No abstract

A solution to the full two-dimensional eigenvalue problem of electrostatic microinstabilities in a tokamak plasma is presented in the framework of gyrokinetic theory. The approach is the generalization of methods previously developed for a cylindrical system [S. Brunner and J. Vaclavik, Phys. Plasmas 5, 365 (1998)]. By solving the spectral problem in a special Fourier space adapted to the curved geometry, orbit width as well as Larmor radius can be kept to all orders. For a first numerical implementation, a large aspect ratio plasma with circular concentric magnetic surfaces is considered. A root finding algorithm for identifying the eigenfrequencies, based on a higher order Nyquist method, enables straightforward implementation on a parallel computer. Illustrative results for ion temperature gradient-related instabilities are presented. These include scaling studies of the radial width, and toroidicity and magnetic shear scans, as well as the effects of nonadiabatic trapped electron dynamics.

The frequency dependence (13.56–70 MHz) of the ion energy distribution at the ground electrode was measured by mass spectrometry in a symmetrical capacitive argon discharge. Reduced sheath impedance at very high frequency allows high levels of plasma power and substrate ion flux while maintaining low levels of ion energy and electrode voltage. The lower limit of ion bombardment energy is fixed by the sheath floating potential at high frequency, in contrast to low frequencies where only the radio frequency voltage amplitude is a determinant. The capacitive sheaths are thinner at high frequencies which accentuates the high frequency reduction in sheath impedance. It is argued that the frequency dependence of sheath impedance is responsible for the principal characteristics of very high frequency plasmas. The measurements are summarized by simple physical descriptions and compared with a particle-in-cell simulation.

Charge-exchange collisions and radio frequency (RF) excitation combine to give peaks in the ion energy distribution measured at the ground electrode of an argon plasma in a capacitive reactor. These peaks are used as a diagnostic to reconstruct the profile of the time-averaged potential in the sheath. Particle-in-cell !?!c) simu!aiions show that t h e method is accurate. The method is applied to investigate the sheath thickness as a function of excitation frequency at constant plasma power. The time-averaged potential is found to be parabolic in both exoerimental measurements and numerical simulations.

Global gyrokinetic particle simulations have been used to search tokamak configurations which are stable against the ion-temperature-gradient-driven (ITG) modes commonly held responsible for the core anomalous ion heat transport. The stable configurations are characterized by strongly reduced or reversed =B drifts on the low-field side. Since excellent transport properties have been observed in high b p (poloidal beta) tokamak experiments, we conjecture, as a result of our simulations, that this may be due to the stabilization of ITG modes by the reduction of the =B drifts.[S0031-9007 (97)03019-6] PACS numbers: 52.55.Fa, 52.35.Kt, 52.55.Dy, 52.65.TtIon-temperature-gradient-driven (ITG) instabilities are now commonly held responsible for the turbulence giving rise to anomalous ion heat transport in the core of tokamaks; ITG-based transport models have been claimed to predict successfully the plasma thermal transport over a wide range of parameters [1][2][3]. The reduction of this transport would be of great help in the achievement of a fusion reactor; configurations that are free of these instabilities are of very high interest. An effort has recently been made to find such configurations; e.g., oblate plasmas [4] and plasmas with sheared rotation (e.g., [2] and [5]) have been discussed as routes to ITG-mode stabilization. We propose here another route to ITG stability: the reduction of the =B drift on the low-field side of the torus.In this Letter, we study finite-pressure effects on ITG instabilities using the first global gyrokinetic particle-in-cell codes which work with full finite-pressure MHD equilibrium data. We note that microinstabilities in general equilibria were previously studied in the ballooning limit [6]. With respect to global ITG stability, we find that the dominant effect is related to the particle =B drift on the low-field side of the torus: The growth rates decrease with decreasing =B drift. In fact, the configurations are stable when =B is close to reversal.The ITG instability takes its free energy from the ion temperature gradient. It can be destabilized by the parallel motion (slab ITG), the poloidal magnetic drifts due to toroidicity (toroidal ITG), and the trapped-ion precession (trapped ion mode). The latter two destabilizing mechanisms are related to the magnetic drift,where B denotes the magnetic field, V the ion cyclotron frequency, y k and y Ќ the parallel and perpendicular particle velocities, and p the plasma pressure. From a waveparticle interaction diagnostic implemented in one of our codes, we find that particles with y Ќ ¿ y k contribute most to the instability. In a simple analysis which helps to understand the simulation results to be presented, the second term in Eq. (1) can therefore be neglected.For k k 0 and k c 0, the wave vector reduces to k nq=x 2 n=w, x being the straight-field-line poloidal angle [7], w the toroidal angle, n the toroidal mode number, and q the safety factor. In the local and fluid limits, the drift frequencies may then be defined asandwhere T i is t...