Generation of shaped cold electron bunches for ultrafast electron diffraction

Abstract

This thesis presents the development of a new electron source with the goal of single-shot Ultrafast Electron Diffraction (UED) of biological samples. A source capable of UED should be both bright and coherent. These properties are both enhanced by reducing the electron temperature. A Cold Atom Electron Source (CAES) produces cold electron bunches by near-threshold ionisation of laser-cooled atoms. A Magneto-Optical Trap (MOT) is used to cool and trap rubidium atoms to a temperature of 70 μK, and electrons are liberated from the cold atoms using two-colour photoionisation. The temperature of the photoelectrons is as low as 10 K, limited by the extraction process.

Upon ionisation, a charged particle cloud is created from the cold atoms. An investigation into the origin of electron temperature is presented. The effect of ion position correlations within the charged particle cloud is shown to play a small role, but for low density, the major contribution is from the scattering of electrons from their parent ion. A model for the extraction of an electron from a atom in a Stark potential is developed and used to explain the observed distributions of photoelectrons. The effects of finite electron temperature on beam parameters relevant for diffraction are presented.

Electron beam quality can be degraded by Coulomb interactions within the bunch. Such effects can be ameliorated by controlling the initial electron density distribution to produce uniform ellipsoidal electron bunches. Ellipsoidal bunches have internal fields which are linear as a function of position, which upon evolution do not degrade the beam coherence, and the Coulomb expansion can be completely reversed using linear optics. The cold atom source has the unique capability to shape the initial electron density distribution in three dimensions. Control over the ionisation volume is achieved via spatially modulating the intensity of the light fields used for ionisation. A method for the production of arbitrarily shaped electron bunches was developed and implemented. Ellipsoidal electron bunches were produced, and in addition, were used to determine a source temperature of 15 K.

The ability to shape the initial electron bunch allowed for a novel implementation of the “pepper-pot” high-precision emittance measurement technique. The brightness of a beam is fundamentally limited by the initial phase space density of the source. Careful characterisation and optimisation of the initial emittance is therefore vital, and the unique shaping abilities of a CAES allow these measurements and optimisation to be performed in real time.

Cold Atom Electron Sources have previously been limited to production of electron pulses with duration of order nanoseconds, too long for UED. A method for reducing the pulse length to a few hundred picoseconds, short enough for Radio Frequency (RF) cavity compression to sub-100 fs, required for UED, is presented. The production of short electron pulses relies on the use of a femtosecond laser pulse and quasi-coherent two-colour photoionisation which reduces the pulse length. Counterintuitively, the effect of the high bandwidth of the laser pulse does not adversely affect the transverse beam qualities, and the electron pulses remain highly coherent.

The intrinsically high coherence of the electrons provided by a CAES, combined with the production of short, ideally distributed electron bunches, should allow for the realisation of a source capable of single-shot diffractive imaging of weakly scattering molecules.