Radiation pressure acceleration: The factors limiting maximum attainable ion energy

Abstract

Radiation pressure acceleration (RPA) is a highly efficient mechanism of laser-driven ion acceleration, with with near complete transfer of the laser energy to the ions in the relativistic regime. However, there is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. The tightly focused laser pulses have group velocities smaller than the vacuum light speed, and, since they offer the high intensity needed for the RPA regime, it is plausible that group velocity effects would manifest themselves in the experiments involving tightly focused pulses and thin foils. However, in this case, finite spot size effects are important, and another limiting factor, the transverse expansion of the target, may dominate over the group velocity effect. As the laser pulse diffracts after passing the focus, the target expands accordingly due to the transverse intensity profile of the laser. Due to this expansion, the areal density of the target decreases, making it transparent for radiation and effectively terminating the acceleration. The off-normal incidence of the laser on the target, due either to the experimental setup, or to the deformation of the target, will also lead to establishing a limit on maximum ion energy.

Laser-driven acceleration of quasi-monoenergetic, near-collimated titanium ions via a transparency-enhanced acceleration scheme

“…With the recent development of high-power lasers, several new mechanisms have been identified. Two of the most promising advanced ion acceleration mechanisms are Radiation Pressure Acceleration (RPA) [27][28][29][30][31] and Relativistically Induced Transparency (RIT) [32][33][34][35][36][37][38]. RPA can potentially produce monoenergetic ion beams of higher energy than TNSA (E I max 2…”

“…Using a circularly polarized laser [39] and plasma mirrors for reduced absorption and better laser contrast can relax some of these issues and reduce the intensity threshold to trigger RPA. Then, when the tightly-focused main pulse ( 10 W cm 20 2 > -) interacts with the ultra-thin foil, finite size spot effects and transverse expansion of the plasma can also cause the maximum electron density to rapidly decrease [28,40]. Moreover, because of the increased electron inertia due to relativistic effect, the plasma frequency decreases below the laser frequency, allowing light to propagate further inside the target and causing the occurrence of RIT [34].…”

Relativistically Intense Laser–Microplasma Interactions

“…because of their transverse size exceeding far beyond the focal spot-size and electron currents neutralizing the target, they typically produce complex and ambiguous signals. Meanwhile, recent efforts have identified the dynamic target expansion as one central issue for laser-ion-acceleration from thin plane foil targets as well [220,221].…”

Parametric study of cycle modulation in laser driven ion beams and acceleration field retrieval at femtosecond timescale

“…The fundamental mechanisms of laser driven ion acceleration have been studied for nearly two decades. Special attention was drawn to the Target Normal Sheath Acceleration (TNSA) [6] (and references therein) and the Radiation Pressure Acceleration (RPA) [7,8]. TNSA is, to some extent, a robust process; while RPA yields highest energies of accelerated ions.…”