Dissipative Trapped Electron Modes in a Heliac

Abstract

The objective of the present paper is to study the dissipative trapped electron modes (DTEM) in a stellarator configuration. A perturbative-iterative process is used to study the non-adiabatic response of the electrons in the dissipative regimes on the drift wave instability. A ballooning mode formalism is used to drive the drift eigenvalue equation for adiabatic electron which is solved numerically. The eigenmodes are marginally stable. The non-adiabatic response of the electrons is calculated perturbatively which is used to re-solve the eigenvalue equation. The process is iterated until electrons response converges. The effects of different parameters such as ηe, the ballooning angle θk, normalised density scale length ϵn, radial label s, and χ are studied. The mode localisation is correlated with the geometrical properties of the magnetic field. It is found that the wave functions of the most unstable modes are highly localised in a single helical ripple with the possibility of a variety of both helical and toroidal trapping. It is further fond that a perturbative calculations of the growthrate generally is not reliable and can give substantially lower or higher growthrate than the iterative method.