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Plasma shaping may have a stronger effect on global turbulence in tight-aspect-ratio tokamaks than in conventional-aspect-ratio tokamaks due to the higher toroidicity and more acute poloidal asymmetry in the magnetic field. In addition, previous local gyrokinetic studies have shown that it is necessary to include parallel magnetic field perturbations in order to accurately compute growth rates of electromagnetic modes in tight-aspect-ratio tokamaks. In this work, the effects of elongation and triangularity on global, ion-scale, linear electromagnetic modes are studied at National Spherical Torus Experiment (NSTX) aspect ratio and high plasma β using the global gyrokinetic particle-in-cell code XGC. The effects of compressional magnetic perturbations are approximated via a well-known modification to the particle drifts that was developed for flux-tube simulations [Joiner et al., Phys. Plasmas 17, 072104 (2010)], without proof of its validity in a global simulation, with the gyrokinetic codes GENE and GEM being used for local verification and global cross-verification. Magnetic equilibria are re-constructed for each distinct plasma profile that is used. Coulomb collision effects are not considered. Within the limitations imposed by the present study, it is found that linear growth rates of electromagnetic modes (collisionless microtearing modes and kinetic ballooning modes) are significantly reduced in a high-elongation and high-triangularity NSTX-like geometry compared to a circular NSTX-like geometry. For example, growth rates of kinetic ballooning modes at high-β are reduced to the level of that of collisionless trapped electron modes. [ABSTRACT FROM AUTHOR]

Electromagnetic gyrokinetic simulation of high temperature plasma is required to predict confinement in magnetic fusion devices and has posed challenges for existing codes. In this paper, we demonstrate successful global gyrokinetic simulation of the ion temperature gradient-driven mode-kinetic ballooning mode transition in a toroidal fusion plasma test case using the mixed variables/pullback transformation (MV/PT) scheme with the particle-in-cell codes XGC and ORB5, and compare to results from a conventional continuum code from the literature. The MV/PT scheme combines explicit time integration with mitigation of the well-known electromagnetic gyrokinetic "cancelation problem." We calculate eigenmodes in the electrostatic and parallel vector potentials, and find good agreement in growth rate, real frequency, and the normalized plasma pressure of mode transition. [ABSTRACT FROM AUTHOR]

We use the global gyrokinetic stellarator code XGC-S to look at turbulence physics in geometry of the proposed quasi-axisymmetric stellarator, QUASAR. The XGC-S code is first verified for nonlinear simulations of ion temperature gradient-driven turbulence by a successful benchmark with the established nonlinear tokamak code XGC1 in a CYCLONE-like tokamak case. We then apply the XGC-S code to QUASAR for nonlinear turbulence physics. We find that the highly poloidally localized linear mode gives way to a more evenly distributed nonlinear turbulent structure. Meanwhile, the radial extent of the mode increases and occupies a large fraction of the volume, despite the initially localized temperature profile, as the profiles relax. [ABSTRACT FROM AUTHOR]

XGC (X-point Gyrokinetic Code) is a whole-volume, total-f gyrokinetic particle-in-cell code developed for modeling tokamaks. In recent work, XGC has been extended to model more general 3D toroidal magnetic configurations, such as stellarators. These improvements have resulted in the XGC-S version. In this paper, XGC-S is benchmarked in the reduced delta-f limit for linear electrostatic ion temperature gradient-driven microinstabilities, which can underlie turbulent transport in stellarators. An initial benchmark of XGC-S in tokamak geometry shows good agreement with the XGC1, ORB5, and global GENE codes. A benchmark between XGC-S and the EUTERPE global gyrokinetic code for stellarators has also been performed, this time in the geometry of the optimized stellarator Wendelstein 7-X. Good agreement has been found for the mode number spectrum, mode structure, and growth rate. [ABSTRACT FROM AUTHOR]

Whole volume simulations of stellarators are necessary to address a number of important physics and engineering issues, including turbulent transport prediction and optimising divertor operation. In this work, advances in the development of a whole volume stellarator capability for the global gyrokinetic particle-in-cell code XGC are detailed. A 3D interpolation of the equilibrium magnetic field to the last close flux surface, calculated using the VMEC ideal magnetohydrodynamic equilibrium code, has been implemented. Orbit tracing with the XGC code is verified for the Wendelstein 7-X stellarator by comparison with the BEAMS3D and EUTERPE codes. The XGC stellarator version has then been used to consider collisionless alpha particle confinement in representative quasi-isodynamic and quasi-axisymmetric designs. It has been shown that, at least without further optimisation for energetic particle confinement, the Wendelstein 7-X-like quasi-isodynamic reactor design exhibits better alpha particle confinement. [ABSTRACT FROM AUTHOR]

