Hemispherical electron plasma waves generated from ultraintense laser interacting with a solid target having a subcritical preplasma is studied using particle-in-cell simulation. As the laser pulse propagates inside the preplasma, it becomes self-focused due to the response of the plasma electrons to the ponderomotive force. The electrons are mainly heated via betatron resonance absorption and their thermal energy can become higher than the ponderomotive energy. The hot electrons easily penetrate through the thin solid target and appear behind it as periodic hemispherical shell-like layers separated by the laser wavelength.
Yang, X. H., Ma, Y. Y., Xu, H., Shao, F. Q., Yu, M. Y., Yin, Y., Zhuo, H. B., & Borghesi, M. (2013). Generation of hemispherical fast electron waves in the presence of preplasma in ultraintense laser-matter interaction. Laser And Particle Beams, 31(3), 379-386. https://doi.org/10.1017/S0263034613000359