Experimental determination of the magnetic field spectrum in the HelicallySymmetric Experiment using passing particle orbits

The leading terms of the magnetic field spectrum for the Helically Symmetri c Experiment [Fusion Technol. 27, 273 (1995)] at low magnetic field are det ermined by analyzing the orbits of passing particles. The images produced b y the intersection of electron orbits with a fluorescent mesh are recorded with a charge coupled device and transformed into magnetic coordinates usin g a neural network. To obtain the spectral components, the transformed orbi ts are then fit to an analytic expression that models the drift orbits of t he electrons. The results confirm for the first time that quasihelical stel larators have a large effective transform that results in small excursions of particles from a magnetic surface. The drift orbits are also consistent with a very small toroidal curvature component in the spectrum. An external magnetic perturbation, nearly resonant with the transform, is shown to ind uce a large excursion of the particle orbit off a flux surface. (C) 2001 Am erican Institute of Physics.