Fast electron energy deposition in aluminium foils: Resistive vs. drag heating

Abstract

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 1019 Wcm-2 laser pulses interacting with Al foils of thicknesses 10-200 ?m. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ?407 nm (the second harmonic of the laser light) and ?500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (Thtail ? 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (Thbulk = 500±100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2?0 emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 ?m. Beyond that depth, because of the beam divergence, the incident current goes under 1012 Acm-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 ?m depth. © EDP Sciences and Springer 2009.