This thesis presents details of a new type of cold atom electron source (CAES) that uses an alternating current magneto-optical trap (AC-MOT). Cold atom electron sources produce electron bunches by threshold photoionisation from a population of laser-cooled atoms. The electrons extracted from the atoms are made into a beam which has an ultra-low transverse velocity distribution. The low velocity distribution increases the brightness and reduces the energy spread of the beam, making CAESs potential sources for future electron diffraction and electron spectroscopy studies.
The newly commissioned source detailed here produces focused electron bunches with characteristic temperatures as low as 12.6 ± 0.8 K, at a rate of up to 10 Hz, and with a bunch charge of 2.2±0.2fC, comparable to other state-of-the-art cold atom electron sources. The AC-MOT CAES can also produce cold bunches with energies <200 eV, which is lower than has previously been reported for this type of source.
The electrons originate from a population of laser-cooled 85Rb atoms held in an AC- MOT. The AC-MOT CAES design allows the magnetic trapping fields to be switched to zero in 32 μs, more than ��175 times faster than is possible using a comparable DC-MOT source. This means that electron bunches can be created and extracted in a magnetic field-free environment without any reduction in trap density and hence bunch charge, as occurs in conventional DC-MOT sources.
The design, development, characterisation, and operation of the systems needed for the new AC-MOT CAES are detailed in this thesis. This includes details of the atom trapping system, the photoionisation system, the electron extraction system and the various timing systems for creation and characterisation of the resultant cold electron bunches.
Finally, the characterisation of the AC-MOT CAES itself is presented, and the measured beam metrics are used to compare the new AC-MOT CAES with other CAES designs which are under development.
Abstract We report the generation of multiple electron beams using a cold atom electron source (CAES) combined with a spatial light modulator (SLM). In the experiment, neutral 87Rb atoms are confined in a magneto-optical trap and cooled to approximately 700μK. By shaping the intensity profile of the excitation laser via the SLM, the spatial distribution of the excited atoms can be precisely controlled. The excited atoms are then ionized by a dye laser, and the ionized electrons are extracted and focused by an electric field to form multiple electron beams. As a demonstration, we generate regular arrays of 3×3,4×4,5×5,6×6 beams, each with a spot size of about 100μm and a normalized transverse emittance of ∼0.4×10−9m⋅rad⋅eV. This programmable electron beam technique holds potential for applications in electron-beam lithography and wafer inspection.
Pulsed photoionization of laser-cooled atoms in a magneto-optical trap (MOT) has the potential to create cold electron beams of few meV bandwidths and few ps pulse lengths. Such a source would be highly attractive for the study of fast low-energy processes like coherent phonon excitation. To study the suitability of MOT-based sources for the production of simultaneously cold and fast electrons, we study the photoionization dynamics of trapped Cs atoms. A momentum-microscope-like setup with a delay-line detector allows for the simultaneous measurement of spatial and temporal electron distributions. The measured patterns are complex, due to the Lorentz force inducing spiral trajectories. Ray-tracing simulations reproduce the main features. We find that the production of electron bunches with bandwidths of a few meV is straightforward; however, pulses in the ps-range are more demanding and require beam blanking or partial blocking.