Multiplicity of electron- and photon-seeded electromagnetic showers at multipetawatt laser facilities

Electromagnetic showers developing from the collision of an ultraintense laser pulse with a beam of high-energy electrons or photons are investigated under conditions relevant to future experiments on multipetawatt laser facilities. A semianalytical model is derived that predicts the shower multiplicity, i.e., the number of pairs produced per incident seed particle (electron or γ photon). The model is benchmarked against particle-in-cell simulations and shown to be accurate over a wide range of seed particle energies (from 100 MeV to 40 GeV), laser relativistic field strengths (10<a_{0}<1000), and quantum parameter χ_{0} (ranging from 1 to 40). It is shown that, for experiments expected in the next decade, only the first generations of pairs contribute to the shower while multiplicities larger than unity are predicted. Guidelines for forthcoming experiments are discussed, considering laser facilities such as Apollon and ELI Beamlines. The difference between electron and photon seeding and the influence of the laser pulse duration are investigated.