In Search of Core Transport

1. In Search of Core Transport D. E. Newman, C. Greenfield and K. Gentle Many thanks to P. Terry and E. Synakowski and all the speakers and participants We apologize profusely to everyone we overlooked (or misrepresented)

2. Areas Discussed • Preview talks • Barriers and Improved Confinement • Basic Transport Predictions and Challenges • Electron transport • Innovative measurements • Transient Transport

3. Preview talks We think the “Experiment” was a success • Pat Diamond- Discussed the issues in what appears to be one of the major open questions - Stressed that no model is well enough understood to be called the dominant candidate … opportunity for much work at all levels • Keith Burrell - Reminded us of the rich variety of transport and MHD phenomena both caused by and leading to flows in fusion plasma experiments. • Bill Nevins - Stressed the importance of proper diagnosis of both experiments and modeling in order to better compare the results and elucidate the physics. Can use models as a proxy for experiment in some cases (might be cheaper sometimes??). • Jack Connor - Stimulated discussion by presenting challenges and opportunities in Burning plasmas.

4. Barriers and improved confinement Progress is being made in the study and characterization and control of enhanced confinement regimes Need both good diagnostics and control knobs • Catherine Fiore, Recent Results of Internal Transport Barrier Studies on Alcator C-Mod - ITB can be triggered using combination of on and off axis ICRF as well as other mechanisms … In enhanced confinement regimes toroidal rotation goes through 0 to cause a neg. radial electric field... rotation profile is assumed based on central measurement results in ExB shearing rate not thought to be large enough for suppression … velocity profile measurements are coming soon … density peaks when barrier forms => Ware pinch dominates when D is reduced Extensive diagnostics were/are essential for the following • Hidenobu Takenaga, Particle and Heat Transport in JT-60U Plasmas with Internal Transport Barrier - Light impurities did not accumulate heavy impurities did .. Low diffusivities … but acceptable for helium exhaust … heavy impurities are a control issue • John Sarff, Improved Confinement in the RFP and The Role of The Magnetic Fluctuation Spectrum - Greatly improved confinement tE increased up to 10 times … basic understanding of how the enhanced confinement is caused (reduction in the overlapping magnetic surfaces) and how to control it using PPCD … MST now on the tokamak ELM-y H-mode scaling

5. Barriers and improved confinement - cont. • Edward Doyle, Progress in Understanding The Physics of the Quiescent Double barrier Regime on DIII-D - Further detailed measurements on impurity transport => similar results to JT-60 … transient modification of Te inside barrier allows density profile to relax which can reduce the impurity concentration … better understanding of EHO and how it regulates particle transport at the edge • David Newman, Creation and Dynamical Co-Evolution of Electron and Ion Channel Transport Barriers - Added electron dynamics to a dynamic transport model to investigate barriers in both channels and their interaction and initiation

6. Electron transport Progress is being made but many opportunities exist for further work - need more diagnostics/experiments and theory work • Cary Forest, Electron Thermal Transport in the MST RFP and Comparison to Rochester-Rosenbluth Theory - Rochester-Rosenbluth over estimates transport in core…region where surface overlap does not exist…gets transport correct just inside reversal surface…region where overlap does exist …when PPCD is used that region also becomes very good • Jon Menard, Stochasticity Enhanced Collisional dissipation of ETG Turbulence by Thermal Ions- Interesting stochastic heating mechanism suggested as a possible explanation of the apparent anomalous ion heating seen in some NSTX discharges... Mechanism involves stochastic damping of ETG streamers on the ions. • Marco de Baar, Numerical Simulations of turbulence in RTP discharges with dominant off-axis ECH - CUTIE model is used to simulate the complicated profiles and profile evolution on RTP … model is able to reproduce the profiles with their flat regions and the hollow electron temperature profile • Robert O'Connell, Bulk and Fast Transport During Improved Confinement Plasmas - Run away electrons now seen in MST due to restored flux surface…energies suggest connection l;ength of 100km… D~1m2/sec same as for bulk electrons suggestive of ES transport not magnetic

7. Transport predictions and challenges Important advances have been made in our understanding of the various basic aspects underlying core transport. Theory (analytic and computational) as well as experiment are needed to make progress on the fundamental physics • Paul Terry, Nonlinear Damping of Zonal Flows in Collisionless Trapped Electron Mode Turbulence - New approach to zonal flow analysis through spectral representation … connecting to cyclones … nonlinear coupling to damped branch damps the zonal flows and gives saturation (nonlinear coupling to electrons gives indirect damping) • Roger McWilliams, Ion Transport and Fluctuation Suppression in a Sheared Zonal Flow - Using LIF was able to directly measure diffusivities by tagging particles…in shear region turbulence is suppressed Experiment - theory interaction can work even on short time scale • Clarisse Bourdelle, Stabilizing Impact of Beta on Microturbulance in NSTX - high beta prime is chief stablizer in NSTX more then low aspect ratio and high beta … might be important for ITB also (connection to B. Leblanc) • Benoit Le Blanc, Transport Studies with High Harmonic Fast Wave on NSTX - Used HHFW to modify Te/Ti…used this to test stability calculations…example of a good collaboration

