Developing predictive capability for the H-mode pedestal ...€¦ · J.W. Hughes et al. 52nd APS-DPP, Chicago IL, 8—12 Nov 2010. TP9.0060 2 Predictive capability for the H-mode

Developing predictive capability for the H-mode pedestal:

Experimental contributions from Alcator C-Mod

J.W. Hughes, A. Dominguez, M. Greenwald, A.E. Hubbard, Y. Ma, J. Terry, N. Tsujii, G.

Wallace, D. Whyte, S. Wolfe MIT PSFCR.J. Groebner, P.B. Snyder, General

Atomics52nd Annual Meeting of the APS Division of

Plasma PhysicsChicago, IL

11 November 2010

J.W. Hughes et al. 52nd APS-DPP, Chicago IL, 8—12 Nov 2010. TP9.0060 1

Abstract

Experiments on Alcator C-Mod characterize the edge pedestal in several high confinement regimes, as part of a community experimental and theoretical effort to improve predictive capability for the pedestal. This is the subject of the FY11 FES Joint Research Target existing among the major domestic fusion devices and theoretical groups. Pedestal structure in both transiently evolving and stationary enhanced Dα (EDA) H-modes is studied and examined alongside edge fluctuations (e.g. quasi-coherent modes), allowing us to seek evidence for transport-driven mechanisms limiting pedestal width and gradients. The pedestal in EDA H-mode is altered with the application of lower hybrid waves, apparently as a result of enhanced edge particle transport. This effect has been extended to discharges which are inaccessible to propagation of the LH through the core plasma. The pedestal structure in H-modes with edge-localized modes (ELMs) is likely to be governed both by transport and by intermittent ELM relaxation. ELMy H-modes are diagnosed and used to test directly models for pedestal height/width, e.g. EPED.

This work is supported by USDoE award DE-FC02-99-ER54512.

A copy of this poster will be available at http://www.psfc.mit.edu/research/alcator/pubs/APS/index.html

http://www.psfc.mit.edu/research/alcator/pubs/APS/index.html


Predictive capability for the H-mode pedestal is the subject of the FY11 JRT

(1) Improve the understanding of the physics mechanisms responsible for the structure of the pedestal and compare with the predictive models described in the companion theory milestone. Perform experiments to test theoretical physics models in the pedestal region on multiple devices over a broad range of plasma parameters (e.g., collisionality, beta, and aspect ratio). Detailed measurements of the height and width of the pedestal will be performed augmented by measurements of the radial electric field. The evolution of these parameters during the discharge will be studied. Initial measurements of the turbulence in the pedestal region will also be performed to improve understanding of the relationship between edge turbulent transport and pedestal structure.

(2) A focused analytic theory and computational effort, including large-scale simulations, will be used to identify and quantify relevant physics mechanisms controlling the structure of the pedestal. The performance of future burning plasmas is strongly correlated with the pressure at the top of the edge transport barrier (or pedestal height). Predicting the pedestal height has proved challenging due to a wide and overlapping range of relevant spatiotemporal scales, geometrical complexity, and a variety of potentially important physics mechanisms. Predictive models will be developed and key features of each model will be tested against observations, to clarify the relative importance of various physics mechanisms, and to make progress in developing a validated physics model for the pedestal height.

Statement of FY11 FES Joint Research Target:

This poster highlights C-Mod research which supports this milestone.


C-mod edge diagnostic set emphasizes mm-resolution profiles, fluctuations

• Thomson scattering (Te, ne)• CXRS (TI, vIθ, vIφ, Er)

– Inner wall toroidal views (passive and gas-puff assisted)

– Pedestal beam-based CXRS (toroidal and poloidal views)

• Scanning Mach probes, HFS+LFS (Te, ne, v)

• Electron cyclotron emission (Te)• Visible bremsstrahlung (neZeff1/2)• Soft x-rays (nI)• Neutral emissivity measurements• Reflectometer (ne fluctuations, localized)• Phase-contrast imaging (ne flucts, chord-

integrated) • Gas puff imaging (ne flucts, 2D localized)

