School of Physics - Theses
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ItemSuppression of Emittance Growth in a Cold Atom Electron and Ion Source( 2020)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.
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ItemElectron diffraction using a cold atom source( 2017)Observing matter on atomic length and time scales simultaneously is now routinely achieved in ultrafast electron and X-ray imaging techniques, but continued advances in both approaches promise to deliver huge leaps in our understanding of all kinds of atomic structures and processes. Both technologies rely on the generation of ultrabright, ultrashort duration electron bunches, with these bunches being used directly to probe the sample in electron diffraction, or to generate ultrabright X-ray pulses in X-ray free electron lasers. The Cold Atom Electron and Ion Source (CAEIS) was conceived as a possible way to generate ultrafast electron bunches that are brighter than can currently be produced, with the aim of enabling next-generation structural determination techniques, particularly those based on electron diffraction. The CAEIS generates electrons by near-threshold photoionisation of an atomic gas, which has been shown to produce electron bunches with temperature as low as 10K. Such cold electron bunches have the potential to be much brighter than those generated from solid photocathode sources, which typically have temperatures in the thousands of Kelvin range. Extremely cold ions are also generated in the CAEIS, which show great potential for use in ion microscopy and milling. This thesis presents work on a number of different aspects of the continued development of the cold atom electron and ion source, with a particular emphasis on progress towards ultrafast single-shot electron diffraction based experiments. The brightness degrading effects of space-charge repulsion are investigated using nanosecond duration ion bunches as analogues of ultrafast, picosecond duration electron bunches. Ion bunch shaping was achieved through tailoring of the spatial profile of lasers used in ionisation of the atomic gas. It was found that atomic fluorescence could substantially reduce the fidelity with which the ion bunch profile could be controlled, but methods were developed to circumvent the fluorescence problem. The improved shaping procedures allowed generation of uniformly filled ellipsoidal bunches, which theoretically will not suffer emittance degradation under space-charge expansion. Emittance measurements following space-charge driven expansion showed that these uniformly filled ellipsoids did indeed have reduced emittance growth compared to other profiles. Photoexcitation and field-ionisation processes involved in generation of cold electrons on ultrafast timescales were investigated, with the aim of determining the mechanisms that affect the ultimate electron bunch duration. Bunch duration was measured for a range of excitation conditions, with the finding that previously assumed ultrafast excitation pathways in fact generated fairly slow nanosecond long bunches. Ionisation time could also be a million times slower than assumed if atoms were excited below the classical ionisation threshold. Identification of the conditions required for ultrafast excitation and ionisation ultimately allowed generation of ultrafast cold electron bunches with duration of tens of picoseconds. Electron diffraction using nanosecond long electron bunches was achieved in both transmission and reflection modes for a variety of large samples of inorganic crystals. Bunches were of sufficiently high charge to allow identification of features of a crystal structure using only a single shot. Bragg peaks could also be identified by averaging together many images formed using ultrafast, but low-charge bunches. Simulations were performed to determine the feasibility of using electrons generated in the CAEIS for electron coherent diffractive imaging of nanoscale apertures. It was found that it should be possible to successfully reconstruct the object plane wavefield, even taking into account realistic experimental parameters for partial coherence and noise.
