Table-Top Laser-Based Source of Femtosecond, Collimated, Ultrarelativistic Positron Beams

The generation of ultrarelativistic positron beams with short duration (${\tau }_{e^{+}}\simeq 30\mathrm{fs}$), small divergence (${\theta }_{e^{+}}\simeq 3\mathrm{mrad}$), and high density ($n_{e^{+}}\simeq {10}^{14}–{10}^{15}{\mathrm{cm}}^{-3}$) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and $\gamma$ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

Figure 1

(a) Top view of the experimental setup. The magnet is rotated by 90° for the sake of illustration. (b) Typical positron signal as recorded by the image plate for 4.2 mm of Ta. The dark region on the upper part is predominantly exposed by the $\gamma$ rays escaping the solid target. (c) Typical signal of the electron beam as recorded on the LANEX screen, without a solid target (see Fig. 1 of the Supplemental Material [20] for a ten-shot series). The dashed white lines depict a full width at total maximum of 2.5 mrad (corresponding full width at half maximum of 1.4 mrad). (d) Extracted spectrum and relative best fit ($\mathrm{\text{linear}}+\mathrm{\text{Gaussian}}$) used as an input for FLUKA simulations.

Figure 2

Experimental (solid lines) and simulated (dashed lines) positron spectra for (a) 5.3 mm of Cu, (b) 6.4 mm of Sn, (c) 2.8 mm of Ta, and (d) 4.1 mm of Pb. Each experimental spectrum results from an average over ten consecutive shots.

Figure 3

(a) Measured positron yield, in the energy range $90<E_{e^{+}}(\mathrm{MeV})<120$ for Ta (blue full circles) and Pb (red empty circles) for different target thicknesses. Lines give the best quadratic fits. (b) Measured positron yield, in the energy range $90<E_{e^{+}}(\mathrm{MeV})<120$, for different materials but constant areal density as a function of $Z^2/A$. The dashed line represents the best quadratic fit. In both graphs, the values result from an average over ten consecutive shots and the error bars account for the shot-to-shot fluctuation of the primary electron beam ($\le 10\%$ in overall charge, see Fig. 1 of the Supplemental Material [20]).