Beam Characterization at the KAERI UED Beamline

TL;DR

This paper presents a novel method for accurately measuring beam emittance in ultrafast electron diffraction setups, and introduces a new charge measurement system capable of detecting femtosecond pulses at very low charge levels.

Contribution

It develops a thick-lens quadrupole scan technique for emittance measurement that does not require large drift spaces, improving precision and applicability.

Findings

Validated the thick-lens emittance measurement method.

Developed a charge measurement system with 10 fC sensitivity.

Successfully measured femtosecond electron bunches at 3.3 MeV.

Abstract

The UED (ultrafast electron diffraction) beamline of the KAERI's (the Korea Atomic Energy Research Institute's) WCI (World Class Institute) Center has been successfully commissioned. We have measured the beam emittance by using the quadrupole scan technique and the charge by using a novel measurement system we have developed. In the quadrupole scan, a larger drift distance between the quadrupole and the screen is preferred because it gives a better thin-lens approximation. A high bunch-charge beam, however, will undergo emittance growth in the long drift caused by the space-charge force. We present a method that mitigates this growth by introducing a quadrupole scan with a short drift and without using the thin-lens approximation. The quadrupole in this method is treated as a thick lens, and the emittance is extracted by using the thick-lens equations. Apart from being precise, our method can be readily applied without making any change to the beamline and has no need for a big drift space. For charge measurement, we have developed a system consisting of an in-air Faraday cup (FC) and a preamplifier. Tests performed utilizing 3.3-MeV electrons show that the system was able to measure bunches with pulse durations of tens of femtoseconds at 10 fC sensitivity.