Abstract
We discuss requirements on relativistic-irradiance (I0 > 1018 W/cm2) 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 ~100 nm (~λ/8). Further, the angular dispersion should be kept considerably smaller than the diffraction divergence, i.e., μrad level for 100–300-mm beam diameters. The corresponding angular chirp should not exceed 10−2 μrad/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.
Highlights
In the new regime of coherent X-ray generation [1,2], intense (>1018 W/cm2) high-power femtosecond laser pulses focused onto gas targets propagate through underdense plasma and produce an electron-free cavity and bow wave [3]
We found that for our wavefront sampling and wavefront distortion statistics, the value η ≈ 1.85 gave a good fit to the data; see Figure 4, where we show the normalized irradiance, I/I0, obtained from the Point-Spread Function (PSF) calculated from the measured wavefront
We presented the dependence of coherent X-ray generation by relativistic plasma singularities, via the BISER mechanism, on the laser irradiance
Summary
In the new regime of coherent X-ray generation [1,2], intense (>1018 W/cm2) high-power (multi-TW) femtosecond laser pulses focused onto gas targets propagate through underdense plasma (electron density ne ~1019 cm−3) and produce an electron-free cavity and bow wave [3]. The robustness and structural stability of these singularities are explained with catastrophe theory [4,5] These singularities, oscillating under the action of an intense laser field, emit coherent high-frequency (up to the soft X-ray spectral region) radiation via the Burst Intensification by Singularity-Emitting Radiation (BISER) mechanism [6]. This emission originates from extremely localized, nanometre-scale regions where the local electron density is several orders of magnitude higher than the original plasma density. The large number of photons together with the small source size and attosecond duration provide very high brightness, e.g., 1027 photons/mm2·mrad2·s in 0.1% bandwidth at a wavelength of 18 nm [6]
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