Compact ultracold electron source based on a grating magneto optical trap

TL;DR

This paper introduces a compact ultracold electron source using a grating magneto-optical trap that simplifies setup and maintains low emittance, enabling high-quality electron beams with energies up to 10 keV.

Contribution

The development of a simplified, compact GMOT-based ultracold electron source that requires only one laser beam and allows high electric fields for electron extraction.

Findings

Electron beams with energies up to 10 keV were achieved.

The normalized transverse emittance was measured at 1.9 nm.

The source temperature is in the few-10K range, much lower than conventional sources.

Abstract

The ultrafast and ultracold electron source, based on near-threshold photoionisation of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge. Its use is however still limited because of the required cold-atom laser-cooling techniques. Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module. This makes the technique more widely accessible and increases its applicability. We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches. The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition. The electron beams extracted from the GMOT have been characterised. Beam energies up to 10 keV were measured using a time-of-flight method. The normalised root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in $\epsilon = 1.9 \rm{nm}$. The root-mean-squared transverse size of the ionisation volume is $30 \mu\rm{m}$ or larger, implying an electron source temperature in the few-10K range, $2-3$ orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.