Coherent high-power RF wakefield generation by electron bunch trains in a metamaterial structure

We present an experimental study of coherent high-power wakefield generation in a metamaterial (MTM) structure at 11.7 GHz by 65 MeV electron bunch trains at the Argonne Wakefield Accelerator (AWA), following a previous experiment, the Stage-I experiment, at AWA. Both the Stage-II experiment, reported in this paper, and the Stage- I experiment were conducted using MTM structures, which are all-metal periodic structures with the period much smaller than the wavelength. Differences between the two experiments include: (1) Structure length (Stage-I 8 cm, Stage-II 20 cm); (2) Number of bunches used to excite the structure (Stage-I with 2 bunches, up to 85 nC of total charge; Stage-II with 8 bunches, up to 224 nC of total charge); (3) Highest peak power measured (Stage-I 80 MW in a 2 ns pulse, Stage-II 380 MW in a 10 ns pulse). The high-power radiofrequency (RF) pulses were generated by reversed Cherenkov radiation of the electron beam due to the negative group velocity in the MTM structures. Because the radiation is coherent, a train of bunches with a proper spacing can build up to achieve a high peak power. The observed output power levels are very promising for future applications in direct collinear wakefield acceleration or in transfer to a second accelerator for two beam acceleration.