Suppression of Emittance Growth Using a Shaped Cold Atom Electron and Ion Source

D. J. Thompson, D. Murphy, R. W. Speirs, R. M. W. van Bijnen, A. J. McCulloch, R. E. Scholten, and B. M. Sparkes

Phys. Rev. Lett. 117, 193202 – Published 3 November 2016

Abstract

We demonstrate precise control of charged particle bunch shape with a cold atom electron and ion source to create bunches with linear and, therefore, reversible Coulomb expansion. Using ultracold charged particles enables detailed observation of space-charge effects without loss of information from thermal diffusion, unambiguously demonstrating that shaping in three dimensions can result in a marked reduction of Coulomb-driven emittance growth. We show that the emittance growth suppression is accompanied by an increase in bunch focusability and brightness, improvements necessary for the development of sources capable of coherent single-shot ultrafast electron diffraction of noncrystalline objects, with applications ranging from femtosecond chemistry to materials science and rational drug design.

Received 20 May 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.193202

Physics Subject Headings (PhySH)

Atom & ion trapping & guiding Space charge in beams

Atomic, Molecular & OpticalAccelerators & Beams

Authors & Affiliations

D. J. Thompson¹, D. Murphy¹, R. W. Speirs¹, R. M. W. van Bijnen², A. J. McCulloch¹, R. E. Scholten^1,*, and B. M. Sparkes¹

¹School of Physics, The University of Melbourne, Victoria 3010, Australia
²Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

^*scholten@unimelb.edu.au

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Vol. 117, Iss. 19 — 4 November 2016

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Images

Figure 1
(a) Two-color laser excitation scheme used to ionize laser-cooled ${}^{85}{Rb}$ atoms. (b) Cold atom electron and ion source with bunch shaping. The intensity profile of the excitation laser coupling the $5 S$ ground state to the $5 P$ intermediate state was shaped using a spatial-light modulator (SLM) with iterative feedback provided via a CMOS camera [25]. Atoms were ionized with a 5 ns pulsed blue laser, focused to a narrow ribbon perpendicular to the excitation laser. The ions were accelerated into a drift region and focused with an einzel lens. A knife edge was inserted into the bunch around the focus to determine the transverse focal spot width. Spatial bunch profiles and bunch charges were measured with a phosphor-coupled microchannel plate (MCP) detector combined with a CCD camera (not shown). (c) Measured radially averaged excitation laser profiles (solid lines) and desired profiles (dashed lines), plotted as the relative excitation probability. Insets show desired transverse bunch density profiles as shaded false-color renderings. All radial averages and density profiles are individually normalized.
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Figure 2
(a) Experimentally measured transverse ion beam density profiles $ρ_{N} (x, y)$ for ion number $N = 2 000$ and 71 000, for HS, GS, FT, and CN initial distributions (scale bar, 2 mm). (b) Radial expansion factors against ion number for each shape individually, with circle, plus, times, and square corresponding to the transverse radii containing 50%, 75%, 90%, and 95% of the bunch charge, respectively. The divergence of the expansion factors at high ion numbers indicate nonlinear space-charge forces, most prevalent in the GS and CN bunches. Simulated expansion factors $e_{95}$ (dashed lines) and $e_{50}$ (solid lines) for each shape are also shown. Measured radii are averaged from 100 ion bunches.
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Figure 3
(a) Example knife-edge plot of relative transmission (points) and erf fit (dashed line) to determine the transverse rms width $σ_{r}$ at a given $z$ position. (b) Example $z$ scan of knife-edge transmission around the focus. Points indicate the knife-edge measurement and the dashed line is a weighted parabolic fit to determine the minimum rms width $σ_{f}$ . Error bars are 95% confidence intervals determined from the fit in (a). (c) Experimentally measured minimum rms width (left-hand axis, points) and simulated emittance of freely expanding bunches (right-hand axis, lines) as a function of ion number for the four transverse spatial profiles: HS (blue, circles, solid line), GS (red, squares, dashed line), FT (green, crosses, dotted line), and CN (purple, stars, dashed-dotted line). Uncertainty in ion number is determined from standard deviation of ion numbers from all knife-edge measurements used to determine $σ_{f} (z)$ , uncertainty in $σ_{f}$ is determined from standard error of fitted parabolas in (b). Ion temperature for simulations was taken to be $T = 1 mK$ .
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