Suppression of Emittance Growth in a Cold Atom Electron and Ion Source

Abstract

Coulomb interactions within charged particle bunches manifest themselves through microscopic statistical Coulomb effects and macroscopic Coulomb explosion, also known as space-charge expansion. Coulomb explosion can lead to unwanted increases in the phase-space density or emittance of the source, which reduces overall focusability and brightness. Therefore, the ability to control, suppress and potentially eliminate space-charge-induced emittance growth in charged particle beams is of critical importance for applications in high-energy accelerator injection, high-brightness X-ray sources, electron and ion microscopy, and ultrafast electron diffraction (UED). The capacity to perform single-shot UED and coherent diffractive imaging experiments of protein membranes and biological samples is of particular interest; the "holy grail" of structure determination techniques. Such an experiment requires high bunch charge and short pulse durations, conditions that result in severe Coulomb explosion.

Conventional electron sources cannot simultaneously achieve the high brightness and high coherence properties required to dynamically image biomolecules, due to limitations imposed by Coulomb effects. Recently, a new generation of Cold Atom Electron and Ion Sources (CAEISs) have been developed and show promise in this regard, utilising low temperature to generate high brightness and coherence. The Melbourne CAEIS produces electron or ion bunches via two-colour near threshold photoionisation of laser-cooled rubidium atoms in a magneto-optical trap. The photoionisation laser can be tuned to excite electrons to the continuum with almost no excess energy, resulting in electron and ion bunch temperatures of approximately 10 K and 1 mK respectively, orders of magnitude lower than that of conventional field emission or photocathode sources. Without obfuscation from thermal diffusion, space-charged-induced effects that evolve within a bunch can be measured and alleviated by carefully tailoring the initial density profile.

Specifically, the ideal bunch is a three-dimensional (3D) uniform density ellipsoid of charge, which exhibits linear and therefore reversible Coulomb expansion and minimal emittance growth under acceleration and propagation. Such objects were first realised for radio frequency (rf) photocathode sources, whereby a prompt, half-spherical radial laser intensity distribution and strong accelerating field were used to generate and extract a pancake electron bunch from the cathode surface, which automatically evolves into a 3D uniform ellipsoidal bunch, provided certain criteria are met. This formalism is adapted to the Melbourne CAEIS using a spatial light modulator for transverse laser beam shaping to create ion bunches that undergo linear space-charge expansion. Nanosecond ion bunches are investigated as they exhibit strong space-charge effects that are analogous to picosecond electron dynamics, on time-scales relevant for UED.

An experimental framework is introduced to allow comparisons between CAEIS-generated half-spherical bunches and other common bunch distributions, namely Gaussian, flat-topped, and conical. By measuring Coulomb expansion for the chosen bunch shapes as a function of increasing density, growth factors are calculated and linear space-charge expansion is verified in a CAEIS for the first time. Particle tracking simulations are used to calculate emittance and emittance growth of cold, shaped bunches, with comparisons made to a thermal source. Transverse bunch focusing experiments are also presented which demonstrate suppression of space-charge-induced emittance growth via bunch shaping. By simulating an rf cavity in the CAEIS beamline for longitudinal bunch compression, 3D reversal of Coulomb explosion is explored and also confirms emittance suppression and brightness enhancement for particular shaped bunches. The concept, design and performance of a novel cateye external cavity diode laser for continuous CAEIS development is also described in this work.