Magnetic field diagnostics for an FEL wiggler

Magnetic field measurements have been performed on a 5-m wiggler magnet array that forms part of a free-electron laser experiment currently in operation at LLNL. The pre-installation tests used a Hall probe to map the transverse field on and near the beam axis, and integrating coils to determine the cancellation errors that tend to steer the beam. For on-line diagnostics, the authors use a bank of 31 rotating-coil gaussmeters that are mounted off-axis but from which the on-axis field is inferred. The gaussmeters form part of a closed-loop control system through which the operator can set and automatically maintain a desired field profile to within 0.1%. The paper discusses pre-installation measurement and calibration techniques and results, and operational experience with the closed-loop control system