Electromagnetic design of the transition section between modules of a wakefield accelerator

The electromagnetic design of a transition section consisting of couplers for extracting the 180-GHz TM01 accelerating mode and 190-GHz HE11 dipole mode of a cylindrical corrugated waveguide used as a collinear wakefield accelerator is presented. Extraction of the high power accelerating mode reduces the power dissipation in the corrugated accelerating structure and allows the much weaker HE11 mode to be isolated and used to monitor the stability of the 10-nC electron drive bunch. The final design demonstrates wide bandwidth and high coupling efficiency of two couplers. The design was also optimized to have the surface electric and magnetic fields that do not exceed those in the corrugated waveguide. Finally, coupling between the drive electron bunch and the HE11 dipole mode in the corrugated waveguide is considered and the utility of the TE11 coupler for detecting the electron beam oscillations in the wakefield accelerator is demonstrated using a few representative examples of oscillations below and above a threshold of the beam breakup instability.