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 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 density fluctuation spectra[a.u.] 0.5 0.4 0.3 0.2 0.1 New GS 2 k R spectrum original GS 2 k y spectrum 0.0 0 2 4 6 8 Wavenumber [cm -1 ] Measured P CI k R spectrum Synthetic PCI spectrum shows agreement with experiment. (Ernst et al.) 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, R/L n < R/L T ; choice of magnetic shear (Ŝ) regime can determine R/L T. From linear ITG calculations IFS-PPPL model Kotchenreuther et al, 1995 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 Is there an important magnetic component in turbulence or transport? 0.03 Micro-tearing Magnetic flutter τ E (sec) Measure B fluctuations with polarimeter 0.02 0.01 0.00 0.0 0.5 1.0 1.5 <n e > (10 ) 7
Highlights: Self-Generated Flows and Momentum Transport Strong, co-current self generated toroidal rotation in H-modes 100 Toroidal Rotation Profile Evolution 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 Toroidal Rotation Velocity [km/s] 50 0-50 0.83 0.81 0.79 0.77 0.73 0.71 0.75 0.70 0.75 0.80 0.85 Major Radius [m] Evolution of velocity profiles following onset of ICRF heating. Changes begin in the edge and propagate into the core 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 selfgenerated flow profiles and crossfield 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 Rotation data from 3 rd generation high-resolution x-ray diagnostic 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 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) Density fluctuations Ion direction electron direction 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 density regimes Key activity model testing Detailed comparisons of profiles and fluctuations with gk simulations Comparisons with Thermodiffusion and Turbulence Equipartition models, mag. shear effects Effects of TEM, ITG interplay, strong electron heating, ionelectron coupling LHCD: Experiments with E φ = 0 New laser blow-off system for impurity transport Multi-pulse laser for multiple injections per discharge Optical Table (See Slide A) Computer for Operating The Control Software for Linear Translation and Mirror Movement Optical Components (See Slide A) Dell Impurity Injector Setup Electronics Racks and Control Equipment On Two Shelves Hirex and Other Diagnostics Are Located Here Under the Rack 4 feet Laser Large Supports with Some Vibration Reduction Vacuum System And Measurement (See Slide C) Ruffing Pump Support Arm For Ruffing Pump Shelf Main Vacuum System Components. (See Slide B) Horseshoe Shaped Supports to Reduce Vibration and Hold Main Vacuum System To the Gate Valve and Plasma. -This Diagram Provides a Side View Of the Impurity Injection System. -This Setup Goes roughly 2.5 feet Into the Page. 13
Highlights: Internal Transport Barrier Physics Investigations of barrier trigger Via B T scan, ICRF resonance location is varied. The ITB threshold can be correlated with a decrease in the normalized temperature gradient ICRF Resonance Location (m) 0.68 0.70 0.72 0.74 0.76 0.78 0.80 No ITB 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 non-itb ITB 14
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 15
Internal Transport Barrier Physics How are internal transport barriers accessed? Focus on L S, L T, L n 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? 16
Plans: Internal Transport Barrier Physics 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 17
Diagnostics Are The Key To Transport Research Important Upgrades Polarimetry (including J(r), B fluctuations, improved n e profiles and R/L n ) 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, CO 2 18
We re Well Aligned With ITER High-Priority Transport Issues 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 19
Joint ITPA Experiments Currently Planned Description JOINT Experiments Notes on C-Mod Contributions Confinement scaling, ν* scans CDB-4 Initial experiments at fixed n/n G performed, higher β operation 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 Impurity transport in peaked density H-modes Scaling of spontaneous rotation with no momentum input CDB-9 Under discussion TP-6.1 Initial data sets provided, parameter extension required Joint experiments under discussion by working group Exploit improved profile measurements 20
Schedule 21
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 22