TEM turbulence optimisation in stellarators

With the advent of neoclassically optimised stellarators, optimising stellarators for turbulent transport is an important next step.The reduction of ion-temperature-gradient-driven turbulence has been achieved via shaping of the magnetic field, and the reduction of trapped-electron mode (TEM) turbulence is adressed in the present paper.Recent analytical and numerical findings suggest TEMs are stabilised when a large fraction of trapped particles experiences favourable bounce-averaged curvature.This is the case for example in Wendelstein 7-X [C.D. Beidler et al Fusion Technology 17, 148 (1990)] and other Helias-type stellarators.Using this knowledge, a proxy function was designed to estimate the TEM dynamics, allowing optimal configurations for TEM stability to be determined with the STELLOPT [D. A. Spong et al Nucl.Fusion 41, 711 (2001)] code without extensive turbulence simulations.A first proof-of-principle optimised equilibrium stemming from the TEM-dominated stellarator experiment HSX [F.S.B.Andersonet al, Fusion Technol.27, 273 (1995)] is presented for which a reduction of the linear growth rates is achieved over a broad range of the operational parameter space.As an important consequence of this property, the turbulent heat flux levels are reduced compared with the initial configuration.