Tests with an Isochronous Recirculation System

An isochronous recirculation system is in operation at the MIT-Bates Accelerator Center. It is used to transport the electron beam from the output of the LINAC back to the input, to be reinjected so that the electrons pass through the accelerator a second time. The design and the general operation of the recirculation system and the Linac has been discussed elsewhere. The Linac accelerates electrons to 400 MeV in one pass with a duty factor near 1%, within an energy spread of always less than 0.3% and an emittance of 10 /el mm-mr. With the recirculation system the accelerator energy capability has been extended to 750 MeV. The accelerator is designed with a progressive series of stop bands to prevent the propagation of transverse modes, thus raising the calculated threshold of beam break-up (BBU) to more than an order of magnitude above operation needs for both single pass and recirculated operation. The recirculation transport system (RTS) is isochronous and highly achromatic allowing for an energy band pass of 6% in which the beam is fully transmitted through the accelerator twice. The admittance of the accelerator is about 0.3 mm-mrad for the recirculated beam. In this paper we describe, qualitatively, observationsmore » of the operation of the recirculation system under a variety of conditions. In particular, both simultaneous and head-to-tail recirculation have been accomplished. The former was done under a variety of phasing conditions of the second pass beam relative to the first. The instrumentation used in the evaluation of the tests include a beam current monitor (toroid) which measures the peak current, and the Rf loads of the output of the traveling wave waveguides which measure effectively the product of the current in the waveguide and the energy gain in that section of waveguide.« less