The tightly focused multi-PW laser pulse of APOLLON will allow entering deeply in the ultra-relativistic regime of laser-plasma interaction where the so-called classical nonlinearity parameter a0 = EL/EC strongly exceed 1 (EL being the laser electric field amplitude et EC = me c ω0/e the so-called Compton field above which an electron gain a relativistic energy in less than a laser optical cycle – e and me are the electron charge and mass respectively, c the light speed and ω0 the laser frequency).
In this regime, electrons are strongly and suddenly accelerated and/or decelerated. They radiate away a significant part of their energy emitting g photons and exchanging momentum with the electromagnetic field. This process, accompanied by the absorption/scattering of many laser photons, is called nonlinear Thomson or nonlinear Compton scattering, depending on whether quantum effects are negligible or not.
The importance of purely-quantum effects on an electron is controlled by the so-called quantum nonlinearity parameter χ0=(ε/c-p//)/(mc)EL/ES, which depends on both the electron energy ε, the longitudinal momentum p// and the laser field amplitude. for χ0~1, pure quantum effects such as the electron recoil due to the emission of particularly energetic photons or electron-pair creation start to kick-in.
Thanks to his high-intensity brightness, the scientific community of the field can study
- High energy photon emission and its back-reaction in laser-plasma interaction;
- Non-linear Compton / Thomson scattering from laser-created electron beams;
- Pair production in the presence of strong Coulomb fields;
- Electron acceleration from vacuum.