Research Objectives
This project focuses on the development and application of low frequency particle-in-cell simulation models for the study of fluctuation-driven heat and particle transport in magnetically confined fusion plasmas, such as the tokamak, and addresses a problem recognized as a grand challenge in plasma physics.
Computational Approach
Our simulation approach is based on the particle-in-cell method applied to the gyro-drift dynamical equations of motion for charged particles in external electric and magnetic fields, along with the self-consistent fields created by the particles. Collisional interactions between like and unlike species of particles were analyzed by Monte Carlo methods. The model has been implemented on a number of shared and distributed memory computing platforms such as the Cray C-90 and the T3E at NERSC. The distributed memory version of the model uses a domain decomposition method for the particles and fields along with message passing, and this allows us to take advantage of the enormous amount of memory, as well as speed, on these machines. It is a computationally intensive model and exploits all 512 processors on the T3E.
Accomplishments
We have recently demonstrated that full-scale, global simulations of tokamak transport with parameters typical of current high temperature experiments such as DIII-D at General Atomics, San Diego, are possible on massively parallel distributed computing architectures such as the Cray-T3E at NERSC. As we are now in the production mode, we are investigating the complex interplay between short and long wavelength fluctuations. We have found a considerable amount of radial structure and propagation effects in the turbulence which has not been included in local simulations. We have also explored the possibility of suppressing turbulent fluctuations by locally reversing the radial variation of the magnetic field line pitch (or magnetic shear) as well as adding small amounts of impurity ion species such as neon or argon.
Both of these effects are shown to suppress the turbulent fluctuations to a certain degree and are similar to the experimental observations. Finally, we have been contributing to the validation and benchmarking of fluid-based codes with kinetic closure schemes, such as the gyro-Landau fluid model which has been applied to local turbulent transport simulations and is currently being developed into a global model.
Significance
One of the primary scientific goals for fusion is to obtain a predictive capability of turbulent transport in different magnetic configurations so that effects from turbulence can be minimized and a cost-effective design for the next generation fusion devices can be achieved. With full-scale simulations of anomalous transport now possible, we are beginning to approach this goal.
Publication
Sydora, R. D., V. K. Decyk, and J. M. Dawson. 1996. Plasma Phys. Control. Fusion 38:A281.
| Electrostatic Potential Fluctuations (Poloidal Cross-Section) | |
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Amplitude of electrostatic potential fluctuations taken at two different time slices during the evolution of the turbulence. The turbulent eddies cause transport of heat and particles. These simulations were performed on the T3E and had a reverse magnetic shear equilibrium. Pre-Saturation Phase (left), Post-Saturation Phase (right) |