Laser wakefield acceleration in Kr-He plasmas and its application to positron beam generation

We present the generation of quasi-monoenergetic electron beams via ionization injection in a laser wakefield acceleration by using a krypton-helium plasma. We obtained higher-energy electron beams of low divergence as compared with beams from the nitrogen-helium plasma at low plasma densities. This is attributed to an immediate ionization, trapping and acceleration of many electrons from several inner shells of the krypton atoms. On the other hand, there are very high ionization potentials of the K-shell nitrogen electrons which require the pulse (in our laser-plasma parameters) to experience a self-focusing (after 1.6 mm of propagation) at first before any ionization injection and acceleration of those two electrons per atom can occur. Based on those high-quality electron beams from the Kr-He plasmas, we generated ultra-relativistic positron beams with energies up to 100 MeV by placing a lead slab converter with a thickness of the order of its radiation length in the electron beam path after the gas jet. Experimental and Mont-Carlo simulated energy spectra of the positron beams are compared for the energy range of 30 MeV <= Ee+ <= 100 MeV along with the positron yields. In addition, various parameters of the positrons are investigated against the primary electron beam parameters. Such electron-positron cascading experiment could be helpful in future development for the composition of an all-optical electron-positron collider.