Gyrokinetic simulations of magnetohydrodynamic modes in stellarator plasmas

For small-shear helical-axis stellarators, linear ideal-magnetohydrodynamic (MHD) stability calculations and full-torus, nonlinear, electromagnetic gyrokinetic (GK) simulations (the latter with this unprecedented combination of objectives in stellarator GKs) in their linear phase are shown to yield well agreeing spatio-temporal structures of unstable, globally extended perturbations. Likewise, good agreement is found for their dependence on the plasma pressure and the vacuum-field magnetic well in plasma equilibria with identical gradient lengths of the temperature and density profiles. In the nonlinear phase, these perturbations with MHD signatures entail deformations of the magnetic surfaces, growing magnetic islands which rotate in the electron diamagnetic direction and, eventually, lead to ergodisation of a larger part of the magnetic surfaces.