Theory Weekly Highlights for June 2009

June 26, 2009

A complete expression was obtained for the poloidal variation of the electrostatic potential in the banana regime for large aspect ratio flux surfaces using the method of matched asymptotic expansions. The result exhibits a finite discontinuity at the innermost point of a flux surface instead of a divergence as previously reported. Using this expression in combination with the solution of the linearized drift kinetic equation with a model collision operator, the part of the toroidal angular momentum flux due to the poloidal electric field is calculated. The result is larger than the one in existing works, which neglect the poloidal electric field, by the order of the square root of the aspect ratio.

Dave Schissel and Gheni Abla attended the 7th IAEA Technical Meeting on Control, Data Acquisition, and Remote Participation for Fusion Research in Aix En Provence, France, from Jun.15th through 19th. They gave oral presentations on “An Investigation of Secure Remote Instrument Control” and “Customizable scientific web-portal for DIII-D nuclear fusion experiment”, respectively.

June 19, 2009

Substantial computational work has been done through collaborations in the RF SciDAC community to more accurately model the DIII-D ICRF heating experiments by fully accounting for finite orbit effects on wave-particle interactions. To accomplish this, the ORBIT-RF code was successfully coupled with the full-wave code AORSA in a self-consistent way and the distribution function computed from the transport code TRANSP is coupled to ORBIT-RF as an initial condition. Preliminary ORBIT-RF/AORSA results for DIII-D ICRF heating experiments suggest that finite orbit width effects may explain the enhancement in the outward radial shift of the spatial profile measured by the Fast Ion D-alpha Array (FIDA) when compared to CQL3D/ray-tracing zero-orbit predictions. However, a discrepancy remains in that the outward shift computed from ORBIT-RF/AORSA at larger radius of R > 2 m is much smaller than the measurement from FIDA. The FIDA data is averaged over a fairly long time window to get better statistics for the steady-state discharge. A similar improvement in the simulation statistics, a better equilibrium model, and a convergence study of the ORBIT-RF/AORSA iterations are in progress to resolve the difference.

June 12, 2009

The Trapped-Gyro-Landau-Fluid (TGLF) linear stability and quasilinear transport code is now available for public distribution. The package includes a stand-alone driver for computing linear drift wave eigenmodes or quasilinear fluxes. The TGLF model has been extensively verified against fully gyrokinetic linear stability calculations and non-linear turbulence simulations using the GYRO code. The TGLF model has proven to be a more accurate model of the gyrokinetic results than its widely used predecessor GLF23. As reported at the 2007 APS conference, the TGLF model predicts the core temperature profiles for a database of more than 100 discharges from three tokamaks with greater accuracy than GLF23. After filling in a user agreement form, a copy of the TGLF source code can be downloaded from the webpage

Vincent Chan, Phil Snyder, and Alan Turnbull attended the week-long ReNeW meeting in Bethesda Md.

June 05, 2009

The first nonlinear saturated states of transport driven by energetic particles (EP) have been demonstrated in new GYRO electromagnetic simulations of the GA-standard ITG/TEM case. The current focus is on three flux-tube simulations at about 1/3 the high-n MHD beta limit: un-driven, marginal, and pressure gradient 80% over the EP threshold for the high-n TAE/EPM modes. The drift-Alfvenic mode turbulence at very long wavelengths (k*ρs < 0.1) is embedded in the moderate short wavelength turbulence spectrum (k*ρs < 1.0 ITG/TEM). Below threshold, the EP diffusivity is about 5% of the ion energy diffusivity and the finite beta of ion and electrons are unaffected. Above threshold, the EP transport doubles and shifts to the locus of the EP drive at lower-k and the background ions and electron transport increases marginally. Surprisingly, even the marginal transport from EP drive is not as intermittent as expected. Eric Bass presented these preliminary results at the 4th IAEA-TM Meeting on Plasma Instabilities, Kyoto, Japan Week May 18.

Dr. Srinivasan returned to the Institute Of Plasma Research in India after a productive one year visit to GA, where he collaborated on modernization of the EFIT computational structure and IMFIT development.

These highlights are reports of research work in progress and are accordingly subject to change or modification