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

TL;DR

This paper introduces a novel heterodyne diagnostic setup for precisely measuring the frequency and shape of coherent transition radiation from proton bunches in the AWAKE experiment, enabling better characterization of plasma wakefield acceleration.

Contribution

The paper presents a new waveguide-integrated heterodyne diagnostic capable of simultaneous, high-resolution measurement of CTR frequency and shape in the 90-300 GHz range for plasma wakefield experiments.

Findings

Successful measurement of CTR in the 90-270 GHz range.

High spectral resolution of 1-3 GHz achieved.

Diagnostic setup validated in AWAKE experiment.

Abstract

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 will describe the measurement principle, the experimental set-up and a benchmarking of the diagnostic in AWAKE.