We propose a short period undulator based on the electromagnetic field pattern in a THz-driven split ring resonator structure. An analytical model is developed that allows us to assess the key performance parameters of the undulator and to estimate the emitted radiation spectrum. Different geometric configurations are compared in detail using numerical simulations. A 100 MeV electron bunch with 5 pC charge is shown to emit narrow band 83 eV photon pulses with a peak brightness of approximately 10$^{19}$ photons/(s mrad$^2$ mm$^2$ 0.1% BW) when passing through the 100 mm long undulator with a 1 mm period.

Physical review accelerators and beams 24(1), 010703 (2021). doi:10.1103/PhysRevAccelBeams.24.010703

Published by American Physical Society, College Park, MD