Collimation of high-current fast electrons in dense plasmas with a tightly focused precursor intense laser pulse

High-current fast electrons at the mega-ampere level provide a unique way to generate high-energy density states of matter, which are related to many applications. However, the large divergence angle of fast electrons typically over 50 degrees is a significant disadvantage. The guiding effect of the self-generated azimuthal magnetic fields on fast electron current is found to be very limited due to the cone-shaped spatial structure of the fields. In this work, we present a new understanding of the collimation conditions of fast electrons under such a magnetic field structure. It is shown that the transverse peak position of the magnetic field layer plays a more crucial role in collimating the fast electrons than its magnitude. Based upon this, a new two-pulse collimating scheme is proposed, where a guiding precursor pulse is adopted to form proper azimuthal magnetic fields and the main pulse is for fast electron generation as usual. The present scheme can be implemented relatively easily with the precursor lasers at the 10 TW level with a duration of 200 femtoseconds, with which the divergence angle of fast electrons driven by the main pulse can be confined within a few degrees. Practical applications of our scheme can be found in high-energy density science.