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.
Highlights
In the past decade new tools have emerged that allow investigation of structural dynamics with atomic spatial and temporal resolution, i.e., as small as 0.1 nm and 100 fs: ultrafast electron microscopy [1,2,3,4], ultrafast electron diffraction [5,6,7,8] (UED) and x-ray crystallography using free electron lasers (XFELs) [9,10,11]
In this work we present a novel compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam
IV we will show that it is possible to operate a GMOT with a hole in the center of the grating and with large electric fields applied across the trapping region, which are both required for extracting electron bunches
Summary
In the past decade new tools have emerged that allow investigation of structural dynamics with atomic spatial and temporal resolution, i.e., as small as 0.1 nm and 100 fs: ultrafast electron microscopy [1,2,3,4], ultrafast electron diffraction [5,6,7,8] (UED) and x-ray crystallography using free electron lasers (XFELs) [9,10,11] This revolution would not have been possible without the spectacular development of ultrafast pulsed electron sources [12]. In the UCES the initial transverse angular momentum spread is decreased which results in increased beam coherence for a given source size [19,20,21] This significantly reduces Coulomb effects at the source which allows extraction of more charge, required for single-shot measurements. The rms transverse source size is 30 μm or larger, implying an electron source temperature less than 25 K
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