Inherent magnetic field error measurements in ELMO Bumpy Torus

The inherent magnetic field error and its radial distribution in the closed field line device of ELMO Bumpy Torus (EBT) have been measured with an electron beam probe. It is found that the field error is almost radially uniform and is about ΔB≤4 G at an average field of B≂7.2 kG. This yields ΔB/B≂5×10− 4, which is large enough to open the confined drift surfaces of the toroidally circulating electrons as a result of field‐error‐dominated large radial shifts. By using two pairs of correction coils (loops) around the device, this inherent field error has been successfully reduced to a level of 10− 4 or less.