Laser acceleration in argon clusters and gas media

We experimentally investigated the generation of high-energy electron beams from laser-driven wakefield acceleration in argon gas jets by using tens of terawatt 30 fs ultrafast laser pulses that were focused to a relatively large-spot size, unmatched with the laser–plasma parameters. In this interaction, and depending on the Ar gas jet density, we could distinguish two different regimes for electron acceleration in the argon medium. In the high-density argon gas jet where argon clusters are formed, upon interaction with the laser electron beams having as high a charge as 3nC are generated. However, the energy spectra of those electron beams were continuous. On the other hand, high-quality quasi-mono-energetic electron beams with a modest charge of tens of pC are observed at much lower argon gas jet densities. The generation of such a high-quality electron beam is attributed to the ionization injection mechanism in which the electron injection takes place over only a few hundred micrometers of the laser–plasma interaction length, leading to the generation of high-quality electron beams.