ERROR FIELD MEASUREMENT AND ANALYSIS FOR A PURE PERMANENT-MAGNET FEL WIGGLER

A two meter long pure permanent magnet FEL wiggler with a 4 cm period and a 2.4 kG field on-axis has been fabricated and tested for the random field error parameters which are relevant to high gain FEL performance. The wiggler is part of a submillimeter-wave alpha particle Thomson scattering diagnostic for fusion plasmas. The integrated magnetic field along the electron beam trajectory was measured for each individual wiggler magnet. The magnets were sorted into a configuration which minimizes the random field-error-induced electron beam steering and displacement. A description of the magnetic field error requirements for the FEL, the error field measurement technique, measurement results and magnet sorting algorithms are presented. After sorting, a measured rms field error of less than 0.5% was obtained.