The Advanced Toroidal Facility is a large torsatron device with a major radius R0 = 2.1 m, an average plasma minor radius a ≈ 0.3 m, and a magnetic field B0 ≤ 2T. The sheared magnetic configuration [τ(0) ≈ 0.3, τ(a) ≈ 1] is produced by an l = 2, M = 12 field period helical winding set and associated circular vertical field coils. The segmented helical windings were constructed with a tolerance of ±1-mm deviation from the ideal winding law using computer-aided manufacturing and assembly techniques. Nevertheless, in the initial operating period, it was found that field errors produced significant magnetic islands (island width ≈6 cm at τ = ½), which reduced the effective plasma radius by ∼30%. The main cause of these islands was the toroidally asymmetric field perturbation produced by the geometry of the electrical coil feeds. After “symmetrization” of the buswork, the dominant magnetic islands were reduced in size to ≤1 cm at the operating field of 1 T.