Angle-dependent modulated spectral peaks of proton beams generated in ultrashort intense laser-solid interactions

Abstract

Proton acceleration from 4 m thick aluminum foils irradiated by 30-TW Ti: sapphire laser pulses are investigated using an angle-resolved proton energy spectrometer. We find that a modulated spectral peak at ~0.82 MeV is presented at 2.5° off the target normal direction. The divergence angle of the modulated zone is 3.8°. Two-dimensional particle-in-cell simulations reveal that self-generated toroidal magnetic field at the rear surface of the target foil is responsible for the modulated spectral feature. The field deflects the low energy protons, resulting in the modulated energy spectrum with certain peaks. I. INTRODUCTIONLaser-driven ion beams hashave advantages of short pulse duration, high brightness and small source size. For their potential applications such as proton radiography 1 , proton-driven fast ignition 2 , tumour therapy 3 , proton-driven nuclear reactions 4 , etc., monoenergetic spectral distributions of protons are preferred. However, most of the experimentally generated proton beams present exponential-like proton energy spectra. To produce proton beams with modulated spectral distributions, several mechanisms, such as radiation pressure acceleration 5 , break-out afterburner 6 , and laser-driven shock acceleration 7 , have been proposed, and successfully demonstrated by numerical simulations and experiments 8,9 . However, to implement these mechanisms the drive laser pulses must have very high contrast ratio better than 10 -10 and high focused intensity higher than 10 21 W/cm 2 , typically. Such requirements are great challenges for the laser systems in commission.With the current laser parameters, the target normal sheath acceleration (TNSA) has been the typical mechanism found in experiments for laser-driven ion acceleration 10,11 . How to generate proton beams with modulated spectral distributions by TNSA mechanism has been investigated by many groups in the last decade. In the interaction of a 20-TW/0.8 ps laser with a hydrogen-desorbed palladium foil target, Hegelich . From particle-in-cell (PIC) simulations, they find two self-generated toroidal magnetic fields at the rear surface of the ~nm ultrathin foil. The field near the target center will focus protons, while the field a little far from the target center will deflect protons off the target normal direction. According to their explanation, the focusing field can self-select protons with certain energies, leading to the spectral peaks.For micron-thickness targets, the focusing field will disappear and no spectral peaks are observed in the normal direction.In this paper, we will present our observation of proton beams emitted from micron-thickness foils irradiated by 30-TW Ti: sapphire laser pulses by using an angle-resolved magnetic spectrometer. We find that modulated spectral peaks are presented at 2.5°off the target normal direction for 4 m thick aluminum foil targets. Our PIC simulations show that a strong toroidal magnetic field is also generated at the rear target surface. This field will deflect (rather than focus) the low en...