Novel magnet structures for free-electron lasers

The high-energy rare-earth permanent magnets afford iron-free wiggler and twister field sources with greater field strengths in less massive structures than are possible when iron pole pieces and flux conductors are employed. However, the iron-free configurations have been notoriously difficult to adjust, as operational efficiency is very sensitive to the influence of small defects incurred in magnet manufacture and assembly. A general method for the compensation for such defects with small strategically placed magnetic dipoles is discussed and applied to structures of square, triangular, and circular cross section. The various configurations are compared with respect to design and convenience of defect compensation. The compensation of permanent magnet structures with mechanically variable field strength is also described together with a procedure for reversibly changing the function of such structures from wiggler to twister.