On-target contrast diagnostic via specular reflectivity measurement

Pirozhkov, A. S.; Choi, I. W.*; Sung, J. H.*; Lee, S. K.*; Yu, T. J.*; Jeong, T. M.*; Kim, I. J.*; Hafz, N.*; Kim, C. M.*; Pae, K. H.*; Noh, Y.-C.*; Ko, D.-K.*; Lee, J.*; Robinson, A.*; Foster, P.*; Hawkes, S.*; Streeter, M.*; Spindloe, C.*; McKenna, P.*; Carroll, D. C.*; Wahlstrm, C.-G.*; Zepf, M.*; Dromey, B.*; Markey, K.*; Kar, S.*; Li, Y. T.*; Xu, M. H.*; Nagatomo, Hideo*; Mori, Michiaki; Yogo, Akifumi; Kiriyama, Hiromitsu; Ogura, Koichi; Sagisaka, Akito; Orimo, Satoshi; Nishiuchi, Mamiko; Sugiyama, Hironori*; Esirkepov, T. Zh.; Okada, Hajime; Kondo, Shuji; Kanazawa, Shuhei; Nakai, Yoshiki; Akutsu, Atsushi; Motomura, Tomohiro*; Tanoue, Manabu*; Shimomura, Takuya; Ikegami, Masahiro; Daito, Izuru; Kando, Masaki; Kameshima, Takashi; Bolton, P. R.; Bulanov, S. V.; Daido, Hiroyuki; Neely, D.*

High-power laser contrast is challenging to measure, especially in the real target irradiation conditions. We present a convenient and relatively simple contrast diagnostic technique based on the measurement of target specular reflectivity. The reflectivity remains high even at irradiances above W/cm in the case of a high-contrast prepulse-free laser. On the contrary, the specular reflectivity drops at high irradiances in the case of an insufficient contrast, due to the beam break-up and increased absorption caused by the preformed plasma. The technique was demonstrated using three laser systems with different contrast conditions: Astra (CLF, RAL), TiS laser at APRI, GIST, and J-KAREN (APRC, JAEA).