C-Mod Core Transport Program Presented by Martin Greenwald C-Mod PAC Feb. 6-8, 2008 MIT – Plasma Science & Fusion Center
Practical Motivations for Transport Research • Overall plasma behavior must be robustly predictable – Could we design Demo based on empirical scaling of τ E and P LH ? – 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) 2
How Do We Take Advantage of C-Mod Characteristics to Best Address Critical Problems? • Exploit unique characteristics – Higher field, density, ( ν *, ν ei τ E ) coupled electrons and ions and T i ~ T e – 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 3
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 4
Model Testing/Validation • Development of predictive model is a key 0 . 5 goal for program N e w GS 2 N e w GS 2 density fluctuation spectra[A.U.] density fluctuation spectra[A.U.] k R s p ec t r u m s p ec t r u m 0 . 4 – What are the critical elements of the o r igin a l GS 2 o r igin a l GS 2 models? k y s p ec t r u m s p ec t r u m 0 . 3 – Requires careful thought about design M eas u r e d P C I 0 . 2 k R s p ec t r u m of experiments, measurements • Quantitative comparisons will stress more 0 . 1 mature topics – drift-wave theories for ion 0 . 0 0 2 4 6 8 and electron thermal transport W ave nu m b e r [ c m - 1 ] – Deployment of fluctuation diagnostics Synthetic PCI spectrum shows agreement with experiment. – Development of synthetic diagnostics (Ernst et al.) – Development of appropriate metrics – Significant priority for run time 5
Validation Experiments: Role of magnetic shear Exploit LHCD • With T e ~ T i , γ > ω ExB , Z EFF << Z I , From linear ITG calculations – IFS-PPPL model R/L n < R/L T ; choice of magnetic shear Kotchenreuther et al, 1995 ( Ŝ ) regime can determine R/L T . • We can exploit LHCD to allow direct manipulation of shear. – Test drift-wave models by evaluating change in R/L T , R/L n and fluctuations as we modify Ŝ • There is additional work planned on effects of magnetic shear in pedestal and edge using other techniques 6
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 k R up to 50-60 cm -1 , spatial localization, separate k r , k θ – Compare with predictions for mixed scale turbulence – LH operation + cryopump will lead to more operation at low density, with strong electron heating 0.04 0.04 • Is there an important magnetic component 0.03 0.03 in turbulence or transport? E (sec) (sec) – Micro-tearing 0.02 0.02 – Magnetic flutter τ 0.01 0.01 – Measure B fluctuations with polarimeter 0.00 0.00 0.0 0.0 0.5 0.5 1.0 1.0 1.5 1.5 > (10 20 20 ) <n e > (10 <n 7
Highlights: Self-Generated Flows and Momentum Transport • Strong, co-current self generated Toroidal Rotation Profile Evolution Toroidal Rotation Profile Evolution toroidal rotation in H-modes 100 100 – Momentum transferred from Toroidal Rotation Velocity [km/s] Toroidal Rotation Velocity [km/s] 0.83 0.83 edge to core (pinch?) 50 50 – Significant rotation gradients in 0.81 0.81 torque-free regions 0 0.79 0.79 • Strong coupling in L-mode to SOL 0.77 0.77 0.73 0.73 0.71 0.71 flows 0.75 0.75 -50 -50 – Complex L-mode behavior 0.70 0.70 0.75 0.75 0.80 0.80 0.85 0.85 Major Radius [m] Major Radius [m • Counter-current rotation driven by Evolution of velocity profiles following LHCD onset of ICRF heating. Changes begin in • Similarity experiments with DIII-D the edge and “propagate” into the core • Multi-machine database assembled and 0-d dimensionless scaling begun 8
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 9
Plans: Self-Generated Flows and Momentum Transport • 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 � Rotation data from 3 rd generation modifying profiles • high-resolution x-ray diagnostic Test feasibility of IC and IBW flow drive with mode converted ICRF � Note V Φ gradient in torque free region 10
Highlights: Particle and Impurity Transport • Peaked density profiles observed in low collisionality H-modes – Confirms results from AUG, JET – Breaks covariance between ν EFF and n e /n G – Predicts moderate peaking for ITER n e (0)/<n e > ~ 1.4-1.5 – Potential effects on fusion yield, MHD stability and divertor operation need to be explored. Density fluctuations • Density transport in ITBs Ion direction electron direction – 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) 11
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 n e profiles 12
Plans: Particle and Impurity Transport • Further exploration of peaked • New laser blow-off system for density regimes impurity transport • Key activity – model testing • Multi-pulse laser for multiple – Detailed comparisons of injections per discharge profiles and fluctuations with gk simulations Impurity Injector Setup – Comparisons with Vacuum System And Measurement (See Slide C) Optical Components Laser Main Vacuum System (See Slide A) Thermodiffusion and Components. (See Slide B) Optical Table (See Slide A) Turbulence Equipartition Dell models, mag. shear effects Horseshoe Shaped Supports to Reduce Vibration and Hold Computer for Operating Electronics Racks Ruffing Pump Main Vacuum System The Control Software for and Control Equipment – Effects of TEM, ITG interplay, Linear Translation and Mirror On Two Shelves Movement Support Arm For Ruffing Pump Shelf To the Gate Valve strong electron heating, ion- and Plasma. Hirex and Other Diagnostics Are Located Here Under the Rack electron coupling Large Supports with Some -This Diagram Provides a Side View Vibration Reduction Of the Impurity Injection System. -This Setup Goes roughly 2.5 feet Into • the Page. LHCD: Experiments with E φ = 0 4 feet 13
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