Presented by Martin GreenwaldC-Mod PAC Feb. 6-8, 2008

MIT – Plasma Science & Fusion Center

C-Mod Core Transport Program

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Practical Motivations for Transport Research

• Overall plasma behavior must be robustly predictable

– Could we design Demo based on empirical scaling of τE and PLH?

– External controls are diminished - self heating, Bootstrap, CD dominate

• All transport channels are important and must be understood

– In a reactor electrons and ions are coupled

– Density profile set by transport, not sources

– Rotation profile mainly set by transport not sources

• Transport Barriers must be predictable and controlled

– Impact on fusion gain and, through profiles, are important for stability and bootstrap current

• Note: Strong physics coupling to pedestal, edge and SOL transport (We stress programmatic connections)

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How Do We Take Advantage of C-Mod Characteristics to Best Address Critical Problems?

• Exploit unique characteristics

– Higher field, density, (ν*, νeiτE) coupled electrons and ions and Ti ~ Te

– Standard operation with no core particle or momentum source

– Decoupling between density profile and power deposition

• Exploit facility capabilities

– Efficient off-axis current drive for manipulation of magnetic shear

– Diagnostic set: improvements in profile and fluctuation measurements

– Upgraded computer cluster – for local nonlinear GK simulations

• Provide strong support for ITER: dimensionless scaling, etc…

• At the same time: C-Mod exploits multi-institutional strengths of transport program via formal and informal collaboration

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Proposed Major Themes For C-Mod

• Overarching - Model Testing and Code Validation

– Systematic and quantitative comparisons with nonlinear turbulence codes

– Quantitative where codes and models are more mature

◊ Role of magnetic shear

◊ Electron transport

• Particle and Impurity Transport

– How to predict fueling, density profile and impurity content?

– Now within capabilities of gyrokinetic codes

• Self-Generated Flows and Momentum Transport

– How to extrapolate to source-free, reactor-like conditions?

• Internal Transport Barriers

– Access conditions and control, especially in absence of dominant ExB

– Important element in advanced scenarios research

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• Development of predictive model is a key goal for program

– What are the critical elements of the models?

– Requires careful thought about design of experiments, measurements

• Quantitative comparisons will stress more mature topics – drift-wave theories for ion and electron thermal transport

– Deployment of fluctuation diagnostics

– Development of synthetic diagnostics

– Development of appropriate metrics

– Significant priority for run time

Model Testing/Validation

Wavenumber [cm -1 ]

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original GS 2original GS 2

ky spectrum spectrum

New GS 2New GS 2

kR spectrum spectrum

Measured P CI

kR spectrum

Synthetic PCI spectrum shows agreement with experiment. (Ernst et al.)

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• With Te ~ Ti , γ > ωExB , ZEFF << ZI, R/Ln < R/LT; choice of magnetic shear (Ŝ) regime can determine R/LT.

• We can exploit LHCD to allow direct manipulation of shear.

– Test drift-wave models by evaluating change in R/LT, R/Ln and fluctuations as we modify Ŝ

• There is additional work planned on effects of magnetic shear in pedestal and edge using other techniques

Validation Experiments: Role of magnetic shear Exploit LHCD

From linear ITG calculations – IFS-PPPL modelKotchenreuther et al, 1995

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• Can we identify the fluctuations contributing to electron heat transport?

– Diagnostics are critical here – Use PCI with kR up to 50-60 cm-1, spatial localization, separate kr, kθ

– Compare with predictions for mixed scale turbulence

– LH operation + cryopump will lead to more operation at low density, with strong electron heating

• Is there an important magnetic component in turbulence or transport?

– Micro-tearing

– Magnetic flutter

– Measure B fluctuations with polarimeter

Validation Experiments: Test models for electron channel turbulence and transport in low-density regimes

0.00.0 0.50.5 1.01.0 1.51.5<n<ne> (10> (102020)

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Highlights: Self-Generated Flows and Momentum Transport

• Strong, co-current self generated toroidal rotation in H-modes

– Momentum transferred from edge to core (pinch?)

