C-Mod Transport Program

PAC 2006Presented by

Martin GreenwaldMIT – Plasma Science & Fusion Center

1/26/2006

C-Mod PAC 2006 10/26/2006 M. Greenwald

Introduction – Programmatic Focus

• Transport is a broad topic – so where do we focus?

– Where C-Mod has unique capabilities, runs in unique regimes or observes unique or unusual phenomena.

– τei<<τE, Ti = Te , ν*, no core particle or momentum sources

– Where we can make important comparisons with other devices

• Further tests of “standard” model for ion energy transport

– Dependence on collisionality - ν* and magnetic shear - Ŝ• Studies of other transport channels are of increasing interest

– Momentum – stabilizing effects in low torque, low ρ* plasmas?

– Particles – what will the density profiles be with no core source?

– Electron energy – no ignorable energy channel

C-Mod PAC 2006 10/26/2006 M. Greenwald

Comparisons With Theory And Modeling Form A Critical Part Of The Program

• Prediction and control are the ultimate goals of transport studies.

– Validation of codes is an emerging theme in the transport community

– Experiments and theory have progressed to the point where

meaningful, quantitative tests are being made.

– Increasing activity - synthetic diagnostic development

– Theory plays critical role in motivation and design of most of our

experiments.

• We have close collaborations with theory and modeling groups at

MIT and elsewhere – these will continue.

C-Mod PAC 2006 10/26/2006 M. Greenwald

Transport Themes For 2006-07 Campaigns

• Role of magnetic shear including

– Marginal stability in “conventional” regimes

– ITBs

• Collisionality effects

• Self-generated rotation and momentum confinement

• Core fluctuations including electron transport

• Particle transport

• Pedestals (discussed previously)

C-Mod PAC 2006 10/26/2006 M. Greenwald

LHCD Should Allow Steady-state Control Of Magnetic Shear

• Shear is predicted to be important for ITG, TEM, ETG stability (linear and nonlinear dependences are complicated)

• One of only a very few “free” parameters that (are predicted to) determine R/LT

• Experiment: Test ITG models by evaluating change in R/LT and fluctuations as we modify Ŝ.

Fixe

d q,

saw

toot

hing

disc

harg

es

(Mikkelsen)

0

0.10.1

0.20.2

0.30.3

4 5 64 5 6

IFS-PPPL analytic critical R/LTi

Zeff

= 1.5, s = 1.2^

R/LR/LTiTi

Max

. gro

wth

rat

e (n

orm

aliz

ed)

Max

. gro

wth

rat

e (n

orm

aliz

ed)

Zeffeff

= 1.5= 1.5s=0.6s=0.6^

Zeffeff

= 1.5= 1.5s=1.2s=1.2 ^

GS2 simulations GS2 simulations Electrostatic Electrostatic

C-Mod PAC 2006 10/26/2006 M. Greenwald

HECE With Field Scan Allows Highly Accurate Measurement of LTe

(Phillips)

1010

8

6

4

2

0

a/a/LTeTe

0.900.900.850.850.800.800.750.750.700.70RMIDRMID

1MA 1.9x1020/m3

0.65MA 1.6x1020/m3

0.65MA 0.8x1020/m3

• Crucial for R/LT scaling studies.

C-Mod PAC 2006 10/26/2006 M. Greenwald

Modification of Magnetic Shear Should Broaden ITB, Hybrid-Mode Research Area As Well

• C-Mod internal barriers are apparently created and controlled

through the interplay of R/LT and R/Ln.

– Up til now, ITBs in C-Mod have only a weak effect on

temperature profile – due to electron-ion coupling?

• At very low or reversed shear, growth rates are predicted to be

much lower (also - suppression of sawteeth with q > 1)

– With Ŝ near or below 1, can we create and maintain ITBs with

strong central heating? (and weak ExB flow shear!)

– Simultaneous electron and ion transport barriers?

– Increase core temperature gradient?

– Exploit barrier control opportunities?

C-Mod PAC 2006 10/26/2006 M. Greenwald

Note Recent Results: ITB Foot Location Controlled by BT and IP (magnetic shear effect?)

