Frustrated Brunel heating by relativistic gyromagnetic effects in ultra-intense laser-matter interactions

We report experimental and numerical investigations of a novel laser absorption mechanism, termed frustrated Brunel heating, arising from relativistic gyromagnetic effects in ultraintense laser-plasma interactions. Hot electron bunches were generated at a step-like plasma boundary irradiated by a high-contrast 150 TW, 25 fs laser pulse and propagated into a foil target. These electrons were diagnosed via coherent transition radiations (CTR), revealing a unique feature: two spatially separated CTR spots, at both fundamental and second-harmonic frequencies, observed simultaneously. To elucidate the underlying mechanism, we conducted a series of 1-D and 2-D particle-in-cell (PIC) simulations. The 1-D boosted frame simulation revealed that Brunel heating is frustrated when the relativistic electron gyrofrequency exceeds the laser frequency, leading to magnetic trapping and suppression of vacuum heating process. Complementary 2-D simulations reproduced the angular distribution of hot electrons, consistent with experimental observation. These results uncover a previously underappreciated role of laser-induced magnetization in modifying electron trajectory and energy absorption at steep plasma boundaries. Our findings provide deeper insight into relativistic laser energy coupling mechanisms and the generation of directional hot electron sources