# Monochromaticity of coherent Smith-Purcell radiation from finite size grating

## TL;DR

This paper investigates the spectral properties of coherent Smith-Purcell Radiation from finite gratings through experiments and simulations, revealing that spectral line monochromaticity improves with diffraction order and comparing it with transition radiation.

## Contribution

It provides experimental and numerical analysis of SPR spectral line shapes from finite gratings, demonstrating increased monochromaticity at higher diffraction orders and comparing SPR with transition radiation.

## Key findings

- Spectral lines become more monochromatic at higher diffraction orders.
- Experimental results agree well with CST simulations.
- Brightness of SPR is comparable to transition radiation in sub-THz range.

## Abstract

Investigation of coherent Smith-Purcell Radiation (SPR) spectral characteristics was performed both experimentally and by numerical simulation. The measurement of SPR spectral line shapes of different diffraction orders was carried out at KEK LUCX facility. A pair of room-temperature Schottky barrier diode (SBD) detectors with sensitivity bands of $60-90$~GHz and $320-460$~GHz was used in the measurements. Reasonable agreement of experimental results and simulations performed with CST Studio Suite justifies the use of different narrow-band SBD detectors to investigate different SPR diffraction orders. It was shown that monochromaticity of the SPR spectral lines increases with diffraction order. The comparison of coherent transition radiation and coherent SPR intensities in sub-THz frequency range showed that the brightnesses of both radiation mechanisms were comparable. A fine tuning feasibility of the SPR spectral lines is discussed.