Isochronous beam recirculation magnet system

The beam energy capability of an electron linear accelerator can be extended by using a magnetic transport system to return the beam after passing once through the accelerator, to the injector end and then recirculating the beam through the accelerating structure several times. The most important property of the transport system is that it be a good achromat, since operability requirements and beam loading transients usually require at least a few percent energy bandwidth over which the beam will successfully traverse the accelerator during recirculation. The system must also be nearly isochronous, with linear phase-energy correlation corresponding to not more than a fraction of an RF wavelength over a few percent bandwidth and with a small second-order phase-energy correlaton coefficient. Such a system to be described in this paper has been designed and is currently under construction for the purpose of recirculating the beam of the 400 MeV Bates Linear Electron Accelerator