– Significant rotation gradients in torque-free regions

• Strong coupling in L-mode to SOL flows

– Complex L-mode behavior

• Counter-current rotation driven by LHCD

• Similarity experiments with DIII-D

• Multi-machine database assembled and 0-d dimensionless scaling begun

Evolution of velocity profiles following onset of ICRF heating. Changes begin in the edge and “propagate” into the core

Toroidal Rotation Profile EvolutionToroidal Rotation Profile Evolution

0.700.70 0.750.75 0.800.80 0.850.85Major Radius [mMajor Radius [m]

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• Questions Raised by Observations

– Can we understand momentum transport and origin of self-generated rotation?

◊ How is momentum transport driven by turbulence?

◊ Can we get at this at the level of fluctuations?

– How does it extrapolate into reactor regime? (zero torque, low ρ*)

– Will rotation be sufficient to affect micro- or macro-instabilities?

– Can significant flows be driven with RF waves?

• Need for additional theory

• Comparisons will necessarily be qualitative in the near future

Self-Generated Flows and Momentum Transport

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• Major upgrade in profile diagnostic: unprecedented measurements in source-free discharges

• Near-term concentration of FES Joule milestone

• Compare measured self-generated flow profiles and cross-field fluxes with emerging theory and models. Compare fluctuation levels, spectra, correlation lengths and times

• Role of LHH and LHCD in modifying profiles

• Test feasibility of IC and IBW flow drive with mode converted ICRF

Plans: Self-Generated Flows and Momentum Transport

Rotation data from 3rd generation high-resolution x-ray diagnostic

Note VΦ gradient in torque free region

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Highlights: Particle and Impurity Transport

• Peaked density profiles observed in low collisionality H-modes– Confirms results from AUG, JET– Breaks covariance between νEFF and

ne/nG

– Predicts moderate peaking for ITER ne(0)/<ne> ~ 1.4-1.5

– Potential effects on fusion yield, MHD stability and divertor operation need to be explored.

• Density transport in ITBs – Fluctuations compared with ITG/TEM

simulations– Mode spectrum and direction of

propagation suggest TEM responsible for barrier “saturation” increase in particle diffusivity. (consistent with linear-gs2 but not nonlinear-gyro)

Ion direction electron direction

Density fluctuations

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• What is the interplay between various forms of drift-wave turbulence that determines particle transport?

• At the fluctuation level, what is the relation between ion energy, momentum and particle transport?

• What plasma conditions lead to a significant inward pinch and density peaking?

– Collisionality is important controlling parameter – what is the physics?

– What’s the role of magnetic shear?

• What are the conditions in which impurity transport might lead to concentration of impurities and unacceptable radiation levels?

– Connection to heat, momentum and particle transport

– Z scaling of impurity transport, especially for peaked ne profiles

Particle and Impurity Transport

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• Further exploration of peaked density regimes

• Key activity – model testing

– Detailed comparisons of profiles and fluctuations with gk simulations

– Comparisons with Thermodiffusion and Turbulence Equipartitionmodels, mag. shear effects

– Effects of TEM, ITG interplay, strong electron heating, ion-electron coupling

• LHCD: Experiments with Eφ = 0

Plans: Particle and Impurity Transport

Dell

4 feet

Hirex and Other DiagnosticsAre Located Here Under the Rack

Electronics Racksand Control EquipmentOn Two Shelves

Computer for OperatingThe Control Software forLinear Translation and MirrorMovement

Laser

Optical Table(See Slide A)

Large Supports with SomeVibration Reduction

Horseshoe ShapedSupports to Reduce Vibration and HoldMain Vacuum System

Support Arm ForRuffing Pump Shelf

Optical Components(See Slide A)

Ruffing Pump

Vacuum System And Measurement(See Slide C) Main Vacuum

System Components.(See Slide B)

To the Gate Valveand Plasma.

Impurity Injector Setup

-This Diagram Provides a Side ViewOf the Impurity Injection System.-This Setup Goes roughly 2.5 feet Into the Page.

