Experimental results of magnetic surface mapping in the stellarator W VII-AS

Before starting plasma experiments in the new modular stellarator W VII-AS the magnetic surfaces have been studied in detail. W VII-AS has small shear, and by means of a superimposed toroidal field the rotational transform τ can be varied between about 0.25 and 0.70. Two different methods have been applied to map the vacuum magnetic surfaces. Both methods use a directed electron beam source that can be positioned on an arbitrary magnetic surface in the poloidal cross section. At magnetic fields Bo>0.2 T and electron energies < 150 eV used in the experiments single electrons emitted parallel to the magnetic field follow almost exactly the magnetic field lines. (author) 3 refs., 8 figs