Tailored laser pulse chirp to maintain optimum radiation pressure acceleration of ions

Abstract

Ion beams generated with ultra-intense lasers-plasma accelerators hold promises to provide compact and affordable beams of relativistic ions. One of the most efficient acceleration setups was demonstrated to be direct acceleration by the laser's radiation pressure. Due to plasma instabilities developing in the ultra-thin foils required for radiation pressure acceleration, however, it is challenging to maintain stable acceleration over long distances. Recent studies demonstrated, on the other hand, that specially tailored laser pulses can shorten the required acceleration distance suppressing the onset of plasma instabilities. Here we extend the concept of specific laser pulse shapes to the experimentally accessible parameter of a frequency chirp. We present a novel analysis of how a laser pulse chirp may be used to drive a foil target constantly maintaining optimal radiation pressure acceleration conditions for in dependence on the target's areal density and the laser's local field strength. Our results indicate that an appropriately frequency chirped laser pulse yields a significantly enhanced acceleration to higher energies and over longer distances suppressing the onset of plasma instabilities.

Effects of pulse chirp on laser-driven proton acceleration

“…Electrons are pulled out in vacuum by the oscillating capacitor field and then re-enter the plasma after half a cycle, delivering their energy 2 . In recent works, chirped laser pulses have been studied in relation to acceleration mechanisms other than TNSA, which may offer higher conversion efficiencies from laser to ion energies in the newly-available PW regime 46 , 47 . When the incident laser field is chirped, the nodes of the SW move in time, with the direction determined by the sign of the chirp: toward the target if the chirp is positive, and away from it in the opposite case.…”