single head

yield poloidal k~4-6 cm-1, inagreement with PCI– Propagation in electron

diamagnetic direction• Analysis of shows that

the mode drives significantradial particle transport acrossthe barrier, Γ~ 1022 /m2 s

• Plumes from probe gas puffsshow Er < 0 at mode location.(Er > 0 at larger radii). B. Labombard

BI1.006, this session

���n E

Φ�

Γ ∝ ���n E

en1 mm

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C-ModMode Has a Strong Magnetic Component

• Pickup coil added to fast-scanning probe.

• Frequency of magneticcomponent is identical todensity fluctuations.

• implies modecurrent density in the pedestal~10 A/cm2 (~10% of edge j).

• Mode is NOT seen on wall,limiter probes(at least 1000x lower)

J. Snipes

−×� 4B ~ 3 10 T

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Rapid decrease of magnetic modeamplitude with R

• Decays as e -1.6 r

• Consistent withkθ~1.6 cm-1.

• Differs from Type III ELMprecursor, which haskθ~0.5 cm-1, and is seenon limiter probes.

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C-ModConditions Favoring EDA

• EDA formation favored by:– Moderate safety factor

• q95 > 3.5 in D• q95 > 2.5 (or lower) in H

– Stronger shaping• δ=> 0.35

– Higher L-mode targetdensity

• ne > 1.2×1020 m-3

– Clean wall conditions(boronization)

• Seen in both Ohmic and ICRFheated discharges

• Seen with both favorable andunfavorable drift direction.

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C-ModHigher density at L-H favours EDA

Low density, ELM-free Higher density, EDA

•Actual threshold may be in neutral density, local ne or gradient orcollisionality (all are correlated; ν*ped < 1 at low ne, 5-10 at high ne)

• 1.2×1020 m-3 quite low for C-mod. ~0.15 nGW , low ne limit ~0.9 ×1020

enen

αDαD

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A Continuum of QC Mode Amplitude,EDA Particle Transport is seen

• ELM-free to EDA transitioncan be progressive(unlike the L-H transition).

• Weakly enhanced particletransport is seen indischarges near EDAboundary.

• As QC mode strengthincreases:– Rate of rise of ne drops

(more steady state,Deff increasing).

– X-ray ped width (~Dimp)increases.

• Similar trends have beenseen in scans of q, δ.

EF steady EDA

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Comparison with other‘small ELM’ regimes

EDA H-mode shares some characteristics of other steadyregimes without large ELMS.

• Low Particle Confinement regime on JET– Appears similar to EDA, but not easily reproduced.

• Quasi-coherent Fluctuations on PDX– Fluctuations similar to those in EDA, present in short bursts in

most H-modes. Coexisted with ELMs.• Type II or Grassy ELMs on DIII-D, JT60U, Asdex UG

– Conditions in q, δ=very similar to EDA– Similar to small ELMs seen in EDA at high βN?– Does a quasi-coherent mode play a role in these regimes?

• Quiescent H-Mode on DIII-D– Globally similar, but longer wavelength mode, different access

conditions (esp density/neutrals).

Loarte, Snipes et alDO1.004, Mon. pm.

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C-ModPhysical origin of EDA, fluctuations

• Since pedestal profiles are not much different in EDA, ELM-free H-modes, it seems likely to be the mode stability criteria whichchange with q,δ, ν* etc.

• One possibility is that EDA is related to drift ballooning turbulence.Diamagnetic stabilization threshold scales as m1/2/q.A lower q threshold wasfound for EDA in H than D.

• Initial scalings of QC mode characteristics show

• Electromagnetic edge turbulence simulations by Rogers et al haveshown a feature similar to QC mode, with .Gyrokinetic simulations of growth rates (GS2 code) are in progress.

� ρ∆

�s

n

nn

0.1 0.2θ ρ −�sk

2 /θ π ∆� pk

A. Mazurenko UP1.105Thurs. am.

M.Greenwald, JO1.004, Tues pmH. Yuh, UP1.098 Thurs. am.

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C-ModSummary

• EDA H-mode regime routinely combines good energyconfinement and low particle confinement in steadystate, without large ELMs.

• Edge pedestals have few mm widths, gradients abovefirst stable limit; but stable with bootstrap currents.

• Time evolution of n, T indicates large, fast drops in D, χ.

• Quasicoherent pedestal fluctuations in density,potential and poloidal B are a key feature of EDA regime

• Mode has been shown to drive large particle transport.

• Safety factor, shaping and density/neutral pressureare key parameters to obtaining EDA.