Correcting for FeL magnetic field distortions: The method of bilinear shimming

A THz free electron laser is being built in Ariel University. Upon completion it is intended to become a user facility. The free electron laser is designed to emit radiation between 1 and 5 THz. It is planned to operate in the super radiance regime. The anticipated output of the free electron laser, is in excess of 150 kW instantaneous power. An essential part of every free electron laser is a device generating a periodic magnetic field denoted as the wiggler. One of the main requirements of a free electron laser instrument is to achieve nominal magnetic field values with a high accuracy, along the main axis of the free electron laser's permanent magnetic periodic undulator. For practical reasons, mainly due to magnets manufacturing, there are deviations of the magnetic field of the magnets bars which construct the wiggler device in reference to the theoretical magnetic field. In many cases, this deviation reduces the efficiency of the radiation. The magnetic bars magnetization can be treated as a random variable for which we assume to have a normal random distribution having a standard deviation from 5% to 10% (depending on the quality of magnet's manufacturing). Our main purpose is to optimize the magnetic field by adding small dipole-like magnets which enable corrections for the noise in practical magnetic fields. This enables us to minimize the unwanted field noise and achieve a more optimal wiggler device.