Testing strong-field QED with the avalanche precursor

The NFS OPAL facility will deliver extreme optical fields that will allow probing, for the first time, a fundamentally new regime of the electromagnetic radiation interaction with matter when the dynamics of electrons is dominated by quantum radiation reaction and light can be transformed into high-brilliance gamma radiation and even electron-positron pairs. Electrons injected into the mutual focus of two multi-PW counter-propagating laser pulses can be repeatedly accelerated by the extreme electric field to GeV-scale energies at a sub-cycle time, with a rapidly increasing probability rate to emit high-energy photons causing strong recoil. With 3D PIC-QED simulations, we show that in two laser pulses with a total power>40PW, the energy transferred to high-energy photons exceeds that of electrons by an order of magnitude. At increasing power, a fractionof photons can also create secondary electron-positron pairs. This renders the precursor of avalanche-type (or selfsustained) QED cascades characterised by an exponential particle number growth. To achieve this, we propose to focus strongly two NSF OPAL Alpha-beams with the peak power in a (transparent for the laser) gas jet of heavy-atomic gas (e.g. Argon). The initial electrons will result from the ionization process. The study of outgoing photon radiation and electron spectral features will allow identifying the radiation-domination regime, whilst registering positrons will give strong evidence of a QED avalanche onset.