C-Mod Core Transport Program - file– “Expand the effort to understand the transport of particles...

Presented by Martin GreenwaldC-Mod 5 Year Proposal Review May 7-9, 2008

MIT – Plasma Science & Fusion Center

C-Mod Core Transport Program

Practical Motivations for Transport Research

• Overall plasma behavior must be robustly predictable– Could we design Demo based on empirical scaling of τE and PLH?

– (These are still major uncertainties for ITER)

– 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, for stability and bootstrap current

• Note strong physics coupling to pedestal/edge and SOL transport including coupling via profiles, flows, turbulence (e.g. L-H threshold and density limit) ⇒ integrated studies stressed on C-Mod!

2C-Mod 5 Year Plan Review, May 2008, M. Greenwald

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


Other Program Drivers (1)

• ITER needs, ITPA joint experiments

– Compiled later in this presentation and in spreadsheets

• 2005 Priorities panel - address topical questions T4 and T5

– T4 How does turbulence cause heat particles and momentum to escape from plasmas?”

– T5 How are electromagnetic fields and mass flows generated?

• Priorities Panel recommendations for enhanced funding/activity

– “Carry out additional science and technology activities supporting ITER…”

– “Expand the effort to understand the transport of particles and momentum”

– “Mount a focused enhanced effort to understand electron transport”

– “Predict the formation, structure and transient evolution of edge transport barriers.”


Other Program Drivers (2)

• 2007 Planning panel recommendations – issues to be resolved during “ITER era”

– A1. Measurement: Make advances in sensor hardware, procedures and algorithms for measurements of all necessary plasma quantities with sufficient coverage and accuracy needed for the scientific mission, especially plasma control.

– A2. Integration of high-performance, steady-state, burning plasmas: Create and conduct research, on a routine basis, of high performance core, edge and SOL plasmas in steady-state with the combined performance characteristics required for Demo.

– A3. Validated Predictive Modeling: Through developments in theory and modeling and careful comparison with experiments, develop a set of computational models which are capable of predicting all important plasma behavior in the regimes and geometries relevant for practical fusion energy. (Turbulent transport stressed)


Proposed Major Themes For C-Mod Transport

• 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 research6C-Mod 5 Year Plan Review, May 2008, M. Greenwald

Model Testing/Validation

• Development of predictive model is a key goal for U.S. Fusion 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

Wavenumber [cm -1 ]

0.5

0 2 4 6 80.0

0.1

0.2

0.3

0.4

dens

ity fl

uctu

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ectr

a[A

.U.]

dens

ity fl

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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.)


Validation Experiments: Role of magnetic shear Exploit LHCD

• 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

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


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

• 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


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

0.000.00

0.010.01

0.020.02

0.030.03

0.040.04

τE

(sec

) (

sec)

L. Lin (PhD Student)

Self-Generated Flows and Momentum Transport

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

– Momentum transferred from edge to core

– 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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eloc

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oroi

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Vel

ocity

[km

/s]

0.710.710.730.73

0.750.75

0.770.77

0.790.79

0.810.81

0.830.83

A. Ince-Cushman (PhD Student)


Self-Generated Flows and Momentum Transport

• 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


Plans: Self-Generated Flows and Momentum Transport


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

Note VΦ gradient in torque free region

• Major upgrade in profile diagnostic: unprecedented measurements in source-free discharges (PPPL collaboration)

• 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

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

L. Lin (PhD Student)

Particle and Impurity Transport

• 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


Plans: Particle and Impurity Transport

• 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

• New laser blow-off system for impurity transport

• Multi-pulse laser for multiple injections per discharge


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.

N. Howard (PhD Student)

Highlights: Internal Transport Barrier Physics

Investigations of barrier trigger


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

K. Zhurovich (PhD Thesis)

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

Plans: Internal Transport Barrier Physics

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

• Access/trigger conditions in terms of local physics variables

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

– Use LHCD, trigger via modification of magnetic shear

– Exploit new core profile measurements

– Quantitative comparisons with simulations

– Change in fluctuation characteristics

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

– Are these predictable from characteristic scales lengths?


Plans: Internal Transport Barrier Physics

• Control of barrier location via q profile

– Magnetic Shear?

– Effect of rational q surfaces?

• Measure transport within barrier

– 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


Transport Research Objectives (1)

Research Goal Intermediate ObjectivesRole of electron heating and current drive in modifying self-generated rotation profiles Compare measured self-generated flows, and cross-field fluxes with emerging theories and models Compare fluctuation levels, spectra and correlations with emerging models Test feasibility of IC and IBW flow drive Identify the portion of k space important for anomalous electron heat transport in low density OH plasmas Extend studies to strongly heated (LHH) plasmas at low densities Test models for mixed scale (ion-electron) turbulence

Better Understanding of electron transport in decoupled regimes

Improved understanding of self-generated rotation and momentum transport and extrapolation to future devices with low input torque



Research Goal Intermediate ObjectivesCompare profiles, fluxes, fluctuation levels and correlations with existing gyrokinetic simulation codesStudy of particle transport in regimes without neoclassical pinchQuantitative assessment of the role of magnetic shear in setting density profilesCorrelation of particle transport and ion thermal transport in a variety of confinement regimesInstall impurity injection system and begin experiments Characterize impurity fluxes and their correlation with particle, energy and momentum transport in a variety of confinement regimes. Compare anomalous and neoclassical impurity fluxes

Scaling of impurity fluxes with Z of impurity

Characterize anomalous and neoclassical impurity transport.

Detailed comparisons with models with experimental measurements of particle transport



Research Goal Intermediate ObjectivesValidate modeling predictions for role of ion temperature gradient in ITB onset Test predictions for nature of density fluctuations during ITB, especially after addition of central ICRF Assess roles of magnetic and flow shear in C-Mod ITB Measure ITB transport behavior with respect to impurity diffusion and electron heat pulse propagation Quantitative assessment of the role of magnetic shear in setting critical temperature gradients

Detailed comparisons of ion thermal transport with gyrokinetic models Comparison with particle and momentum transport

channels

Better understanding of access conditions, transition dynamics and control of internal transport barriers

A summary of our experimental approach, new diagnostics and modeling required to meet these objectives can be found in the proposal on pages 3-21 – 3-23


Diagnostics Are The Key To Transport Research

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


We’re Well Aligned With ITER High-Priority Transport Issues(Shimada/ITPA)

• “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”


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 β

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


Schedule


HIREX

Full Power LHCD

Imp Injector

Full Power LHCD

Summary

• 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


C-Mod Core Transport Program - file– “Expand the effort to understand the transport of particles...

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