Distance from LCFS (cm)-3 -2 -1 0 1

Te

Er

L-modeI-modeH-mode

kV/m

-20

20

0

40

-40

0.2

0

0.4

0.6

0.8

R.M. McDermott, APS-DPP 2008


We highlight C-Mod progress toward the topics of the FY11 JRT • Assessing

predictions of EPED for pedestal height, width in ELMyELMy HH--modemode– Clear contact point

with other devices• Small or no-ELM

regimes are also studied– EDA H-mode

• Saturation of fluctuations, pedestal gradients in H-mode evolution• Modification of the pedestal with lower hybrid waves

– I-mode physics• Not covered here; See Hubbard, PI2.6.

0.1 10

200

400

600

800

1000

Te,9

5 (

eV)

30.3

pedestal collisionality ν*95

I-mode

L-mode

H-mode: ELM-free

H-mode: ELMy

H-mode: EDA

D.G. Whyte et al., Nucl. Fusion 50 (2010) 105005.

ELMy H-mode studies and comparisons to EPED


EDA/small ELMs replaced by larger more regular ELMs with atypical shaping shown

10801300071080304029

Line-averaged density

ICRF power

Pedestal Te

D-alphaδlower>0.75δupper~0.15

R. Maingi et al., submitted to Nucl. Fusion (2010).


Density reduction crucial for large ELM access

• ELMs observed with this shape over 3.1


Increased operation in ELMy H-mode: new regime for pedestal width physics

• Pedestal width nearly invariant under typical C-Mod operating conditions (EDA/ELM-free)– No apparent scalings with

ρφ, ρθ, βpol– Does not scale with neutral

fueling depth– Indications that magnetic

shear plays a role • However . . .

• Width scaling in Type I ELMy H-mode is consistent with βpol1/2 scaling – consistent with DIII-D results– Consistent with qualitative expectations from kinetic

ballooning mode arguments

DIII-D scaling


ELMy H-modes on C-Mod used to test and improve EPED model for pedestal

• EPED (P. Snyder) determines a maximum pressure pedestal height and width in ELMingplasmas– Peeling-ballooning stability

constrains the height– A βpol,PED dependence

constrains the width (calculated from kinetic ballooning mode stability arguments in EPED 1.6)

• Experiments ongoing to test EPED predictions • 2009 pedestal data used to validate improvements to EPED• EPED derived pressure limit not necessarily valid in the EDA

regime with small ELMs, but its performance is being tested

0 10 20 30 40 500

10

20

30

40

50

EPED1.x Predicted Pedestal Height (kPa)

Measu

red P

edest

al H

eig

ht (k

Pa)

EPED1EPED1.5EPED1.6


C-Mod contribution extends EPED predictions to higher pedestal pressure

100 101 102100

101

102

EPED1.6 Predicted Pedestal Height (kPa)

Mea

sure

d P

edes

tal H

eigh

t (kP

a) DIII-D (pedscale + ITER demo)JET (rhostar)C-Mod (1090914)ITER prediction (base)

• C-Mod has obtained record plasma pressures in EDA H-mode• ELMy H-mode pedestal pressure within a factor of ~3 of ITER

requirement for Q=10 scenario

Snyder et al. IAEA (2010)


Very recent experiment (26 Oct) extend IPscan for EPED1 predictions

• Pressure pedestal continues to be well predicted over the range of 650—950kA

• Why the discrepancy at higher current? Closer examination needed

600 700 800 900 1000 1100

Plasma Current (kA)

0

10

20

30

40

50

60

Pe

de

sta

l He

igh

t (k

Pa

)Measured EPED1.6 Model

New measurements

?

Preliminary results


Status and plans for ELMy H-mode studies

• What is known:– Small ELMs favored by operating in EDA H-mode and obtaining

typically βN>1.3, Te,95>0.5keV– Shaping (triangularity, magnetic balance) plays a role in

regulating pedestal transport• Affects pedestal parameters obtained at given machine parameters• Magnetic geometry could play a role in determining ELM quality,

non-linear evolution– Larger, but still weakly perturbing ELMs obtained in atypically

shaped plasmas in which quasi-coherent modes are suppressed– ELITE calculations suggest that ELMy regimes on C-Mod are

near peeling-ballooning boundary; for larger ELMs EPED model projects pressure pedestal correctly within uncertainties

• Outstanding questions– How is peeling-ballooning stability boundary affected by

improved models for diamagnetic stabilization?– Is linear stability sufficient to explain onset conditions for both

small and large ELMs?

