Correcting for FEL Magnetic Field Distortions: The Method of Bilinear Shimming

  Asher Yahalom  ,  Itzhak Chaimov   
Ariel University

One of the main requirements of a Free Electron Laser (FEL) instrument is to achieve nominal B-field values with a high accuracy, along the main axis of the FEL’s permanent magnetic periodic undulator, known as Wiggler device of the Halbach configuration. From practical reasons, mainly due to magnets manufacturing, there are deviations of the magnetic field of the magnets bars which construct the Wiggler device with reference to the theoretical magnetic field - in most cases, this deviation reduces the efficiency of the radiation extraction - reducing the delivered power and energy extracted from the device and hence, giving rise to undesirable heat absorbed at the device, which in some cases can cause a de-magnetization affect at the magnet bars. The magnetic bars magnetization could be treated as a random variable for which we assume to have a normal random distribution having a standard deviation from 5% to 10% (depends on the quality of magnet’s manufacturing). Our main purpose is to optimize the magnetic field by adding small dipole-like magnets which correct for the noise in practical magnetic fields. This enable us to minimize the unwanted field noise to minimum and achieve a more optimal Wiggler device.