|
|
|
|
|
A
full-torus simulation of turbulent transport scaling shows that
transport driven by microscopic-scale fluctuations (ITG modes) in
present devices can change character and transition from Bohm-like
scaling ~(rIvi)
to Larmor-orbit-dependent "gyro-Bohm" scaling ~(rivi)(rI/a).
|
|
W.
W. Lee, Z. Lin, W. M. Tang, J. L. V. Lewandowski, W. Wang, and S. Ethier,
Princeton Plasma Physics Laboratory
Research
Objectives
Our objective is improvement of the existing 3D Gyrokinetic Toroidal
Code (GTC) by using split-weight df
and hybrid schemes for the electrons for studying the trapped particle
and finite-b
effects. The improved GTC code will be used for studying neoclassical
and turbulent transport in tokamaks and stellarators as well as for investigating
hot-particle physics, toroidal Alfvén modes, and neoclassical tearing
modes.
Computational
Approach
The basic approach for transport
studies in tokamaks and stellarators is the gyrokinetic particle-in-cell
method. GTC, in general geometry with 3D numerical equilibria, is written
in Fortran 90 with MPI/OpenMP and is scalable on MPP platforms. The inclusion
of the vital electron dynamics makes the code computation-intensive because
of the mass disparity between the electrons and the ions.
Accomplishments
We have carried out gyrokinetic particle simulation studies
on turbulent transport using GTC and have gained substantial understanding
of the zonal flow physics. For example, the interplay between the ion
temperature gradient (ITG) driven turbulence, zonal flow generation, and
the collisional effects in the simulation is shown to rise to the bursting
behavior observed in Tokamak Fusion Test Reactor (TFTR) experiments. A
full-torus simulation of turbulent transport in a reactor-sized plasma
indicates a more favorable scaling of transport driven by ITG modes as
the size of the plasma increases. Gyrokinetic calculations of the neoclassical
radial electric field in stellarator plasmas have also been carried out
with the GTC code, which is part of the PPPL's effort for designing the
next generation of stellarators. In addition, we have developed several
versions of the split-weight schemes for the electrons to account for
the trapped-particle and finite-b effects.
Significance
A
key issue in designing a fusion reactor is the realistic assessment of
the level of turbulent transport for reactor-grade plasma conditions.
Up until very recently, this has been done by extrapolating to larger
reactors the transport properties observed in smaller experimental devices.
This approach relies on models of transport scaling that have often stirred
debates about reliability. Taking advantage of the power recently accessible
in new supercomputer capabilities, we have been able to take a major step
forward in understanding turbulent transport behavior in reactor-sized
plasmas by using direct numerical simulations. These advanced simulations
have just become feasible because of the recent development of better
physics models and efficient numerical algorithms, along with the newly
available 5 teraflop/s IBM SP at NERSC.
Publications
Z. Lin and L. Chen, "A kinetic-fluid hybrid electron
model for electromagnetic simulations," Phys. Plasmas 8, 1447
(2001).
Liu Chen, Zhihong Lin, Roscoe B. White, and Fulvio Zonca, "Nonlinear
zonal flow dynamics of drift and drift-Alfvén turbulences in tokamak
plasmas," Nuclear Fusion 41, 747 (2001).
J. L. V. Lewandowski, A. H. Boozer, J. Williams, and Z. Lin, "Gyrokinetic
calculations of the neoclassical radial electric field in stellarator
plasmas," Phys. Plasmas 8, 2849 (2001).
|