EDA studies


Studying pedestal limiting phenomena in EDA H-mode (fluctuations structure)• Studies in C-Mod suggest a critical pressure gradient

in the H-mode pedestal• The quasi-coherent mode (QCM) is a candidate

mechanism for limiting the pressure gradient • Goal of experiment: obtain high quality, temporally-

resolved pressure profiles to study this idea• Determine if time rate of change of pedestal pressure

gradient is dramatically reduced when QCM mode turns on

• Experiment complementary to a study in DIII-D for evidence that kinetic ballooning modes limit the pressure gradient [Yan, GI2.4]


EDA H-mode: Edge transport continuously regulated by QCM --- does it limit grad-p?

Quasi-coherent mode(electromagnetic)(edge-localized)


QCM amplitude has been correlated with enhanced particle transport in pedestal

M. Greenwald et al., Fusion Sci. Technol. 51 (2007) 266.


Database analysis suggests that QCM is linked to higher pressure gradient

J.W. Hughes et al. PoP9 (2002)

J.W. Hughes et al. APS 2006


Time evolution of pedestal in ELM-free H-mode (no QCM!!)

J.W. Hughes et al. APS 2006


Experiments were designed to study pedestal and H-mode evolution

• Set up discharge which develops a robust QCM, evolving for 10s of ms prior to saturating

• Multiple repeatable discharges used to build up record of temporal evolution of pedestal profiles during growth and saturation of QCM

• Inherent jitter in L-H transition time (many ms) timing useful to obtain time history

• Obtained a number of nearly identical H-modes at each target IP– 0.65, 0.8 and 1.0 MA

5.4T, 0.8MA

ne,95

ne


Typical evolution of pedestal & QCM

• Plasma density, edge pressure gradient taken from composite data set at 800kA

• Dα, PCI fluctuations taken from a characteristic H-mode in the data set

• Clear “turn-on” of QCM typically comes 10—20ms after L-H transition, similar to roll-over time of pedestal grad-p

• Continued QCM amplitude rise correlated with further increase in grad-p?

ne


PCI characterization of QCM evolution: a preliminary look

• Distinct broadening of mode as IPincreased• Low amplitude in intensity fit for

1MA cases may be an artifact of the analysis

• Correlation of these mode characteristics with pedestal parameters is a work in progress

Fits of QCM spectrum used to determine frequency, amplitude trends with time

t – tL-H (s)


Questions to ask (no good answers yet)

• Does power in QCM correlate with pressure gradient?

• Is there a separate identifiable density gradient criterion for QCM turn-on?

• Does the access condition for QCM help determine the IP-scaling of the pedestal?

• What are the dynamics of pedestal growth before QCM turn-on?

• Can modeling teach us about the drive terms for the QCM, and associated transport?– M3D simulations beginning (L. Sugiyama)– Exploration with BOUT?

Pedestal studies with lower hybrid


C-Mod exploring means of actively modifying pedestal structure: e.g. LHRF

(a)

PICRFPLHRF

(b)

(c)

(d)

Aux.

Pow

er (M

W)

Cen

tral T

e (ke

V)

WM

HD

(kJ)

time (s)0.6

0

1

2

1

0

2

1.0

1.5

2.0

2.5

3

20

40

60

0.8 1.0 1.2 1.4 1.6

n e (1

020 m

-3)

1080

3060

13

(a)

(b)

(c)

(d)

ψφ1/20 1.00.80.60.40.2

n e (1

020 m

-3)

0

1

2

3

T e (k

eV)

0

1

2

3

n e (1

020 m

-3)

0

1

2

ψφ1/20.80 1.00 1.050.950.900.85

T e (k

eV)

0.0

0.2

0.4

0.6

ICRF only

ICRF + LHRF

• Basic phenomenology established from a small set of EDA discharges in 2008 (mostly 600kA, some 450kA)

– Relaxation in density pedestal, increase in Tped– Prompt effects on SOL density, pedestal toroidal flow;

significant LH power dissipated in edge– Finite damping in core – how important is this to sustaining the

pedestal “pump-out”?

