An intense laser pulse, focused in a gas cloud, instantly ionizes the latter. The laser thus propagates in an ionized gas, ie a plasma. During the propagation, the pulse expels electrons out of its path and creates an ion cavity in its wake. This cavity is associated with very high longitudinal electric fields that can be used to accelerate electrons. This is the principle of laser-plasma acceleration. In practice, the fields produced are three to four orders of magnitude greater than those obtained in conventional accelerators; hence only 20 cm are required to accelerate electrons up to ten GeV, whereas it would take about 500 m with a conventional accelerator.
A tool for studying quantum electrodynamics
The APOLLON laser is expected to accelerate electrons in an energy range from 1 to 20 GeV. Yet, obtaining the highest energies will require the development of new guiding techniques. A major advantage of laser-plasma accelerators is the inherent synchronization of the electron beam with the laser. By using the two beams of the Apollon laser, it will be possible to collide the particle beam with a second laser pulse. It is expected that the electric field in the particle frame is greater than the Schwinger limit (E> 1.3 1018 V/m), ie the electric field that separates electron-positron pairs created from the fluctuations of the vacuum. This will pave the way for the study of several phenomena of quantum electrodynamics.