Novel diagnostic for precise measurement of the modulation frequency of Seeded Self-Modulation via Coherent Transition Radiation in AWAKE

We present the set-up and test-measurements of a waveguide-integrated heterodyne diagnostic for coherent transition radiation (CTR) in the AWAKE experiment. The goal of the proof-of-principle experiment AWAKE is to accelerate a witness electron bunch in the plasma wakefield of a long proton bunch that is transformed by Seeded Self-Modulation (SSM) into a train of proton micro-bunches. The CTR pulse of the self-modulated proton bunch is expected to have a frequency in the range of 90–300 GHz and a duration of 300–700 ps. The diagnostic set-up, which is designed to precisely measure the frequency and shape of this CTR-pulse, consists of two waveguide-integrated receivers that are able to measure simultaneously. They cover a significant fraction of the available plasma frequencies: the bandwidth 90–140 GHz as well as the bandwidth 255–270 GHz or 170–260 GHz in an earlier or a latter version of the set-up, respectively. The two mixers convert the CTR into a signal in the range of 5–20 GHz that is measured on a fast oscilloscope, with a high spectral resolution of 1–3 GHz dominated by the pulse length. In this contribution, we describe the measurement principle, the experimental set-up and a benchmarking of the diagnostic in AWAKE.