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

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 microbunches. 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.

Proton Bunch Self-Modulation in Plasma with Density Gradient

“…and the second harmonic (∼240 GHz) frequencies of the modulation. Bench measurements show that these systems measure frequencies with <1 GHz accuracy [15]. The presence of harmonics in the signal indicates deep modulation of the charge density along the bunch, an indication of the nonlinear stage reached by the SM process, as can be seen in the profiles of Fig.…”

“…One meter downstream, the bunch train enters a metallic screen where it produces coherent transition radiation (CTR). We send the CTR to a heterodyne frequency measurement system that yields modulation frequency along the bunch (spectrogram [15]). Protons then produce backward (incoherent) optical transition radiation (OTR) when entering another nearby metallic screen [3,16].…”

“…We determine the bunch modulation frequency f CT R with the CTR diagnostic [15]. The CTR spectrum of a periodic bunch train consists of peaks at the modulation frequency and at its harmonics.…”

Recent advances of transition radiation: Fundamentals and applications

“…In the following decades, more optical components (e.g. wire grid, Michelson interferometer) are adopted to provide not only the longitudinal and transverse distributions of electron beams but also their divergent angle with the help of coherent transition radiation [72][73][74][75][76].…”

“…A brief review of the development of transition radiation is provided, including its interesting history and recent advances. To highlight the importance of transition radiation in science and technology, several typical applications of transition radiation are discussed in depth, ranging from high-energy particle detectors [56][57][58][59][60][61][62][63][64][65], coherent radiation source [66][67][68][69][70][71], beam diagnostics [72][73][74][75][76], to excitation of surface plasmon [77][78][79][80][81][82][83][84]. Due to the recent rapid development in material science and nanotechnology, an outlook on how to flexibly control the behavior of transition radiation is given by exploiting exotic artificially-engineered materials and nanostructures [85][86][87][88][89][90], including gain materials, spatial-temporal materials, meta-boundaries, and layered structures with a random stacking.…”