Magnetic collimation of relativistic positrons and electrons from high intensity laser-matter interactions

Collimation of positrons produced by laser-solid interactions has been observed using an externally applied axial magnetic field. The collimation leads to a narrow divergence positron beam, with an equivalent full width at half maximum beam divergence angle of 4° vs the un-collimated divergence of about 20°. A fraction of the laser-produced relativistic electrons with energies close to those of the positrons is collimated, so the charge imbalance ratio (n_e-/n_e+) in the co-propagating collimated electron-positron jet is reduced from ∼100 (no collimation) to ∼2.5 (with collimation). The positron density in the collimated beam increased from 5×10⁷cm^-3 to 1.9×10⁹cm ^-3, measured at the 0.6m from the source. This is a significant step towards the grand challenge of making a charge neutral electron-positron pair plasma jet in the laboratory.