J.W. Hughes et al. NF 50 (2010) 064001

C-Mod LHCD Posters TP9.77—83


PIC

RF

(MW

)P

LHR

F (M

W)

WM

HD (k

J)

t - tLH,on (s)

n|| = 1.9

n|| = 2.3

T e,0

(keV

)Li

ne-in

t. n e

(1

020 m

-2)

• EDA H-mode density reduced to similar plateau during LH flattop

• Lower LH power was used with same (perhaps improved) effectiveness

• Improved coupling was obtained with reduced n|| in the newer experiment– Provided immediate test of

whether core accessibility matters (it doesn’t)

Successfully reproduced H-mode modification with upgraded LH launcher


Basic phenomenology of pedestal modification reproduced• Density gradient

relaxation, combined with Te,ped boost

• Edge pressure not degraded

• SOL densities appear higher than in previous experiment --- both before and after LH turn on

R - RLCFS (mm)

Te (keV)

pe (kPa)

ICRF only ICRF + LHRF

ne (1020m-3)


In contrast to older discharges, core plasma was inaccessible to launched LH

Shot = 1100804027 Time = 1.1436 [s]

0.65 0.7 0.75 0.8 0.85 0.9

1.2

Major Radius [m]

Shot = 1080306013 Time = 1.3102 [s]

0.65 0.7 0.75 0.8 0.85 0.9

1.2

Major Radius [m]

ICRF + LHRF 1.41.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

Shot = 1100804027 Time = 0.97692 [s]

n || c

rit

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

Shot = 1080306013 Time = 1.0103 [s]

n || c

rit

n|| = 2.3 n|| = 1.9

ICRF only ICRF onlyICRF + LHRF

2

2

2

2

2

2

||,|| 1ce

pe

ce

pepicritnn ω

ωωω

ωω

+−+=>Wave accessibility criterion:


Does effect scale with LH power? Scanned PLHRF in 600kA EDA H-modes

• Effect previously observed over narrow range of PLH (mostly 800—950kW)

• In new experiment, flattop LH power was varied by a factor of ~5x

•• Initial Initial dn/dtdn/dt, final n, fairly insensitive to PLH, down to ~300kW

• Reduced pump-out rates seen at 120—200kW

• Shot with PLH=120kW had an early back-transition to L-mode, ~200ms after LH turn-on

• Control shot with PLH=0 was not obtained

PIC

RF

(MW

)P

LHR

F (M

W)

WM

HD (k

J)T e

,0 (k

eV)

Line

-int.

n e

(102

0 m-2

)

t - tLH,on (s)


H-mode formation with and without LH can be compared as well

• Raising IP from 600 to 800kA gave shorter, more radiativeH-modes (though still with QCMs)

• Deteriorating boronizationlikely partly to blame

• Even so, able to show that with LH on, rate-of-rise of density in H-mode is reduced relative to cases with LH off

PIC

RF

(MW

)P

LHR

F (M

W)

WM

HD (k

J)

t - tLH,on (s)

T e,0

(keV

)Li

ne-in

t. n e

(1

020 m

-2)


Summary of 2010 lower hybrid results in EDA H-mode• Established H-mode/pedestal pump-out with LH• Demonstrated that core wave accessibility is unnecessary• Determined that effect is insensitive to total LH power, at or

above about 300kW• Began to extend study to higher plasma current

– LH showed promise at reducing density buildup during H-mode formation

– Needs to be examined again in more EDA-like targets– Extending effect to IP>1MA could give less collisional, higher-performing

H-modes at low q95• Dependencies left to address

– Net power flux through pedestal• Prior results showed that increased Prad eliminated nped reduction• One gets this naturally with changing wall conditions + injections• Improved range in ICRF power scans would be beneficial

– Co- vs. counter-IP vs. heating phasing• What is the role of toroidal momentum input?

