Supra-bubble regime for laser acceleration of cold electron beams in tenuous plasma

Relativistic electrons can be accelerated by an ultraintense laser pulse in the "supra-bubble" regime, that is, in the blow-out regime ahead of the plasma bubble (as opposed to the conventional method, when particles remain inside the bubble). The acceleration is caused by the ponderomotive force of the pulse, via the so-called snow-plow mechanism. The maximum energy gain, Δ γ∼ γ_ga, is attained when the particle Lorentz factor γ is initially about γ_g/a, where γ_g is the pulse group speed Lorentz factor, and a is the laser parameter, proportional to the laser field amplitude. The scheme operates at a γ_g, yielding Δ_γ of up to that via wakefield acceleration for the same plasma and laser parameters, Δγ∼ γ _g². The interaction length is shorter than that for the wakefield mechanism but grows with the particle energy, hindering acceleration in multiple stages.