Alfvén eigenmodes may be important in driving fast particle transport in magnetic confinement fusion devices, with potentially deleterious results. To explain and predict this behaviour, numerical simulations are necessary. In order to predict transport, modes must be simulated through to their nonlinear saturated state. In this work, the first simulations of non-linear wave-particle interaction between an energetic particle population and a Toroidal Alfvén Eigenmode are performed in which fluctuations responding self-consistently to modification of the fast particle profile are calculated with gyrokinetic treatment of all plasma species. Results from two such gyrokinetic codes are compared with new results from non-perturbative and perturbative fluidgyrokinetic hybrid codes. There is a power-law relationship between the saturated magnetic perturbation amplitude, δB/B0, and the linear mode growth rate, γL. All models show a transition from a higher to a lower exponent regime with increasing γL. Measured values of the higher exponent from different codes fall in a range between 1.45 and 1.79, while the lower exponent falls in a range between 0.47 and 0.79. There is a consistent difference of 1.0 between the higher and lower exponents independent of the model. The absolute level of saturated δB/B0 is determined by the damping rate. In the fluid-gyrokinetic hybrid codes, an ad-hoc damping is applied, while in the gyrokinetic case the measured damping is consistent with the estimated rate of physical electron Landau damping. [ABSTRACT FROM AUTHOR]

We have extended our study of the competition between the drive and stabilization of plasma microinstabilities by sheared flow to include electromagnetic effects at low plasma β (the ratio of plasma to magnetic pressure). The extended system of characteristic equations is formulated, for a dissipative fluid model developed from the gyrokinetic equation, using a twisting mode representation in sheared slab geometry and focusing on the ion temperature gradient mode. Perpendicular flow shear convects perturbations along the field at the speed we denote as Mcs (where cs is the sound speed). is required to make the system characteristics unidirectional and inhibit eigenmode formation, leaving only transitory perturbations in the system. This typically represents a much larger flow shear than in the electrostatic case, which only needs M > 1. Numerical investigation of the region shows the driving terms can conflict, as in the electrostatic case, giving low growth rates over a range of parameters. Also, at modest drive strengths and low β values typical of experiments, including electromagnetic effects does not significantly alter the growth rates. For stronger flow shear and higher β, geometry characteristic of the spherical tokamak mitigates the effect of an instability of the shear Alfvén wave, driven by the parallel flow shear. [ABSTRACT FROM AUTHOR]

Energetic particle transport in toroidal magnetic confinement fusion devices can be enhanced by the particles' interaction with electromagnetic global modes. This process has been modelled numerically. The most extensive work has been with reduced models, which may use a simplified description of the bulk plasma, assuming a perturbative approximation for mode structure evolution, restrict simulation to the linear phase, or some combination. In this work, nonlinear non-perturbative simulations are performed using a fully gyrokinetic and reduced models of the bulk plasma. Previous linear investigation of a simple model tokamak case is extended to show that, at least under some conditions, dramatic qualitative differences in mode structure and saturated mode amplitude can exist due to non-perturbative response in the linear and nonlinear phases that depends upon the bulk plasma physics. This supports analytical work which has shown that the non-perturbative energetic particle response should depend upon the magnetic geometry and kinetic physics. It is also shown that energetic particle modes that dominate in the linear phase can be subdominant to a non-perturbative toroidal Alfvén eigenmode-based global structure in the nonlinear phase. [ABSTRACT FROM AUTHOR]

A fully implicit particle-in-cell method for handling the v ∥ -formalism of electromagnetic gyrokinetics has been implemented in XGC. By choosing the v ∥ -formalism, we avoid introducing the nonphysical skin terms in Ampère's law, which are responsible for the well-known "cancellation problem" in the p ∥ -formalism. The v ∥ -formalism, however, is known to suffer from a numerical instability when explicit time integration schemes are used due to the appearance of a time derivative in the particle equations of motion from the inductive component of the electric field. Here, using the conventional δf scheme, we demonstrate that our implicitly discretized algorithm can provide numerically stable simulation results with accurate dispersive properties. We verify the algorithm using a test case for shear Alfvén wave propagation in addition to a case demonstrating the ion temperature gradient-kinetic ballooning mode (ITG-KBM) transition. The ITG-KBM transition case is compared to results obtained from other δf gyrokinetic codes/schemes, whose verification has already been archived in the literature. [ABSTRACT FROM AUTHOR]