@misc{oai:repo.qst.go.jp:00072470,
 author = {Matsui, Ryutaro and Fukuda, Yuuji and Kishimoto, Yasuaki and 松井 隆太郎 and 福田 祐仁 and 岸本 泰明},
 month = {Sep},
 note = {In laser-driven ion accelerations, the recent advancements in acceleration techniques using thin foil targets now allow the maximum proton energies close to 100 MeV. However, the generation of ion beams with low bandwidth and low divergence at a high repetition rate still remains a critical issue. In addition, from a view point of practical applications, high-purity proton beams are quite advantageous. In experiments using thin foil targets, however, protons from surface contaminants along with the high-z component materials are accelerated together, making the production of impurity-free proton beams unrealistic.
     Here we propose a new way to produce highly-directional, highly-reproducible, impurity free, quasi-monoenergetic proton beams exceeding 200 MeV using micron-size cluster targets. Interaction processes of a PW class, ultrashort (∼40 fs) laser pulses and micron-size hydrogen clusters are investigated using 3D-PIC simulations. We found a special parameter regime that highly directional (divergence angle ~8.5 degree) and quasi-monoenergetic (ΔE/E∼7%) protons with energies up to 290 MeV are accelerated by collisionless shocks propagating inside the micron-size clusters in the relativistically-induced transparency regime. 
 The proposed acceleration mechanism can be considered as a future candidate in laser-driven proton sources exceeding 200 MeV with the upcoming advanced multi-PW lasers., International Conference on Inertial Fusion Sciences and Applications (IFSA 2017)},
 title = {Generation of quasi-monoenergetic protons exceeding 200 MeV via intra-cluster collisionless shocks in a laser-irradiated micron-size H2 cluster},
 year = {2017}
}