Increasing Laser Field Localization by Focusing on a Toroidal Target

Abstract

Conical targets enable refocusing of laser pulses when they cannot be initially precisely focused or when the focusing element cannot be placed close to the target. Flat-walled targets have been well studied, but optimization of their geometry to reduce energy losses remains a pressing issue. A hyperboloid geometry, which can be approximated experimentally by a toroidal target, appears promising for minimizing losses. A two-dimensional PIC simulation (the toroid is replaced by a pair of cylinders) shows that, when interacting with a toroidal target, the field amplitude increases multiple times as the exit aperture decreases. When the exit aperture diameter becomes smaller than the wavelength, the field does not pass through the target. The addition of a thin foil at the target exit results in an increase in the maximum energy of TNSA protons compared to the case of a foil without a toroidal target.