• New laser blow-off system for impurity transport

• Multi-pulse laser for multiple injections per discharge

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Highlights: Internal Transport Barrier Physics

Via BT scan, ICRF resonance location is varied. The ITB threshold can be correlated with a decrease in the normalized temperature gradient

ITB

non-ITB

Linear growth rates calculate by gs2 for the same set of shots The ITB threshold is seen to correspond to an expansion of the region of ITG stability

No ITB ITB

ICRF Resonance Location (m)0.68 0.70 0.72 0.74 0.76 0.78 0.80

Investigations of barrier trigger

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Highlights: Internal Transport Barrier Physics (2)

• Barrier strength controlled by application of on-axis ICRF

– Understood through interplay of ITG and TEM turbulence

– Supported by turbulence measurements

• Width of barrier region found to be controlled via field and current: q

• Hysteresis in power deposition profile associated with transition has been characterized

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• How are internal transport barriers accessed?

– Focus on LS, LT, Ln mechanisms (rather than ExB shear)

– Quantitative comparisons with simulations

– Change in fluctuation characteristics

• What is the structure (width, height) of transport barriers?

• Are these predictable from characteristic scales lengths?

• Can we control barrier location or strength?

– Magnetic Shear?

– Effect of rational q surfaces?

Internal Transport Barrier Physics

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• Investigate core Barriers in reactor relevant regime: no core particle or momentum source, equilibrated ions/electrons & current profile:

• Access/trigger conditions in terms of local physics variables

– Use LHCD, trigger via modification of magnetic shear

– Exploit new core profile measurements

• Control of barrier location via q profile

• Barrier transport properties

– Magnetic shear and heating profile effects

– Impurity and particle transport within barrier

– Measurement of core fluctuations – in barrier zone

– Heat and density pulse propagation across barrier

• Integration with advanced scenarios program

Plans: Internal Transport Barrier Physics

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Important Upgrades

• Polarimetry (including J(r), B fluctuations, improved ne profiles and R/Ln )

• Better view for HECE

• Further upgrades to Reflectometry (higher frequency)

• Doppler reflectometry (Velocity fluctuations, zonal flows)

• Improved resolution for beam diagnostics

• Impurity injection system

• New scattering diagnostic for fluctuations, CO2

Diagnostics Are The Key To Transport Research

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• “Utilize upgraded machine capabilities to obtain and test understanding of improved core transport regimes with reactor relevant conditions, specifically electron heating, Te~Ti and low momentum input, and provide extrapolation methodology”

• “Develop and demonstrate turbulence stabilization mechanisms compatible with reactor conditions, e.g. magnetic shear stabilization, shear flow generation, q-profile. Compare these mechanisms to theory.”

• “Study and characterize rotation sources, transport mechanisms and effects on confinement and barrier formation”

• “Quantitative tests of fundamental features of turbulent transport theory via comparisons to measurements of turbulence characteristics, code-to-code comparisons and comparisons to transport scalings”

• “Understand the collisionality dependence of density peaking”

We’re Well Aligned With ITER High-Priority Transport Issues

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Joint ITPA Experiments Currently Planned

Description JOINT Experiments

Notes on C-Mod Contributions

Confinement scaling, ν* scans at fixed n/nG

CDB-4 Initial experiments performed, higher βoperation required

ρ* scaling along ITER relevant path at both low and high b

CDB-8 Will require further development of low density H-modes at high current.

Density profiles at low collisionality

CDB-9 Initial data sets provided, parameter extension required

Impurity transport in peaked density H-modes

Under discussion

Joint experiments under discussion by working group

Scaling of spontaneous rotation with no momentum input

TP-6.1 Exploit improved profile measurements

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Schedule

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• Prediction and control are the ultimate goals of transport studies

– Experiments and theory have progressed to the point where meaningful, quantitative tests are being made.

– Theory/experiment comparisons motivate the experimental program

• C-Mod operates in unique regime in several important respects –crucial for validation of physics models

• Facility Upgrades - important tools for transport research: heating, current drive, particle control, power handling and impurity control.

• Diagnostics – the tokamak is a scientific instrument

– Over the last 5 year period, previous investment in high resolution diagnostics enabled edge studies.

– Lower Hybrid/AT/ program increases overall emphasis on core plasma

– New and planned profile and fluctuation diagnostics will facilitate a wide range of core transport studies

Summary