### Theory Weekly Highlights for April 2003

##### April 25 2003

The unified users' login environment where users have the same home directories and login files on different platforms HP and OSF has been extended to include Linux. This environment allows users to take advantage of the LSF (Load Sharing Facility) cluster transparently. Substantial tests were done for the analysis tools to make this possible. This has not only opened up the Linux machine (Delphi) in the LSF cluster to all users, but has also paved the way for adding more Linux hosts to the LSF cluster in the future to increase computational power at minimal cost.

##### April 18 2003

At the joint EU-US transport task force meeting in Madison April 2-5, particle transport was an emerging focus of research interest. A paper by Clemente Angioni of IPP demonstrated that the GLF23 transport model, developed at GA, reproduced the anomalous pinch effect leading to density peaking in tokamaks. Detailed comparisons of GLF23 with Asdex-U data under a variety of controlled experimental conditions were made. The dependence of the particle pinch on plasma parameters predicted by the GLF23 model was found to be in good agreement with the data.

##### April 11 2003

GYRO simulations for DIII-D L-mode discharges, using the real electron-to-ion mass ratio, as well as shaped plasma geometry, equilibrium ExB rotational shear, electron collisions, and kinetic finite-beta effects, now yield electron and ion energy diffusivities that match the experimental values well within error bars. In these new simulations, the extreme linear stiffness of the gyrokinetic-Maxwell differential equation system was largely eliminated by a special implicit-explicit Runge-Kutta scheme based on the SSP322 method of Pareschi and Russo. The numerical implementation of SSP322 couples the Poisson and Ampere equations and eliminates the severe, unphysical Courant limit imposed by pathological electrostatic box modes. Previously, this limited DIII-D simulations to less than about 80 gyroradii.

##### April 04 2003

An alternative formulation of the Grad-Shafranov equation has been obtained, in which the free functions that characterize specific tokamak equilibria, are selected on the basis of physics arguments instead of mathematical simplicity. The reformulation is based on the observation that certain source terms for the poloidal flux function, which superficially appear to originate from the plasma current, are really driven by the pressure gradient. A regrouping of the source terms identifies those that arise from pressure gradients and those due to plasma current. This should provide an improved basis for EFIT determination of plasma current profiles since the large local cancellations in the sources can be handled explicitly. The formulation should also be extremely valuable for assuring 100% bootstrap fraction models with perfect current alignment in Advanced Tokamak studies.

**Disclaimer**

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