Theory Weekly Highlights for April 2000

April 28, 2000

The MCGO Monte Carlo guiding center orbit code has been ported to a Pentium III based Linux machine and a preliminary parallelization has been completed using the MPI library. Performance tests on the GA Stella Linux cluster using NFREYA to compute the birth rate profile for the initial conditions of the hot ions showed that for 3000 ions, the CPU time was significantly reduced from the single processor time of 1302 secs down to 21.5 secs using 46 processors. This represents an important step toward achieving practical use of the MCGO code for calculating the neutral beam deposition profiles in the ONETWO code.

A fixed boundary equilibrium code TOQU was created combining the existing version of TOQ, which applies to updown symmetric equilibria, with an elliptic equation solver that does not require symmetry. The code, which applies to general equilibria, now runs on ALPHAS. A plot program TOQUplot was also created for the ALPHA workstations. Interface with EFIT, GATO, and BALOO1.5 have been implemented. Benchmarking of the FF' solver has been successful. The qsolver still experiences convergence difficulties, which are now being addressed.

April 21, 2000

The vacuum magnetic field code VACUUM is used to simulate the DIII-D Mirnov loop measurements. Preliminary results indicate that the phase of the tangential perturbations along the DIII-D vessel shows a strong reversal in the inner major radius region of the shell, a tendency which is also observed in the experiments. A pure m=2 perturbation shows no such reversal. Results of the ideal stability analysis using the GATO code with vacuum magnetic energy from VACUUM also compare favorably with those using GATO own vacuum calculation.

A new version of the equilibrium reconstruction code EFIT with auto-knot option for the spline representation of the current profile has been released. The knot locations are optimized by minimizing a weighted sum of the convergence error, and magnetic, MSE, and kinetic χ2. The availability of this tool will reduce the manual efforts involved to reconstruct a high quality kinetic EFIT equilibrium.

April 14, 2000

In DIII-D discharges, the growth of Neoclassical Tearing Modes (NTMs) often set a soft beta limit and sometimes lead to disruptions. Criteria for the suppression of NTMs by Electron Cyclotron Current Drive (ECCD) have been evaluated theoretically. It is found that the ratio J of the driven current density to the bootstrap current density in the vicinity of the NTM resonant surface is a key figure-of-merit. For J > 0.6, continuous ECCD can reduce NTM island widths to be comparable to the width of the driven current layer. Experimentally, it is anticipated that the width of the driven current layer can be made appreciably less than the width of a saturated NTM island in the absence of ECCD. Application of ECCD should therefore lead to significant island width reductions and corresponding increases in the soft beta limit. Also, for J > 1.6, the calculations actually predict complete stabilization of arbitrarily small islands, provided the ECH source can be modulated at the NTM frequency.

April 07, 2000

Howard Wilson of Culham Science Centre visited GA March 30 through April 7, 2000 and assisted in the implementation of a new poloidal angle variable into the ELITE (Edge Localized Instabilities in Tokamak Experiments) MHD stability code. The new poloidal angle reduces mode coupling, allowing for more efficient numerical calculation of edge stability. The ELITE code is a new tool that is being developed to better understand ELM physics. ELITE has also recently been rewritten in F90 and extended to treat general nonlocal equilibria produced by EFIT and TOQ. The code is now ready for a systematic stability study of moderate to high-n edge modes in DIII-D.

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