Laser-cluster interaction in an external magnetic field: Emergence of a nearly monoenergetic weakly relativistic electron beam

Recent studies [e.g., Sci. Rep. 12, 11256 (2022)] on laser interaction (for a wavelength of 800 nm and intensity greater than 1016 W/cm2) with a deuterium nanocluster in an ambient magnetic field B0 demonstrate that collisionless absorption of laser light occurs in two stages via anharmonic resonance and electron-cyclotron resonance (ECR) or relativistic ECR (RECR) processes. The auxiliary magnetic field B0 enhances the coupling of the laser field to cluster electrons via improved frequency matching for ECR or RECR as well as phase matching for the prolonged duration of the 5-fs (FWHM) broadband pulse. As a result, the average absorbed energy per electron E significantly jumps up approximately 36-70 times its ponderomotive energy Up. In this paper we report the energy dispersion of these energetic electrons and their angular distribution in position and momentum space by performing hybrid particle-in-cell simulations. By simulating bigger clusters (radius R0 approximate to 3-4 nm) at high intensities of approximately 10(16)-10(18) W/cm(2), we find E approximate to 36Up-70Up, which is similar to a small cluster (R-0 approximate to 2 nm), but total energy absorption increases almost linearly with increasing cluster size due to the greater number of available energy carriers. In all cases near ECR or RECR, electrons form a narrow conelike weakly relativistic gyrating beam (about B0) within an angular spread Delta theta < 5(degrees), propagating far beyond 200R0 along B-0. This study may be relevant because an intense, weakly relativistic electron beam has wide applications, including the fast ignition technique for inertial confinement fusion, ultrashort-x-ray sources, and medical applications.