Compact ultracold electron source based on a grating magneto-optical trap

Abstract

The ultrafast and ultracold electron source, based on near-threshold photoionization 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 characterized. Beam energies up to 10 keV were measured using a time-of-flight method. The normalized root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in ε=1.9 nm rad. The root-mean-squared transverse size of the ionization volume is 30 μm or larger, implying an electron source temperature in the few-10 K range, 2–3 orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.