8. Transport predictions and challenges - cont. • Taik Soo Hahm,- Gyrokinetic Studies of Transport Scaling in Tokamaks - Scaling in full torus gyro-kinetic code is found to change from bohm to gyro-bohm scaling with a/i >400 • Wendell Horton, Analysis of the Critical Electron Temperature Gradient in Tore Supra - EM- ETG mode seems to work for Tore Supra and ASDEX in critical gradient model ~Te3/2 • Dan Baker, Experimental determination of a Thermal Diffusivity Due to a Turbulent Transport. - Details of drift kinetic theory to investigate more rigorously the regions of validity of n~qa in transport modeling • Amiya Sen, Experimental Study of Isotope Scaling of Ion Thermal Transport - Slab ITG experiment is able to directly test isotope scaling in diffusivities and thermal conductivities as m-0.5 to 0.7 in contrast to theory and similar to some tokamak results • Andre Rogister, Interpretation of the Velocity Measured in Alcator C-Mod Ohmic H-Mode Discharges - Presented a neoclassical based theory of toroidal rotation with applications to Alcator C-MOD included previously neglected terms

9. Innovative transport measurements Diagnostic developments can be key to understanding and controlling new regimes • Tobin Munsat, Microwave Imaging Reflectometry: :Laboratory Study of The Fundamentals of Reflectometry for Fluctuation Measurements - Performed simple elegant lab test which determined regions of validity for both 1-D reflectometry and MIR …MIR is found to be valid over a wider range of conditions (also see poster by H. Park about system to be put on TEXTOR) • Stephen Terry, Internal Measurement of Core Magnetic Field Fluctuations During Current Profile Modification in the MST Reversed Field Pinch - Using polarimetry measurements of faraday rotation to characterize the time evolving mean profiles of current and B field through a sawtooth cycle… has plan to upgrade to enable both density and B to be simultaneously measured. Closer to Taylor state after a saw tooth with PPCD fluctuations are reduced but profile is not as close to a Taylor state (also poster by W. X. Ding which shows power law similar to that shown by Spangler for interstellar turbulence)

10. Transient Transport • Transients are a powerful tool for elucidating transport • Modulated ECH is measuring electron transport on several devices: >DIII-D (DeBoo) >FTU (Jacchia) >ASDEX-U (Ryter) • Sawteeth have returned -- DIII-D (Gentle)

11. DIII-D (DeBoo) • No clear sign of nonlinear stiffness • Some indication of critical gradient R/LTe~5 • No theoretical model gives good fit to all data, but renormalized GLF23 best

12. FTU(Jacchia) • Off-axis deposition requires heat pinch • Modulation response linear • Modulation response consistent with critical LTe model of Asdex-U and Tore Supra, including s/q dependence

13. New Analysis of Sawteeth (Gentle) • Evidence of reconnective dissipation at crash • Measure ofQe(Te) near equilibrium over range explored by sawtooth cycle. Not obviously diffusive: from hyperstiff at edge to gradient-independent in core.

14. Posters • S. Kaye - NSTX confinement times…evidence is that core Ion transports are low consistent with stability analysis • N. Kirneva - Initialsearch for scaling relation in ITB database… • C. Greenfield - Collection of analysis showing that fast ion instabilities can have a large impact on the perceived transport properties and on the profiles in a tokamak • W. Ferenbaugh - Search for new techniques to characterize dynamics of systems • W. Houlberg - Addresses neoclassical issues in coupling of NBI to plasma • R. Woodard - Presented a series of steps toward understanding radial and poloidal correlation lengths in the plasma frame from measurements in the lab frame • B. Labit - In ETG gyro-fluid model, for Tore Supra heat transport is found to weak depend on Beta unlike other recent studies • C. Holland, N. Crocker, M.Burin - Modelingbasic turbulence dynamics and techniques for understanding them

15. Summary discussion • Good discussion • Endorsed idea of transport initiative …to identify principle physics mechanisms behind electron, ion and momentum transport including their interactions and saturation dynamics… Discussion to continue toward fleshing out what we will “do with our hands and feet” to support this goal - PLEASE PARTICIPATE! • Burning Topics (to advance all of these needs new diagnostics and control tools) • Electron transport • The group recognized that other issues below are just as important, but we are ready to begin to attack electron transport with theory, experiments and diagnostics currently under development • Particle transport • Flows (including momentum generation and transport as well as structures) • Theory-Experiment comparisons • Will work to facilitate inter-machine collaborations (think different) • Need all approaches to attack these problems