Measurements of flux surfaces in the ATF torsatron

Flux surfaces in the Advanced Toroidal Facility (ATF) torsatron have been measured using electron-beam techniques. In initial measurements, the flux surfaces were found to have islands at the location of low-order resonances, e.g., at m/n = 2, signifying the existence of field errors. Measurements made for different values of B₀ between -0.1 and +0.1 T showed that the island width was approximately constant at 5-6 cm, indicating that the error field was proportional to the main field. Modeling studies showed that the major field errors were due to uncompensated current feeds to the helical and vertical field coils producing magnetic dipoles. After modifications to the coil systems were made to compensate for the field errors, the flux surface measurements were repeated at fields up to 1 T. They indicate that the island at m/n = 2 has been largely reduced in size to approximately 2.6 cm at B₀ = 0.1 T, but that others, e.g., m/n = 3, have grown. The surfaces in the confinement zone between are of much better quality, with significantly reduced isolation. The island at m/n = 2 appears to scale with magnetic field in a manner consistent with a field error due only to a constant external field, whereas the island at m/n = 3 does not scale with magnetic field, indicating an intrinsic field error.