– Sensitivity to magnetic topology, particle removal rate• Would like to utilize density control in LSN as well


Planned experiments and modeling efforts for FY11• Peeling/ballooning mode stability

– Evaluate P/B stability with ELITE in ELMy/EDA H-modes, I-modes (P. Snyder)– Continue experiments in ELMy H-mode to clarify βp scaling of pedestal, shape dependence of stability

• Kinetic ballooning modes and other continuous pedestal regulation mechanisms

– Additional tests of EPED1.6 over a greater range of βp, shaping scans --- more experiments next week! (P. Snyder)– Can we look for KBM signatures with reciprocating magnetics probe? Evaluate stability to

KBM numerically?– Continued analysis of gradient saturation in EDA H-mode and I-mode– QCM excitation by dedicated antenna, ICRF amplitude modulation– M3D and other simulations of QCM (L. Sugiyama)

• Enhanced fluctuation characterization with gas puff imaging– Role of ExB shear in turbulence suppression– Zonal flows?

• Tests of models– Paleoclassical (J. Callen): Examine profile scale lengths for trends consistent with theory;

estimate and compare transport coefficients in pedestal – Role of neutral fueling in pedestal formation: Use extensive Lyα arrays, including upgrades

planned for the winter up-to-air. 2D source modeling?– Particle pinches: Interpretive analysis of pedestal evolution data obtained in EDA H-modes– Neoclassical vs. anomalous transport: XGC0 being used to model EDA H-mode in

interpretive manner; Will likely apply to ELMy H-modes as well. (Chang, Pankin)


Leave your e-mail address if you want a copy of this poster:

A copy of this poster will be available at http://www.psfc.mit.edu/research/alcator/pubs/APS/index.html

http://www.psfc.mit.edu/research/alcator/pubs/APS/index.html

Developing predictive capability for the H-mode pedestal: Experimental contributions from Alcator C-Mod AbstractPredictive capability for the H-mode pedestal is the subject of the FY11 JRTC-mod edge diagnostic set emphasizes mm-resolution profiles, fluctuationsWe highlight C-Mod progress toward the topics of the FY11 JRT ELMy H-mode studies and comparisons to EPEDEDA/small ELMs replaced by larger more regular ELMs with atypical shaping shownDensity reduction crucial for large ELM accessIncreased operation in ELMy H-mode: new regime for pedestal width physicsELMy H-modes on C-Mod used to test and improve EPED model for pedestalC-Mod contribution extends EPED predictions to higher pedestal pressureVery recent experiment (26 Oct) extend IP scan for EPED1 predictionsStatus and plans for ELMy H-mode studies EDA studiesStudying pedestal limiting phenomena in EDA H-mode (fluctuations structure)EDA H-mode: Edge transport continuously regulated by QCM --- does it limit grad-p?QCM amplitude has been correlated with enhanced particle transport in pedestalDatabase analysis suggests that QCM is linked to higher pressure gradient Time evolution of pedestal in ELM-free H-mode (no QCM!!)Experiments were designed to study pedestal and H-mode evolutionTypical evolution of pedestal & QCMPCI characterization of QCM evolution: a preliminary lookQuestions to ask (no good answers yet)Pedestal studies with lower hybridC-Mod exploring means of actively modifying pedestal structure: e.g. LHRF Successfully reproduced H-mode modification with upgraded LH launcherBasic phenomenology of pedestal modification reproducedIn contrast to older discharges, core plasma was inaccessible to launched LHDoes effect scale with LH power? Scanned PLHRF in 600kA EDA H-modesH-mode formation with and without LH can be compared as wellSummary of 2010 lower hybrid results in EDA H-modePlanned experiments and modeling efforts for FY11Leave your e-mail address if you want a copy of this poster:

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Developing predictive capability for the H-mode pedestal ...€¦ · J.W. Hughes et al. 52nd APS-DPP, Chicago IL, 8—12 Nov 2010. TP9.0060 2 Predictive capability for the H-mode