Enhanced laser intensity and ion acceleration due to self-focusing in relativistically transparent ultrathin targets

Abstract

Laser-driven proton acceleration from ultrathin foils is investigated experimentally using f /3 and f /1 focusing. Higher energies achieved with f /3 are shown via simulations to result from self-focusing of the laser light in expanding foils that become relativistically transparent, enhancing the intensity. The increase in proton energy is maximized for an optimum initial target thickness, and thus expansion profile, with no enhancement occurring for targets that remain opaque, or with f /1 focusing to close to the laser wavelength. The effect is shown to depend on the drive laser pulse duration.

Influence of target-rear-side short scale length density gradients on laser-driven proton acceleration

“…Energies close to 100 MeV have been achieved via a hybrid acceleration mechanism involving both TNSA and radiation pressure acceleration in ultrathin foils expanding to the extent that relativistic transparency occurs [14]. The maximum ion energy can also be enhanced by self-focusing of the laser light as it propagates within the expanded plasma [15] and properties such as the spatial distribution of the ion beam can also be strongly affected [16,17].…”

School of Mechanical Engineering, Shanghai Jiao Tong University, 200240, Shanghai, China

“…Since RT effect provides a feasible path for deeper penetration of laser fields and enhancing their interaction with plasmas, it has become one of the central issues on the intense lasers interacting with high-density targets [15][16][17][18].…”

Target Characteristics Used in Laser-Plasma Acceleration of Protons Based on the TNSA Mechanism

“…The higher proton energy reported for plastic targets, with several peaks surpassing 60 MeV [3,50,61] can be due to the increased coupling of the laser light into the target as demonstrated by Geng et al [73]. When plotted against the pulse duration, the results are again well separated, most high peaks being obtained for pulses longer than 400 fs, as shown in Figure 3B.…”

“…In Figure 1A one can distinguish between two groups, depending on the peak energy value. The first group is characterized by a high laser intensity Iλ 2 L ≳ 4 × 10 19 W cm −2 μm 2 [3,5,29,31,[33][34][35][36][37][38][39][40]. In this group the peaks have consistently higher energy, between 18 and 50 MeV, with a few exceptions at 5, 10, 11 and 12.6 MeV [35,[38][39][40].…”

“…In an unprecedented effort in the last 2 decades or so, many groups have been striving to push the limit of proton acceleration beyond the 100 MeV threshold. The highest energy attained so far is at about 94-98 MeV [2,3]. Breaking this barrier opens up a full range of potential applications, just to mention a few, from nuclear fusion to radiography, or to medical use in tumour therapy.…”