The Plasma Microturbulence Project
W.M. Nevins 1
1 Lawrence Livermore National Laboratory, Livermore, CA 94551, USA
SummaryOur goal is to to study plasma microturbulence through direct numerical simulation. Plasma microturbulence is a critical issue to the magnetic fusion program because it controls the energy confinement and thereby determines the performance of a burning plasma experiment. Direct numerical simulation is a powerful tool to study plasma microturbulence because it allows far better diagnostics than those available on experiments, while excellent resolution of the microturbulent eddies can be achieved on existing supercomputers. Our effort includes: code development (to enhance the fidelity of our numerical models, increase their efficiency, and couple them to a common system for data analysis and visualization) code validation (against each other, against experimental measurements, and against theory) and expanding our user community through web-based applications and collaborations with the MFE theory and experimental communities.
A key goal of magnetic fusion programs worldwide is the construction and operation of a burning plasma experiment. The performance of such an experiment is determined by the rate at which energy is transported out of the hot core (where fusion reactions take place) to the colder edge plasma (which is in contact with material surfaces). The dominant mechanism for this transport of thermal energy is plasma microturbulence excited by radial gradients in the plasma temperature and density.
The development of more powerful computers and of efficient simulation algorithms provides us with a new means of studying plasma microturbulence — direct numerical simulation. Direct numerical simulation complements analytic theory by extending its reach beyond simplified limits. Simulation complements experiment because non-perturbative diagnostics measuring quantities of immediate theoretical interest are easily implemented in simulations, while similar measurements in the laboratory are difficult or prohibitively expensive. The Plasma Microturbulence Project (PMP) is addressing this opportunity through a program of code development, code validation, and expansion of the user community.
Code Development. We support a 2x2 matrix of plasma microturbulence codes which use either particle-in-cell (PIC) or continuum methods, and model the plasma in either flux-tube or global geometry. We have completed (or are in the process of) implementing multi-species kinetic plasma models including both the turbulent electric and magnetic fields. We have done this in a realistic equilibrium geometry (see Fig. 1). Our global codes are able to simulate plasmas as large as those in present experiments, and the even larger plasmas foreseen in burning plasma experiments.
Figure 1. The (turbulent) electrostatic potential from a GYRO simulation of plasma microturbulence in the DIII-D tokamak.
Code efficiency is a key issue. The PMP is the largest user of computer cycles within OFES. Our algorithms scale nearly linearly with processor number to ~1000 processors. The GYRO code is a benchmark code for the CRAY X-1 at ORNL, while the GTC code has been ported to both the CRAY X-1 and the Earth Simulator in Japan.
Data analysis and visualization is supported in all four PMP codes by providing an interface to GKV — a common set of tools for analyzing data from plasma turbulence simulations and visualizing the results.
Figure 2. Data from the four PMP codes, as analyzed by GKV, produce the same correlation function for the potential.
Code Validation is a critical step in our program to understand plasma micro-turbulence. The PMP code validation effort includes comparisons both between simulation codes (see Fig. 2) and between these codes and experiments.
Developing a Wider User Community. A large user community is necessary if the magnetic fusion community is to benefit from the PMP codes. This user community has been engaged through our efforts to validate the PMP codes against experiments, has been trained in workshops held in Dec. 2002 (for the GYRO code) and January 2003 (for the GS2 code and the GKV post-processor). Members of this wider PMP user community have presented 7 invited talks at major meetings over the last 3 years based on work they did with PMP codes.
Key Accomplishments and Plans. The principal accomplishments of the project so far are: (1) the successful implementation of electromagnetic drift-wave turbulence algorithms in both continuum and PIC codes; (2) the use of these codes to simulate experimentally relevant discharges, and to perform parameter studies; (3) successful validation between PMP codes; (4) the propagation of PMP codes (GS2 and GYRO) to a wider community of fusion researchers for routine use in modeling experiments; (5) detailed investigations of the physics of velocity-shear stabilization and its relation to the internal transport barriers; (6) study of the dependence of turbulent transport on system size; (7) initiation of studies of electron-driven instabilities; and (8) application of a PMP code to the problem of ion heating in an astro-physical context. These accomplishments resulted from computing resources and research support by both SciDAC and OFES.
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