Effects of oblique incidence and colliding pulses on laser-driven proton acceleration from relativistically transparent ultrathin targets

Journal article, 2020

The use of ultrathin solid foils offers optimal conditions for accelerating protons to high energies from laser-matter interactions. When the target is thin enough that relativistic self-induced transparency sets in, all of the target electrons get heated to high energies by the laser, which maximizes the accelerating electric field and therefore the final ion energy. In this work, we first investigate how ion acceleration by ultraintense femtosecond laser pulses in transparent CH2 solid foils is modified when turning from normal to oblique (45 degrees) incidence. Due to stronger electron heating, we find that higher proton energies can be obtained at oblique incidence but in thinner optimum targets. We then show that proton acceleration can be further improved by splitting the laser pulse into two half-pulses focused at opposite incidence angles. An increase by similar to 30% in the maximum proton energy and by a factor of similar to 4 in the high-energy proton charge is reported compared to the reference case of a single normally incident pulse.