Laser Requirements for High-Order Harmonic Generation by Relativistic Plasma Singularities

We discuss requirements on relativistic-irradiance (I-0 > 10(18) W/cm(2)) high-power (multi-terawatt) ultrashort (femtosecond) lasers for efficient generation of high-order harmonics in gas jet targets in a new regime discovered recently (Pirozhkov et al., 2012). Here, we present the results of several experimental campaigns performed with different irradiances, analyse the obtained results and derive the required laser parameters. In particular, we found that the root mean square (RMS) wavefront error should be smaller than similar to 100 nm (similar to lambda/8). Further, the angular dispersion should be kept considerably smaller than the diffraction divergence, i.e., mu rad level for 100-300-mm beam diameters. The corresponding angular chirp should not exceed 10(-2) mu rad/nm for a 40-nm bandwidth. We show the status of the J-KAREN-P laser (Kiriyama et al., 2015; Pirozhkov et al., 2017) and report on the progress towards satisfying these requirements.