"Understanding Turbulence in Tokamak Plasmas Using Dedicated Experiments and Validated Gyrokinetic Simulations," by Nathan Howard (MIT)

Date: 
Friday, January 22, 2016 - 2:00pm to 3:00pm
Series: 
Plasma Seminars

Plasma Seminar

Physics & Astronomy Building (PAB) Room 4-330
Friday, January 22, 2016
2-3 PM

Guest Speaker: Nathan Howard (MIT Plasma Science and Fusion Center)

Talk Title: "Understanding Turbulence in Tokamak Plasmas Using Dedicated Experiments and Validated Gyrokinetic Simulations"

Abstract:

It is now generally accepted that plasma turbulence is the origin of the ‘anomalous’ transport observed in the core of fusion plasmas.  However, despite years of research, disagreements between experimental electron heat flux and the leading turbulence models are fairly common.  It has been speculated that short wavelength, electron-scale turbulence may play an important role in electron thermal transport.  However, simulation capturing both ion (k_theta rho_s < 1.0) and electron-scale (k_theta rho_s > 1.0) turbulence simultaneously was, until recently, computationally intractable and therefore had not been compared quantitatively against experiment.  In Alcator C-Mod L-mode plasmas, ion-scale simulations often exhibit a robust under-prediction of the experimental electron heat fluxes.  We report results from dedicated experiments designed to probe the origin of this discrepancy.  A set of cutting-edge, multi-scale simulations were performed which capture ITG/TEM/ETG turbulence up to k_theta rho_s = 48.0 using all experimental inputs, impurities, realistic electron mass, ExB shear, and collisions.  These simulations revealed that ion-scale turbulence likely coexists and couples with electron-scale, ‘streamers’ in the tokamak core.  Direct comparisons of the experimental heat fluxes (Qi and Qe) and electron profile stiffness demonstrate that only multi-scale simulation is able to reproduce experiment – pointing to the origin of the electron heat flux discrepancy.  Cross-scale coupling of ion and electron-scale turbulence is found to play a significant role in experimental plasma conditions, increasing electron heat fluxes by up to an order of magnitude of above standard, long wavelength simulation.  Details of the experiment, mechanisms of cross-scale coupling, and the implications for predictive modeling of tokamak plasmas will be discussed.

Event Attachments: 
Location: 
PAB 4-330