Properties of freely decaying and driven turbulence of fusion plasmas using gyrokinetic particle simulation
In a recent work, the effects of nonlinear self-consistent profile relaxation on the ITG turbulence in global delta-f particle simulations were studied [1] using Gyrokinetic Toroidal Code (GTC) [2]. In contrast to several other gyrokinetic turbulence models, GTC shows that the turbulence is insensitive to minute or drastic profile changes and reaches the similar flux levels irrespective of whether or not it is a driven or freely decaying turbulence. In the present work, we address the spectral properties of gyrokinetic turbulence and argue that correct inclusion of parallel velocity space nonlinearity may bring forth certain important late-time features such as Geodesic Acoustic Modes like (GAM) structures.
A Fourier-Hermite Study of Dissipation in 1+1-D Models of Fusion Plasmas
This study investigates dissipation in fusion plasmas confined by strong magnetic fields, as described by 5-D gyrokinetic theory. We first derive a reduced model for the dynamics parallel to the magnetic field in both physical and velocity spaces. This model closely resembles the classic one-dimensional Vlasov-Poisson system, but replaces Poisson’s law with the quasineutrality condition relevant to multi-species fusion plasmas. We construct a numerical model using a truncated Fourier-Hermite representation, validate it against existing results for the Vlasov-Poisson system, and then apply it to fusion plasmas. Similarities and differences between the classic and fusion cases are assessed. We then investigate the effect of unsteady forcing by a neutral particle beam, representing both an actual tokamak energy source and a model for turbulence in the omitted dimensions perpendicular to the magnetic field. We find that even small amplitude forcing may substantially affect the long-time behavior, which may have important consequences in the design of efficient fusion reactors.
An analytical approach to the design of electrodes in high-power, fast-transverse-flow CO2 lasers
Abstract Usually, the design of the electrodes of fast-transverse-flow CO 2 lasers relies more on experimental data than on theoretical analysis. Traditional systems sustain a stable, high-power discharge but the current distribution generates a peaked, non-uniform small-signal-gain pattern. We present a theoretical model suitable for the design of electrodes that achieves a quasi uniform gain distribution. The analysis, based on a two-temperature model and the gas-transport equations, computes the electron density that supports a flat gain pattern. Combining the desired electron density with the electron-balance equations, the model determines the required electric field. The results were used to design a new set of electrodes for a home-made fast transverse flow CO 2 laser. A stable, large volume discharge ( 4 × 8 × 90 cm 3 ) with very good uniformity was obtained. The resulting gain distribution was registered in a bi-dimensional map. The peak gain rose from 1.2 m - 1 , attained with the old electrode set, to 1.6 m - 1 with the new one and the gain showed a homogeneous profile.