Magnetic Alignment of NCSX and Control of Field Errors
Authors: M.C. Zarnstorff, N. Pomphrey, and A. Brooks
Date of PPPL Report: June 2006
Presented at: the Thirty-Third European Physical Society Conference on Plasma Physics, 19 – 23 June 2006, Rome, Italy.
Careful elimination of magnetic field errors is crucial for optimum performance of stellarators and tokamaks. Resonant error fields produce islands in the magnetic topology, reducing confinement. Error fields also damp rotation, and can cause MHD mode locking. In existing experiments, error fields due to millimeter-scale offsets between coils or misshaping of coils have produced measurable effects on the plasma confinement. Similar errors have been deliberately introduced to study their effect. Low-order field errors are the most dangerous, due to their long radial decay lengths. To prevent them, coil positions and shapes are usually measured and aligned mechanically during construction. However, this may be inadequate due to the coil conductors being encased by insulation and support structures of variable thickness. In addition, tolerance errors can build up through multiple assembly steps, producing significant deviations. During operation, the magnetic field can be laboriously measured directly to search for errors and, in stellarators, the resonant errors can be measured using electron-beam mapping. However, once operation has begun, correcting coil misalignment may not be possible.