(Fiore)

• Efficient, off-axis current drive may allow creation of large-volume ITBs

C-Mod PAC 2006 10/26/2006 M. Greenwald

Impact of Collisionality on Transport Has Become An Important Issue

• Physics Issues – nonlinear regulation of turbulence

– ITG - Nonlinear effects through change in electron

dynamics

– Reduction of instability drive (ITG) predicted to be

more important than zonal flow damping?

– TEM – Drives and dissipation? Effects on particle

transport and density profile?

• Previous results at higher collisionality (ν* = 0.2→1)

– BτE ~ ν*-1.0±0.2 in H-mode; BτE ~ ν*-0.4± 0.1 in L-mode

Plasma Profiles

Drift Waves

Zonal Flows

CollisionalDamping

CollisionlessDamping

C-Mod PAC 2006 10/26/2006 M. Greenwald

Cryopump Should Allow Operation at Significantly Lower Collisionality

• At fixed pressure, a small change in density can have a large effect on ν*

• Should provide more overlap with other experiments

• Test predictions of nonlinear simulations – apparent contradiction with high-collisionality results

• Note collaboration w/ McKee – DIII-D

• Zonal flow – pellet diagnostic

(Mikkelsen)

0

20

40

60

80

4 5 6

tota

l hea

t flu

x (n

orm

aliz

ed)

R/LT

Linearcritical

gradients

five times five times lowerlowerν

e & & ν

iactualactualν

e & & ν

i

C-Mod EDA H-mode plasmar/a=0.56

GS2

C-Mod PAC 2006 10/26/2006 M. Greenwald

Test Collisionality Effects on Density Profile – With No Particle Source

• ITER interest – better

fusion performance with

moderate density peaking

• Results from ASDEX, JET

suggest increase in density

peaking at low ν*

• (Most work has significant

beam heating/fueling)

• With cryo-pump, C-Mod

should be able to test in

overlapping rangesAngione et al. – ASDEX-U

C-Mod PAC 2006 10/26/2006 M. Greenwald

Self-Generated Rotation and Momentum Transport

(LaBombard, Rice)

• Rotation important for stabilization

of turbulence and MHD

– But we’re moving toward low

torque, low ρ* regimes (ITER)

• What is the origin and scaling of

self-generated flows?

• What is the role of boundary flows,

neutrals?

• Is there a steep rotation pedestal?

• If so, how is momentum transported

in that region?

-10 0 10

-40

-30

-20

-10

Toro

idal

Vel

ocity

(km

s-1

)Toroidal Projection of Parallel Velocity (km s-1)

Core Ar17+Doppler

ρ = 1 mm Outer Probe

▲USN ▼LSN

Data from Ohmic target plasmas

C-Mod PAC 2006 10/26/2006 M. Greenwald

Momentum Transport Research Plans

(Rice)

• Emphasis will be on

– Multi-machine studies w/ITPA

– Edge-core coupling mechanisms

– Extend range of L-H threshold

studies

– Pedestal rotation profiles and

transport

– Comparison with GK simulations

– (eventually) connection to

fluctuations

C-Mod

DIII-D

JET

Tore Supra

JT60-U

C-Mod PAC 2006 10/26/2006 M. Greenwald

Self-Generated Rotation and The Pedestal

• We need to fill in gap

in measurements

(NeSoX, CXR)

• What is the relation

between pedestal

scaling for Te, ne

and Vφ?

• Does self-generated

rotation contribute to

ExB stabilization of

pedestal?

