Optimized laser ion acceleration at the relativistic critical density surface

Abstract

In the effort of achieving high-energetic ion beams from the interaction of ultrashort laser pulses with a plasma, volumetric acceleration mechanisms beyond Target Normal Sheath Acceleration have gained attention. A relativisticly intense laser can turn a near critical density plasma slowly transparent, facilitating a synchronized acceleration of ions at the moving relativistic critical density front. While simulations promise extremely high ion energies in in this regime, the challenge resides in the realization of a synchronized movement of the ultra- relativistic laser pulse (a0>∼30) driven reflective relativistic electron front and the fastest ions, which imposes a narrow parameter range on the laser and plasma parameters. We present an analytic model for the relevant processes, confirmed by a broad parameter simulation study in 1D- and 3D-geometry. By tayloring the pulse length and plasma density profile at the front side, we can optimize the proton acceleration performance and extend the regions in parameter space of efficient ion acceleration at the relativistic relativistic density surface.

Optical probing of ultrafast laser-induced solid-to-overdense-plasma transitions

“…Rapid progress in laser-driven ion acceleration over the past two decades has demonstrated the production of proton energies up to ~ 100 megaelectronvolts (MeV) 14 , yet protons of ~ 200 MeV are required for radiation oncology 15 . However, theoretical predictions have anticipated the acceleration of protons with energies reaching several hundred MeV 16 , 17 , providing additional impetus for ongoing experimental efforts 14 , 18 , 19 .…”

“…Alternative acceleration schemes proposed in 16 , 17 are based on Relativistic-Induced Transparency (RIT) 25 , 26 . In contrast to surface acceleration as in TNSA and RPA, the laser peak penetrates a near-critical overdense pre-plasma owing to the relativistic increase of the electrons’ mass, which leads to a change in the plasma’s refractive index (RIT effect).…”

Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density

“…The target density is the decisive parameter for the type of laser-matter interaction and therefore determines the ion acceleration mechanism. This ranges from target-normal sheath acceleration (TNSA) 17 – 19 and hole-boring or light-sail radiation pressure acceleration (HB-RPA, LS-RPA) 20 – 23 at solid density over acceleration from targets undergoing relativistic transparency 24 – 27 and synchronized acceleration at the relativistic critical density surface (RTF-RPA) 28 , 29 to collisionless shock acceleration (CSA) 30 and magnetic vortex acceleration (MVA) 31 , 32 at near-critical density. A major advantage of acceleration mechanisms in the near-critical density regime is their promise of higher ion beam energies compared to TNSA.…”

Laser-driven high-energy proton beams from cascaded acceleration regimes

“…L / e 2 , with vacuum permittivity ε 0 , electron mass m e , angular laser frequency ω L and electron charge e. Electrons at n cr are pushed into the target, thereby creating charge separation fields. These fields trigger different acceleration mechanisms such as hole-boring RPA [13,18,23], relativistic transparency front RPA [38] or collisionless shocks [39]. For the sake of simplicity, we refer to this acceleration component as front surface acceleration (FSA), which is mainly induced by the radiation pressure of the laser and is maintained until the expanding target undergoes RIT.…”