Modelling electron deflectometry measurements of magnetic fields in ultrahigh-intensity, femtosecond laser-foil interactions

We examine the processes of magnetic field generation in the recent experiment at LOA, where the B fields induced in a thin (~20 μm) solid foil by a ∼10¹⁹ Wcm⁻², ∼30 fs laser pulse were diagnosed via electron deflectometry. A ~100 MeV-range probe electron beam, produced by an auxiliary LWFA, is injected to the solid foil through its rear side. The mean angular deflection and root-mean-square spread of the beam electrons show nontrivial dependencies on delay time and transverse position with respect to the driving laser pulse.

Our 2D collisional particle-in-cell simulation results reproduce qualitatively the experimental data regarding both mean and rms deflections. We consider the laser’s pedestal 2D preplasma and describe self-consistently the LWFA electrons - induced plasma fields interaction. The two main B-field generation mechanisms found to account are the collisionless current filamentation instability^[1,2], which excites strong (>10³ T), kinetic-scale fields around the laser spot^[3] and the fountain-like motion of fast electrons near the plasma-vacuum boundaries, leading to azimuthal B fields of up to ~100 μm radii^[4].

We shed further light with a quasistatic approach where the small- and large-scale field effects are isolated as a function of the location and time of probing.

References

[1] G. Raj et al., Phys. Rev. Res. 2, 023123 (2020)

[2] A. Bret, et al., Phys. Plasmas 17, 12050 (2010)

[3] J. C. Adam et al., Phys. Rev. Lett. 97, 205006 (2006)

[4] G. Sarri et al., Phys. Rev. Lett. 109, 205002 (2012)