Microwave inverse Cerenkov accelerator

Testing and analysis have been carried out on a dielectrically lined waveguide, which appears to be a suitable structure for accelerating electrons. From the dispersion relation for the TM01 mode, inner and outer radii of a copper-clad alumina pipe (var-epsilon=9.40) have been determined such that the phase and group velocities are 0.9732c and 0.1096c, respectively. Analysis and particle simulation studies for the injection of 6-MeV microbunches from a 2.856-GHz rf gun, and subsequent acceleration by the TM01 fields, predict that an acceleration gradient of 6.3 Mv/m can be achieved with a traveling-wave power of 15 MW applied to the structure. Synchronous injection into a narrow phase window allows trapping of all injected particles. The rf fields of the accelerating structure provide radial focusing, so that longitudinal and transverse emittance growth during acceleration is small and that no external magnetic fields are required for focusing. The acceleration mechanism is the inverse of that in which electrons radiate as they traverse a waveguide at speeds exceeding the phase velocity of the microwaves (Cerenkov radiation) and is thus referred to as a microwave inverse Cerenkov accelerator. For 0.16-nC, 5-psec microbunches, the normalized emittance of the accelerated beam is predicted to be less than 5π mrad. Experiments on sample alumina tubes have been conducted that verify the theoretical dispersion relation for the TM01 mode over a two-to-one range in frequency. No excitation of axisymmetric or nonaxisymmetric competing waveguide modes was observed. High power tests showed that tangential electric fields at the inner surface of an uncoated sample of alumina pipe could be sustained up to 8.4 MV/m without breakdown. These considerations suggest that a microwave inverse Cerenkov test accelerator can be built to examine these predictions using an available rf power source, a 6-MeV rf gun, and an associated beam line