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Slice Emittance Preservation and Focus Control in a Passive Plasma Lens

J. Björklund Svensson, J. Beinortaitė, L. Boulton, B. Foster, J. M. Garland, P. González Caminal, M. Huck, H. Jones, A. Kanekar, G. Loisch, J. Osterhoff, F. Peña, S. Schröder, M. Thévenet, S. Wesch, M. Wing, J. C. Wood, R. D'Arcy

Abstract

Strong, symmetrically focusing plasma lenses are promising for accommodating the small beams associated with plasma-based accelerators and collider final foci. However, while focusing with active and passive plasma lenses has been experimentally demonstrated, compatibility with high-brightness beams relevant for applications has not. In this Letter, we show experimentally that passive plasma lenses can preserve free-electron-laser-quality slice emittance while focusing two orders of magnitude more strongly than quadrupole magnets, and that the focal parameters can be controlled.

Slice Emittance Preservation and Focus Control in a Passive Plasma Lens

Abstract

Strong, symmetrically focusing plasma lenses are promising for accommodating the small beams associated with plasma-based accelerators and collider final foci. However, while focusing with active and passive plasma lenses has been experimentally demonstrated, compatibility with high-brightness beams relevant for applications has not. In this Letter, we show experimentally that passive plasma lenses can preserve free-electron-laser-quality slice emittance while focusing two orders of magnitude more strongly than quadrupole magnets, and that the focal parameters can be controlled.

Paper Structure

This paper contains 16 sections, 7 figures.

Table of Contents

  1. Introduction
  2. Experimental setup
  3. Slice emittance preservation
  4. Resolution limit
  5. Varying the lens parameters
  6. Driver bunch dynamics
  7. Outlook and conclusions
  8. Acknowledgements
  9. Author Contributions
  10. End Matter
  11. The beam emittance
  12. Single-optic chromaticity
  13. Chromaticity correction
  14. Imaging spectrometer
  15. PIC simulations
  16. ...and 1 more sections

Figures (7)

  • Figure 1: a. The main part of the experimental setup. The two bunches, propagating left to right, are pre-focused into the PPL, which is then used to focus further. The bunches are then imaged onto the spectrometer screen. b-d. Spectrometer images at three different quadrupole imaging settings along a measurement scan, with the horizontal projections of all images shown in e.
  • Figure 2: a. Measured RMS beam sizes (dots) and corresponding fits (colored lines) for the incoming (orange) and outgoing (blue) witness bunch, along with the corresponding fit parameters, for energy slice 12. The green line shows the median of Monte-Carlo simulations solving the betatron envelope equation through the Gaussian focusing channel (gray shaded area). Errors show the 95 % confidence interval (CI) of the fits. b. Incoming and outgoing slice normalized emittances. c. Outgoing waist betas. The slice emittances are preserved within the dotted lines, within which the measured, modeled, and simulated waist parameters agree. d. Measured relative emittance growth (c.f. b,c) plotted against measured waist beam sizes.
  • Figure 3: Projected witness-bunch waist parameters as a function of delay between the discharge in argon and beam arrival, thereby changing the plasma density (see End Matter). a. Normalized emittances. b. Waist betas, c. Waist locations. The reference quadrupole focus is a tight focus using the matching quadrupoles, without the PPL. The horizontal displacement between grouped error bars is for ease of visualization only. Errors show the 95 % CI of the fit parameters. d. Measured and simulated waist locations versus waist betas. Error bars removed for clarity.
  • Figure 4: a. Measured projected mismatch parameter between driver and witness as a function of the witness's waist beta. Horizontal error bars show the 95 % CI of the fits, vertical errors show the 95 % CI based on Monte-Carlo sampling of driver and witness waist parameters. b. Snapshot of the simulated beam-plasma interaction in the highest-density working point ($n_\mathrm{p,0}=5\times10^{14}$ cm-3, rightmost point in Fig. \ref{['fig:3']}d). Solid and dashed curves show the on-axis focusing gradient normalized to the nominal blowout gradient for the lowest and highest peak density working points. The transversely elliptical (as opposed to circular) driver causes the relative horizontal (vertical) gradients to overshoot (undershoot) 1 manwani_2025_PRL.
  • Figure 5: LPSs and horizontal slice parameters reconstructed from experimental data, used in the PIC simulations. Beam used with: a. nitrogen (Fig. \ref{['fig:2']}), b. argon (Figs. \ref{['fig:3']} and \ref{['fig:4']}).
  • ...and 2 more figures