High-Flux Femtosecond X-Ray Emission from Controlled Generation of Annular Electron Beams in a Laser Wakefield Accelerator

Annular quasimonoenergetic electron beams with a mean energy in the range 200–400 MeV and charge on the order of several picocoulombs were generated in a laser wakefield accelerator and subsequently accelerated using a plasma afterburner in a two-stage gas cell. Generation of these beams is associated with injection occurring on the density down ramp between the stages. This well-localized injection produces a bunch of electrons performing coherent betatron oscillations in the wakefield, resulting in a significant increase in the x-ray yield. Annular electron distributions are detected in 40% of shots under optimal conditions. Simultaneous control of the pulse duration and frequency chirp enables optimization of both the energy and the energy spread of the annular beam and boosts the radiant energy per unit charge by almost an order of magnitude. These well-defined annular distributions of electrons are a promising source of high-brightness laser plasma-based x rays.

Figure 1

Schematic of the experimental setup. The main beam is focused using an $f/20$ off-axis paraboloid onto a two-stage gas cell. Electrons exiting the cell were either swept by a dipole magnet and imaged using the ESPEC diagnostic or undeflected and imaged on axis at 12 cm from the cell exit using the EPROFILE diagnostic (with the magnet removed in this case). The dash-double-dotted line shows the trajectory of the undeflected electrons.

Figure 2

(Top row) Images of the undeflected two-component beams taken 12 cm from the cell exit. The length of the acceleration stage is (a) 6, (b) 7, and (c) 8 mm. (d) and (e) indicate that the angle of conical emission decreased as either the acceleration length or the energy of the annular beam increased. The dashed lines show the momenta scalings described in Ref. [22]. Error bars in (d) denote the standard deviation.

Figure 3

(a) ESPEC images show the trend toward production of the most distinct, highest-energy annular beams as the pulse duration increases as well as their monoenergetic nature. (b) Increasing the pulse duration (while simultaneously imparting a positive frequency chirp) boosts the energy of the annular beams (square markers) while decreasing the maximum energy of the axial beam (the diamond markers). (c) The energy spread of the annular beams decreases as the pulse duration increases, reaching a minimum of approximately 7%.

Figure 4

(a) Presence of the annuli in conjunction with the axial beam increased the radiant energy per unit charge (the square markers) compared with cases of the axial beam alone (the diamond markers). At a pulse duration of 94 fs, the most distinct annular beams are responsible for almost an order of magnitude increase in the x-ray yield per unit charge. (b) Single photon spectrum of the axial plus annular (solid) and axial-only (dash-dotted) beams are shown for the case of a 94 fs pulse duration. (c) 3D map of charge density in the first bucket of the wakefield after the down ramp with both the axial and annular components of the electron beam (using osiris).