• Based on (incomplete) current measurements,

we hypothesize that a significant portion of the

toroidal velocity gradient is in the pedestal

(Rice)

1.0 1.1 1.2 1.3 1.4 1.5 1.6t (s)

-20

2

46

V Tor (

104 m

/s)

r/a0.60.60.30.30.0

EDA

0.0 0.2 0.4 0.6 0.8 1.0r/a

02

4

68

V Tor (

104 m

/s)

EDAEDA

C-Mod PAC 2006 10/26/2006 M. Greenwald

Enhanced Core Fluctuation DiagnosticsOpening Up Window To Core Transport

(Lin, Porkolab)

• PCI– Spatial Localization with fast

spatial scan– high k (25 cm-1)– high frequency (5 MHz)

• Reflectometer– Set up all channels for

fluctuation measurements– 140GHz channel = 2.4x1020

– Correlation length measurements

• HECE core ne fluctuations via cutoff

Broadband fluctuations propagate in +R direction, QC mode in –R direction

C-Mod PAC 2006 10/26/2006 M. Greenwald

With Localization, PCI Can Distinguish Core Modes From Edge Fluctuations

(Lin, Porkolab)

• In barrier, QC mode propagates in electron direction (previously noted)

• In core, broadband fluctuations propagate in ion direction

• Large part of this due to Doppler shift from core rotation

• Plasma frame propagation?

• L-mode experiments

H-mode at q = 2.9

C-Mod PAC 2006 10/26/2006 M. Greenwald

Electron Energy Transport

C-Mod PAC 2006 10/26/2006 M. Greenwald

• Experiments have begun with enhanced PCI diagnostic

• Start by looking at linear Ohmic regime

• Some differences seen

• But high k fluctuations do not stand out

• Need to compare to nonlinear simulations

• Comparisons with DIII-D, NSTX discussed

(Lin)

Renewed Interest and Emphasis on Particle Transport

• Poorly understood channel

• ITER – What will density profiles be? Ash removal?

– Collisionality dependence (as noted) ?

– Relative importance of TEM and ITG in particle transport?

• Internal Barrier work will continue

• Gyrokinetic studies with gs2

• LHCD should enable repetition of Tore Supra experiments with no

core source or Ware pinch

• Further transient transport experiments planned

C-Mod PAC 2006 10/26/2006 M. Greenwald

• Supported by recent experiments.

• Barrier saturation and control via TEM destabilization

• ITBs In C-Mod Not Dominated By ExB Stabilization

Barrier formation understood as stabilization of ITG via modification of LT.

(Ernst)

(Zhurovich)

No ITB ITB

C-Mod PAC 2006 10/26/2006 M. Greenwald

Particle Transport in ITB In Quantitative Agreement With TEM Predictions

(Basse)Wavenumber [cm -1 ]

0.5

0 2 4 6 80.0

0.1

0.2

0.3

0.4

dens

ity fl

uctu

atio

n sp

ectr

a[A

.U.]

dens

ity fl

uctu

atio

n sp

ectr

a[A

.U.]

original GS 2original GS 2

ky spectrum spectrum

New GS 2New GS 2

kR spectrum spectrum

Measured P CI

kR spectrum

(Ernst, Long)

• HECE measurements indicate these fluctuations are inside barrier

C-Mod PAC 2006 10/26/2006 M. Greenwald

Studies Of Particle Transport In “Baseline” Regimes

• Dependence of density profile on collisionality

• Compare with thermodiffusion (TTD) and turbulence equipartition (TEP) models

• Comparison with simulations of particle transport.

• Transient transport experiments

• Impurity transport via CXR

• Pinch? With EΦ = 0?

n T qorn T q∇ ∇ ∇

∝

C-Mod PAC 2006 10/26/2006 M. Greenwald

H-mode and Pedestal Work Will Continue To Be An Emphasis of Transport Program

• L/H threshold – connection to SOL flows, theories

• ELM types, effects

• QC mode physics

• Pedestal scaling

• Self-generated rotation – momentum transport in pedestal

• Transition/bifurcation dynamics

• Turbulence spreading? – intermediate gradient region in pedestal

Te profile

C-Mod PAC 2006 10/26/2006 M. Greenwald

Summary

• The C-Mod experiment offers excellent opportunities to advance the state of transport science

– Capable and unique facility

– Strong diagnostic set

– Wide collaborations with theory and modeling

• All three of these components will be improved and expanded.

– Cryopump and LHCD present important opportunities

– Significant upgrades in core profile and fluctuation measurements

C-Mod PAC 2006 10/26/2006 M. Greenwald