Formation of Ultrarelativistic Electron Rings from a Laser-Wakefield Accelerator

B. B. Pollock, F. S. Tsung, F. Albert, J. L. Shaw, C. E. Clayton, A. Davidson, N. Lemos, K. A. Marsh, A. Pak, J. E. Ralph, W. B. Mori, and C. Joshi

Phys. Rev. Lett. 115, 055004 – Published 31 July 2015

Abstract

Ultrarelativistic-energy electron ring structures have been observed from laser-wakefield acceleration experiments in the blowout regime. These electron rings had 170–280 MeV energies with 5%–25% energy spread and $\sim 10 pC$ of charge and were observed over a range of plasma densities and compositions. Three-dimensional particle-in-cell simulations show that laser intensity enhancement in the wake leads to sheath splitting and the formation of a hollow toroidal pocket in the electron density around the wake behind the first wake period. If the laser propagates over a distance greater than the ideal dephasing length, some of the dephasing electrons in the second period can become trapped within the pocket and form an ultrarelativistic electron ring that propagates in free space over a meter-scale distance upon exiting the plasma. Such a structure acts as a relativistic potential well, which has applications for accelerating positively charged particles such as positrons.

Received 23 October 2014

DOI:https://doi.org/10.1103/PhysRevLett.115.055004

Authors & Affiliations

B. B. Pollock^1,*, F. S. Tsung², F. Albert¹, J. L. Shaw², C. E. Clayton², A. Davidson², N. Lemos², K. A. Marsh², A. Pak¹, J. E. Ralph¹, W. B. Mori², and C. Joshi²

¹Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA
²University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095, USA

^*pollock6@llnl.gov

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Vol. 115, Iss. 5 — 31 July 2015

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Images

Figure 1
(a) Top-view schematic of the electron spectrometer. The magnetic field is in the direction of the black arrows, causing electron dispersion out of the page. Two image plates (IPs) record the electron flux exiting a vertically slotted vacuum window, which is 2.5 cm (45 mrad) wide and consists of $50 μ m$ -thick Al and $65 μ m$ -thick mylar foils. This window partially blocks the full transverse width of the ring in the spectral images (the dotted white lines). The magnet entrance is 3 cm from the plasma exit, and IP A (B) resides 62.2 (136.1) cm from the plasma exit. (b) 3D rendering from the OSIRIS simulation (see text) of the wake after 1 cm of plasma. The laser has been omitted for clarity. The solid blue-purple contours indicate the charge density of the wake structure, and the yellow-green dots represent the on-axis and ring electrons accelerated in the wake. (c) False-color raw (dispersion, not linearized) images of IP A electron data sorted by the gas composition ( $% He / % N_{2}$ by partial pressure; given above each image) from all of the cases where rings were produced in a 1 cm gas cell. The dotted black circle in the pure He case overlaps the ring. (Table) Laser, target, and electron beam parameters for each image in (c). The charge quoted is that in the visible portion of the image that passes through the vacuum window.
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Figure 2
Spectra for the (a) broad energy on-axis beam with 41 mrad plasma exit angle and (b) the “high-energy” side of the ring electron beam from the $90 / 10$ gas composition data shown in Fig. 1.
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Figure 3
(a),(c),(e) Electron density maps from 3D OSIRIS PIC simulations at 5, 7.5, and 10 mm of laser propagation, respectively. The $E_{⊥}$ color table corresponds to the envelope of the driving laser pulse at the right of the panels. (b),(d),(f) Maps of the focusing force of the wake. White regions represent isocontours of zero focusing force. The key physical features are the electron sheath at the wake perimeter, the splitting of the sheath into outer and inner streamlines resulting in the formation of the toroidal pocket shown in (e), and the electrons trapped into each of the first two buckets close to the laser axis and an annular ring of electrons residing within the pocket.
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Figure 4
After 8.8 mm of laser propagation in the plasma, the (a) phase space plot of the electrons from the second bucket and (b) spectrum of the ring electrons, with an inset showing the 3D simulation ring structure.
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