Dene Murphy

Detailed observation of space–charge dynamics using ultracold ion bunches

Control of Coulomb expansion 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 space-charge effects constrain the temporal and spatial imaging resolution. The development of techniques to reverse space-charge-driven expansion, or to observe shock waves and other striking phenomena, have been limited by the masking effect of thermal diffusion. Here we show that ultracold ion bunches extracted from laser-cooled atoms can be used to observe the effects of self-interactions with unprecedented detail. We generate arrays of small closely spaced ion bunches that interact to form complex and surprising patterns. We also show that nanosecond cold ion bunches provide data for analogous ultrafast electron systems, where the dynamics occur on timescales too short for detailed observation. In a surprising twist, slow atoms may underpin progress in high-energy and ultrafast physics.

Increasing the brightness of cold ion beams by suppressing disorder-induced heating with Rydberg blockade

A model for the equilibrium coupling of an ion system with varying initial hard-sphere Rydberg blockade correlations is used to quantify the suppression of disorder-induced heating in Coulomb-expanding cold ion bunches. We show that bunches with experimentally achievable blockade parameters have an emittance reduced by a factor of 2.6 and increased focusability and brightness compared to a disordered bunch. Demonstrating suppression of disorder-induced heating is an important step in the development of techniques for the creation of beam sources with sufficient phase-space density for ultrafast, single-shot coherent diffractive imaging.

High-coherence electron and ion bunches from laser-cooled atoms.

Charged particle sources based on photoionisation of laser cooled atoms can provide unique properties, in particular high spatial coherence and the ability to create complex three-dimensional spatial density distributions, allowing detailed measurement of the internal charged particle interactions. Cold electrons extracted from laser cooled atoms promise the spatial coherence and high current required for picosecond molecular scale imaging. Similarly, sources of cold ions provide the opportunity of ion microscopy and ion beam milling with unprecedented resolution. We use arbitrary and real-time control of the electron and ion bunch shapes to demonstrate and measure the high spatial coherence of the cold atom electron and ion source.

Disorder-induced heating of ultracold neutral plasmas created from atoms in partially filled optical lattices

We quantify the disorder-induced heating (DIH) of ultracold neutral plasmas (UCNPs) created from cold atoms in optical lattices with partial filling fractions, using a conservation of energy model involving the spatial correlations of the initial state and the equation of state in thermal equilibrium for a one-component plasma. We show, for experimentally achievable filling fractions, that the ionic Coulomb coupling parameter could be increased to a degree comparable to other proposed DIH-mitigation schemes. Molecular dynamics simulations were performed with compensation for finite-size and periodic boundary effects, which agree with calculations using the model. Reduction of DIH using optical lattices will allow for the study of strongly coupled plasma physics using low-density, low-temperature, laboratory-based plasmas, and lead to improved brightness in UCNP-based cold electron and ion beams, where DIH is otherwise a fundamental limitation to beam focal sizes and diffraction imaging capability.

Stimulated Raman adiabatic passage for improved performance of a cold-atom electron and ion source

We implement high-efficiency coherent excitation to a Rydberg state using stimulated Raman adiabatic passage in a cold atom electron and ion source. We achieve an efficiency of 60% averaged over the laser excitation volume with a peak efficiency of 82%, a 1.6 times improvement relative to incoherent pulsed-laser excitation. Using pulsed electric field ionization of the Rydberg atoms we create electron bunches with durations of 250 ps. High-efficiency excitation will increase source brightness, crucial for ultrafast electron diffraction experiments, and coherent excitation to high-lying Rydberg states could allow for the reduction of internal bunch heating and the creation of a high-speed single ion source.

Bunch Shaping for Improved Brightness with a Cold Atom Electron and Ion Source

We create ultracold ion bunches via precisely shaped photoionisation of laser cooled atoms that exhibit linear Coulomb self-field expansion, smaller emittance growth and hence improved brightness under transverse focusing in comparison to standard Gaussian bunches.

High-Coherence Electron and Ion Bunches from Laser-Cooled Atoms

Cold atom electron and ion sources produce electron bunches and ion beams by photoionisation of laser cooled atoms. They offer high coherence and the potential for high brightness, with applications including ultrafast electron diffractive imaging of dynamic processes at the nanoscale. Here we present our cold atom electron/ion source, with an electron temperature of less than 10 K and a transverse coherence length of 10 nm. We also discuss experiments investigating space-charge effects with ions and the production of ultra-fast electron bunches using a femto-second laser. In the latter experiment we show that it is possible to produce both cold and fast electron bunches with our source.

Arbitrarily shaped high-coherence electron and ion bunches from laser-cooled atoms

Charged particle sources based on photoionisation of laser cooled atoms can provide unique properties, in particular high spatial coherence and the ability to create complex three-dimensional spatial density distributions, allowing detailed measurement of the internal charged particle interactions. Cold electrons extracted from laser cooled atoms promise the spatial coherence and high current required for picosecond molecular scale imaging. Similarly, sources of cold ions provide the opportunity of ion microscopy and ion beam milling with unprecedented resolution. We use arbitrary and real-time control of the electron and ion bunch shapes to demonstrate and measure the high spatial coherence of the cold atom electron and ion source.