Cold electrons from cold atoms


Electron sources are the basic components for many key applications in science. These range from electron diffraction and electron microscopy systems, through the study of fundamental quantum interactions, to state-of-the-art scientific research facilities that utilize particle accelerators for x-ray free electron lasers, light sources and high-energy electron-positron colliders.

Cold atom electron sources (CAES) have recently been proposed as an alternative to the photo-injectors and thermionic electron guns [1-3] currently adopted in these applications. The CAES produces electron beams with temperatures as low as 10K (ΔE ~ 1meV), which is several orders of magnitude lower than conventional sources. The resultant beam emittance (which is a measure of the beam quality) is then markedly improved, allowing ultra-high brightness high-resolution electron beams to be produced. These unique features are crucial for the next generation of particle accelerators and free electron lasers, and will result in a step-change in future structure studies using electron diffraction techniques, as widely used in areas from materials science through to biology.

The ultra-short, high brightness electron beams produced by this new source will have applications in many areas. These include time-resolved materials characterisation, through to femtosecond chemistry, electron microscopy, X-ray free electron lasers, and as injectors for accelerators. One of the key properties of ultra-cold electron beams is their diffraction resolution when compared to X-rays. The new CAES based facility being developed in Manchester will provide comparable performance to much larger and more costly single-shot X-ray diffraction facilities. In addition, since electrons interact with matter ~104 times more strongly than X-rays, it becomes possible to make sensitive measurements of structures with brightness requirements orders of magnitude lower than for X-rays. This will allow single-shot time-resolved structure studies to be conducted for the first time.

This is a collaborative project between researchers at the University of Manchester School of Physics & Astronomy, members of the Photon Science Institute, and the Cockcroft Institute for Accelerator Science in Daresbury.

Researchers working in this area are:

Dr Matthew Harvey2,3, Dr. Guoxing Xia1,2, Dr Will Bertsche1,2, Professor Andrew Murray1,3, Dr. Robert Appleby1,2, Dr. Oznur Mete1,2 and Professor Swapan Chattopadhyay2,3

1School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL; 2Cockcroft Institute, Daresbury Laboratory, Warrington, WA4 4AD; 3Photon Science Institute, University of Manchester, Manchester, M13 9PL

The cold atom electron source at Manchester

A fruitful collaboration between researchers at the University of Manchester and Cockcroft Institute is currently developing the cold-atom electron source at the Photon Science Institute (PSI). The cold atom source is nearing completion, and will provide a test-platform to characterise and optimise the production of cold electrons.

Cold electrons will be produced as follows. A dense atomic vapour will be laser-cooled and trapped in the new vacuum system (see figure 1) using one of the ultra-high resolution continuous wave (CW) lasers in the PSI. The atoms will be cooled to <100 µK in an alternating current magneto-optical trap (AC-MOT) [4] so that they have almost zero velocity. These trapped atoms will then be photo-ionized using a second high power laser, which will be tuned so that the resulting photo-electrons have very little excess energy. The plasma that is produced will then be subjected to an electrostatic field to extract, accelerate and shape the cold electrons into a beam. These experiments will make use of several of the central laser systems in the PSI, including CW laser systems for cooling, trapping and ionization, and pico- and femto-second lasers for creation of ultra-short electron pulses.

FIG 1. The new cold electron source at the PSI in Manchester, showing the electron beam acceleration and focussing optics to couple the electron beam into high energy accelerators and Free Electron Lasers.

FIG 2. The new source at the PSI in Manchester, nearing construction. This apparatus will be used to test and optimise cold electron production for input into new accelerators, and for low energy diffraction experiments in the future. The AC-MOT has yet to be installed and so is not shown in this figure. Details of the trapping and cooling processes can be found here.