@misc{oai:repo.qst.go.jp:00072149,
 author = {Fukuda, Yuuji and Jinno, S. and Kanasaki, M. and Tanaka, H. and Sakaki, Hironao and Kondo, Kiminori and Matsui, Ryutaro and Kishimoto, Yasuaki and 福田 祐仁 and 榊 泰直 and 近藤 公伯 and 松井 隆太郎 and 岸本 泰明},
 month = {Oct},
 note = {The laser-driven ion acceleration has been one of the most active areas of research during the last several years, because the accelerated ion beams have unique properties such as ultrashort duration, high brilliance, and low emittance. The recent advancements in laser-driven ion acceleration techniques using thin foil targets now allow the maximum proton energies above 85 MeV. 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.
   Meanwhile, PW class high power lasers now open up a new regime of laser-matter interactions at intensities 10^21-22 W/cm^2. In such a high intensity regime, the Coulomb explosion of clusters results in a pronounced increase in accelerated ion energies, i.e. over 100 MeV proton beams could be produced via the Coulomb explosion of micron-size hydrogen clusters. Such high purity proton beams would be quite advantageous for nuclear physical studies and medical applications. The recent results on electrostatic collisionless shock accelerations of pure protons up to 22 MeV with narrow energy spread obtained with a CO^2 laser (~10. 6 μm) irradiation on near-critical hydrogen gas targets (~10^19 cm^-3) made a significant step towards practical applications of laser-driven proton beams. In this context, high density hydrogen gas jets (~10^21 cm^-3) as well as cryogenically cooled hydrogen based targets (~10^22 cm^-3) have also attracted a great deal of interest as a promising near-critical density media for the development of high-repetitive laser-driven “pure” proton sources using Ti:Sapphire lasers (~0.8 μm).
   Given this situation, we have developed a new cluster target formation system which consists of a pulsed solenoid valve, a conical nozzle, and a liquid helium cooling head, having a potential to produce micron-size hydrogen clusters. The size distribution measurement, conducted by using the Mie scattering method, demonstrates that hydrogen clusters with diameters of 0.5~1.5 μm are produced. The 3D PIC code simulations support a generation of over 100 MeV forward accelerated protons via the Coulomb explosion of a single micron-size hydrogen cluster at a laser intensity of 1×10^22 W/cm^2., International Symposium on Ultrafast Intense Laser Science 15},
 title = {Micron-size hydrogen cluster targets for over 100 MeV, high-repetitive, impurity-free proton beams via relativistic laser-plasma interactions},
 year = {2016}
}