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ItemMeasurement and modelling of intrabeam coulomb interactions in ultracold ion bunches( 2017)Control of Coulomb-induced emittance growth in charged particle beams is of critical importance for applications including electron and ion microscopy, injectors for particle accelerators and in ultrafast electron diffraction, where Coulomb effects constrain the temporal and spatial imaging resolution. The development of techniques to prevent space-charge and disorder-induced emittance growth has been limited by the masking effect of thermal diffusion in conventional beams. In this thesis it is shown that ion bunches from a cold-atom electron and ion source can be used to observe the effects of intrabeam Coulomb interactions with unprecedented detail. Experiments are performed using nanosecond-duration cold ion bunches, which provide data for analogous ultrafast electron systems where the dynamics occur on timescales too short for detailed observation. Cold ion bunches were produced by photoionising a laser-cooled gas. The intensity profile of the photoionising lasers was controlled using a spatial light modulator, which allowed for shaping of the spatial charge density distribution of the ion bunches. Nonlinear space-charge expansion dynamics were observed in the propagation of the ion bunches. Certain aspects of the observed dynamics were inconsistent with initial modelling attempts. In particular, high-density rings were formed in the transverse density distribution, which were not predicted in particle tracking simulations for the calculated initial ion distributions. Through detailed modelling, it was determined that the rings form in the interaction of the expanding ion beam with a diffuse ‘fluorescence halo’ of ions. The fluorescence halos were formed by reabsorption of fluoresced light from the sequential photoexcitation and ionisation process. Modelling of the photoexcitation process and particle-tracking simulations reproduced the experimentally observed beam dynamics, confirming the hypothesis of halo formation. The nonlinear transformation of the beam density profile leading to the formation of the fluorescence halo rings is indicative of loss of beam coherence. The fluorescence halo rings were suppressed by controlling the duration of laser overlap during photoionisation, where a shorter overlap reduces the time available for absorption of fluorescence. Fluorescence halo rings are an issue specific to atomic-gas based sources, but serve as an example of a nonlinear space-charge effect that is observable only because of the non-diffusive cold propagation of the ions. Efforts towards reconstruction of the beam dynamics leading to the ring formation were used to show that the cold ion bunches can be used as a platform to observe space-charge dominated beam dynamics in analogue of high-brightness and ultrafast electron beams. The cold ion beams were then used as a platform to investigate methods to overcome space-charge-induced beam-quality limitations. Modelling and experimental efforts were contributed to proof-of-principle experiments that demonstrated linearisation of the space-charge effects through beam-shaping. The ion bunches were shaped to uniform transverse distributions to linearise the internal electric field, suppressing the nonlinear space-charge effect. Improvements to the focusing properties of the shaped ion beams were measured, as compared to unshaped bunches, directly demonstrating improvement of beam quality through beam shaping for the first time in any charged particle beam. Beyond the linearisable space-charge effects are statistical disorder-induced heating (DIH) effects, which set lower-bound achievability limits on particle beam temperature. Models of the DIH process were used to predict the degree to which DIH can be suppressed in cold ion bunches by introducing interparticle spatial correlations in the cold atoms prior to ionisation. Two different methods of introducing correlations were modelled: first, by exploiting the Rydberg blockade effect in the photoexcitation process to excite and ionise atoms with hard-sphere type spatial correlations limited by close-packing effects; and second, by loading the atoms into optical lattices, which have crystalline structural correlations, limited by partial-filling effects. The models predicted that the heating can be significantly suppressed in the cold ion bunches for experimentally achievable degrees of spatial correlation using either of the two correlation methods. Excitation of Rydberg atoms was implemented in the cold-atom ion source, towards achieving the improvement of beam quality predicted by the DIH modelling. Spectroscopy based on electromagnetically-induced transparency was used to tune photoionising lasers to resonance with Rydberg states. A method was presented for suppressing the formation of fluorescence halos during Rydberg excitation, by using intermediate-state-decoupled stimulated Raman adiabatic passage to excite Rydberg atoms while bypassing population of the fluorescent intermediate state in sequential (ladder) photoexcitation. Measurements of the blockaded photoexcitation dynamics of the Rydberg ion bunches established the presence of spatial correlations, to a degree consistent with a sevenfold increase in beam brightness compared to a disordered distribution, according to the DIH models. The models, simulations, methods and measurements presented in this thesis guide the development of charged particle beams towards attaining the necessary coherence, focusability, and brightness to perform single-shot ultrafast electron diffraction of biological molecules. In a surprising twist, slow atoms may underpin progress in high energy and ultrafast physics.