@misc{oai:repo.qst.go.jp:00066350,
 author = {福田, 祐仁 and 松井, 隆太郎 and 岸本, 泰明 and 福田 祐仁 and 松井 隆太郎 and 岸本 泰明},
 month = {Jun},
 note = {Laser-driven ion acceleration has been one of the most active areas of research over approximately the past decade, because accelerated multi-MeV ion beams have unique properties that can be employed in a broad range of applications including laser-driven nuclear science, hadron therapy, and fast-ignition. The recent advancements in laser-driven ion acceleration techniques using thin foil targets allow the maximum proton energies close to 100 MeV. However, the two other hurdles, low bandwidth and low divergence, are difficult to overcome. In addition, from a view point of practical applications, high-purity proton beams with high reproducibility 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 quasi-monoenergetic, impurity free, and highly reproducible 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 < 10 degree) and quasi-monoenergetic (ΔE/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 multi-PW lasers., Sixth International Conference on Energy Density Physics (ICHED2017)},
 title = {Quasi-monoenergetic proton acceleration exceeding 200 MeV triggered by the intra-cluster collisionless shock in the relativistically-induced transparency regime},
 year = {2017}
}