Richard D. Sydora

Fluctuation-induced heat transport results from a large global 3D toroidal particle simulation model

Turbulent transport simulation results from a large nonlinear three-dimensional (3D) toroidal electrostatic gyrokinetic particle-in-cell simulation model, including global equilibrium effects, are presented. The parallel implementation of the particle simulation model on massively parallel computers has allowed us to perform large-scale simulations of ion temperature gradient-driven turbulence and to include low-n and high-n toroidal mode numbers in a single calculation. Simulation results indicate a strong interaction between these short and long wavelength scales and that this is the possible origin of a Bohm-like ion thermal transport scaling. The inclusion of trapped electron dynamics and self-generated shear flows on the fluctuation dynamics is shown to produce quantitative differences in the ion thermal transport.

Comparison of turbulence measurements from DIII-D low-mode and high-performance plasmas to turbulence simulations and models

Measured turbulence characteristics (correlation lengths, spectra, etc.) in low-confinement (L-mode) and high-performance plasmas in the DIII-D tokamak [Luxon et al., Proceedings Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] show many similarities with the characteristics determined from turbulence simulations. Radial correlation lengths Δr of density fluctuations from L-mode discharges are found to be numerically similar to the ion poloidal gyroradius ρθ,s, or 5–10 times the ion gyroradius ρs over the radial region 0.2<r/a<1.0. Comparison of these correlation lengths to ion temperature gradient gyrokinetic simulations (the UCLA-University of Alberta, Canada UCAN code [Sydora et al., Plasma Phys. Controlled Fusion 38, A281 (1996)]) shows that without zonal flows simulation values of Δr are very long, spanning much of the 65 cm minor radius. With zonal flows included, these decrease to near the measured values in both magnitud...

Binary collision model in gyrokinetic simulation plasmas

Abstract A binary collision model for plasma particle simulation is re-examined with emphasis on the application to gyrokinetic plasma description and efficient implementation on presentday vector computers. The model conserves the number of particles, the total momentum and the total energy quasi-locally. Two efficient implementation schemes are presented. The results of measurement on various relaxation rates in velocity space are shown to agree well with test particle theory. Classical cross-field transport of particles and heat in an inhomogeneous plasma is examined using this model, and the results indicate that good agreement between continuous solution of particle and heat transport and particle simulation can be achieved.

Gyrokinetic simulation of internal kink modes

Internal disruption in a tokamak has been simulated using a three‐dimensional magneto‐inductive gyrokinetic particle code. The code operates in both the standard gyrokinetic mode (total‐f code) and the fully nonlinear characteristic mode (δf code). The latter is a quiet low noise algorithm. The computational model represents a straight tokamak with periodic boundary conditions in the toroidal direction and a square cross section with perfectly conducting walls in the poloidal direction. The linear mode structure of an unstable m=1 (poloidal) and n=1 (toroidal) kinetic internal kink mode is clearly observed, especially in the δf code. The width of the current layer around the x‐point, where magnetic reconnection occurs, is found to be close to the collisionless electron skin depth, indicating the importance of electron inertia. Both codes give very similar nonlinear results, in which full reconnection in the Alfven time scale is observed along with the electrostatic potential structures created during this...

Toroidal gyrokinetic particle simulations of core fluctuations and transport

The generation of low frequency electrostatic fluctuations and the formation of anomalous particle and heat diffusion in the core region of tokamak plasmas is investigated using gyrokinetic particle-in-cell simulations in toroidal geometry. We consider the case of H-mode tokamak plasmas characterized by nearly flat core density profiles. The principle source of fluctuations include ion temperature gradient (ITG)-driven instabilities and ITG-dissipative trapped electron modes. We compare the evolution of both branches of instability and find that the electron diffusivity can be larger than the ion thermal diffusivity in the latter case. An externally imposed radial electric field, which gives rise to sheared poloidal flows, is demonstrated to reduce the radial correlation length of the turbulent transport and hence reduce the anomalous thermal flux.

High‐Performance Computing and Plasma Physics

The physics of ionized gases is a relatively new science. Not until the development of the electrical industry were controlled experiments on ionized gases possible, and so plasma physics is only about 100 years old. The early part of this century saw some pioneering studies of gas discharges and radio propagation in the ionosphere. However, the real impetus came with the initiation of the controlled thermonuclear reaction programs in the 1950s and with the discoveries of the Van Allen belts and the solar wind in the 1960s. Studies in these areas showed that plasma behavior is much more complex than had been anticipated.

A vlasov particle ion zero mass electron model for plasma simulations

Abstract We have extended a particle-MHD model of discrete fluid ions and massless fluid electrons to the fully kinetic ions and massless fluid electrons so that such important kinetic effects, e.g., the finite Larmor radius effects, are incorporated; 2D as well as 3D versions exist. As a result the model now exhibits ion Bernstein waves. We have also developed a linear theory of the model and obtained the linear dispersion relation. The dispersion relation is in excellent agreement with results from the model. Two important improvements in numerical procedures have been implemented; the first is the elimination of unphysical short wavelength disturbances by a smoothing technique; the second is the implementation of the Richardson extrapolation (extrapolation to Δt = 0) technique in the velocity difference equations. These improvements increased the range of stability of the code greatly and enabled us to use time steps which are 10 times larger. As one application, we use the model to simulate the interaction of a neutral gas and an ambient plasma and observe the generation of waves with frequencies at integral as well as half-integral ion cyclotron harmonics for all k in accordance with some recent experimental observations in the JET tokamak.

Three‐dimensional gyrokinetic particle simulation of low‐frequency drift instabilities

The nonlinear behavior of drift‐wave fluctuations driven unstable by trapped particles and pressure gradients is studied using three‐dimensional gyrokinetic plasma simulation methods. In the linear stages of instability, the growth rates and the radial and poloidal ballooning mode structures agree reasonably well with theory. Saturation of the unstable modes occurs principally through quasilinear profile modification and mode coupling. Results of the growth and saturation phase of the instability are compared with a bounce‐averaged electron drift model. The resultant anomalous particle and energy diffusion are estimated using the saturated mode spectrum.

Effect of externally imposed and self-generated flows on turbulence and magnetohydrodynamic activity in tokamak plasmas

The effects of externally imposed and self-generated poloidal flows on turbulence and magnetohydrodynamic (MHD) activity are examined in the context of the possible Electric Tokamak (ET) [Phys. Plasmas 6, 4722 (1999)] plasmas and (circularized) DIII-D-like [Fusion Technol. 8, 441 (1985)] discharges. Global gyrokinetic particle simulations and reduced MHD calculations respectively show that ion temperature gradient driven turbulence (ITGDT) and resistive internal kink MHD activity can be reduced and/or suppressed with experimentally achievable externally imposed flows for possible ET start-up plasmas. Global gyrokinetic particle simulations of ITGDT also serve to demonstrate that self-generated flows are necessary to yield experimentally relevant radial correlation lengths in the case of DIII-D-like discharges.

Magnetic reconnection driven by current repulsion

The evolution of an equilibrium consisting of two magnetic islands with oppositely directed currents embedded in a strong magnetic field is investigated, using numerical simulation methods. The rapid development of an ideal magnetohydrodynamic instability is observed, which first rotates and then expels the islands. The growth rate is on the order of the inverse of the Alfven transit time and is much higher than that for magnetic island coalescence. In the nonlinear stage, resistivity becomes important as the reconnection process ensues and dissipates the magnetic energy. The growth rate of the instability is a weak function of the plasma beta and other plasma parameters such as S, the magnetic Reynolds number. An energy principle analysis, based on eigenfunctions obtained from the simulation, confirms the existence of the instability.