Effect of magnetic drifts on global ion-temperature-gradient modes in helical configurations

Ion-Temperature-Gradient (ITG) modes are studied in straight stellarator configurations with a global approach based on a time-evolution, Particle-In-Cell, finite element formulation of the gyrokinetic equations. The global code GYGLES [I] has been adapted to the helical symmetry and will also serve as a benchmark for a 3-D gyrokinetic code now under development [2]. In this paper we consider a straight heliac configuration characterized by Virtually zero shear, elongated bean-shaped cross sections, and a helical curvature of the magnetic axis. We study in particular the marginal stability points and the transition from "slab-like" or "Trapped-Ion-Mode" (TIM) to interchange-like ("helical-ITG") regimes for different kll, LT and mode number values. The helical-ITG has a large critical gradient, whereas the other modes have a stability behaviour that critically depends on k(parallel to) resonances and is strongly affected by del B drifts.