Non-invasive characterisation of a laser-driven positron beam

Abstract

We report on an indirect and non-invasive method to simultaneously characterise the energydependent emittance and source size of ultra-relativistic positron beams generated during the propagation of a laser-wakefield accelerated electron beam through a high-Z converter target. The strong correlation of the geometrical emittance of the positrons with that of the scattered electrons allows the former to be inferred, with high accuracy, from monitoring the latter. The technique has been tested in a proof-of-principle experiment where, for 100 MeV positrons, we infer geometrical emittances and source sizes of the order of e + ≈ 3 µm and D e + ≈ 150 µm, respectively. This is consistent with the numerically predicted possibility of achieving sub-µm geometrical emittances and micron-scale source sizes at the GeV level.

Sarri et al. Reply:

“…A source size of 180 μm is obtained, in line with the experimental results in Ref. [4], the estimate in Ref.…”

“…A source size of approximately 150 μm has since been confirmed by a series of experimental measurements recently reported by part of our collaboration [4]. In similar conditions, a positron source size of ð230 AE 100Þ μm was experimentally observed [Fig.…”

“…We disagree with this statement. The applicability of LWFA-driven positrons to advanced accelerator concepts is currently subject of investigation, and our recent work [3,4,6], together with that of independent groups [7,8], confirms that there is significant potential in this area. We have also confirmed that such positron beams can be used for some specific studies in laboratory astrophysics, and we have already experimentally measured plasmalike behavior of LWFAdriven electron-positron beams [9][10][11][12].…”

Laser Generation of Near-GeV Low Emittance Positron Beams

“…Laser-driven generation of ultra-relativistic positron beams has recently gathered significant attention from the research community with several landmark results already reported, including: positron energies in the region of 100 MeV [1][2][3], generation of high-density and quasineutral electron-positron beams [4], and first experimental observation of pair-plasma dynamics [5]. Besides the application of these beams to the investigation of phenomena of importance to laboratory astrophysics (see, for instance, Ref.…”

“…The slit selected a narrow band of energies from the dispersed positron beam, which were collimated onto an additional scintillator screen by the magnetic field. The lead slit was trans- The electron and positron energy-resolved emittances were characterised using a 5.0 mm thick tungsten mask composed of horizontal slits with a period of 1100 µm (550 µm gaps), placed into the beam 290 mm behind the rear face of the converter (similar to the setup used in [2]). Particles hitting the solid bars of the mask were scattered to form a smooth background on the detector screen 1000 mm after the mask.…”