Fig. 1. Experimental scheme (not to scale). The J-KAREN-P laser pulses (Ø 280 mm, 33 fs, 10 J, λ₀ ~ 820 nm, p-polarized) were focused into an r_eff ~ 1.3 μm focal spot on a 5–20 μm stainless-steel tape target^[26] mounted at a 45º incident angle. The observation direction of the back-reflection diagnostics at 1ω is shown by the red arrow. Several reflection diagnostics (1ω and 2ω imagers, 1ω–4ω fiber spectrometer) measured the reflected beam footprint on a screen mounted perpendicular to the ‘specular reflection’ direction; a three-channel flat-field XUV spectrograph (3FF) was mounted behind a hole in the screen. The first hard X-ray spectrometer, HXRS-1, was mounted 98º off the main laser pulse direction. The second hard X-ray spectrometer, HXRS-2, and an electron spectrometer (ESM) were along the direction of the main laser pulse, while the imaging XUV spectrograph was 12º off this direction. The symbols represent dipole magnets removing electrons from HXRS-1 and HXRS-2 and dispersing electrons in the electron spectrometer ESM. Two soft X-ray spectrometers with spatial resolution (FSSR) were mounted out-of-plane on the target front (-F) and rear (-R) sides, respectively. The insets show spatial and temporal J-KAREN-P laser pulse profiles. A tape target of 20 mm width was mounted on a double-rotating-reel setup, which could be translated linearly along the laser axis with a 0.1-μm step size (the ‘+X’ denotes direction away from the OAP mirror).

Fig. 2. The fields of view of the 1ω (a) and 2ω (b) cameras imaging a PTFE screen mounted perpendicular to the ‘specular reflection’ direction. The geometric center of the reflected beam is marked with white circles. The dashed ellipses denote the spectrometer observation area. (c) Typical absolutely calibrated reflected spectrum. The energy values calculated within the (n ± 0.25)ω₀ spectral bandwidths (colored) are given for harmonic orders n = 1, 2, 3.

Fig. 3. Normalized energy from the four reflection beam diagnostics versus the target position X (‘–’ denotes the direction towards the OAP mirror, and X₀ corresponds to the best focus position). All values are normalized by the on-target pulse energy E₀. The plots in (a) and (b) are for the 1ω and 2ω diagnostics, respectively, where the upper data (black) are from the imagers, while the lower data (red) are integrated from the 1ω–4ω spectrometer within (1 ± 0.25)ω₀ and (2 ± 0.25)ω₀, correspondingly. (c) The 3ω data integrated within the (3 ± 0.25)ω₀ band from the 1ω–4ω spectrometer. (d) The normalized back-reflected energy.

Fig. 4. (a) Typical spatially resolved XUV spectrum; λ = 0 denotes the zeroth diffraction order. (b) Dependence of the integrated zeroth order on the target position (‘–’ is towards the OAP mirror) for 5-μm- and 15-μm-thick targets, and their Lorentzian fits. (c) Dependence of the integrated ESM yield on the target position for a 15-μm-thick target. The dashed line shows the ESM noise level.

Fig. 5. HXRS scintillator plate signals versus target position X (‘–’ is towards the OAP mirror, X₀ is the best focus). (a)–(c) HXRS-1 (off-axis), (d)–(f) HXRS-2 (on-axis). (a) HXRS-1 plate #0, 10-μm-thick SUS. (b) HXRS-1 plate #5, 10-μm-thick SUS. (c) HXRS-1 plate #5, 5-μm-thick SUS. (d) HXRS-2 plate #0, 10-μm-thick SUS. (e) HXRS-2 plate #3, 10-μm-thick SUS. (f) HXRS-2 plate #2, 5-μm-thick SUS. Lorentzian fits are shown, where applicable. Scintillator plate #0 was the closest to the interaction point. The error bars in all the frames are due to the CMOS camera noise.

Fig. 6. (a) A typical FSSR-F spectrum recorded in the vicinity of the best in-focus target position. The spectrum covers wavelengths from 0.165 to 1.63 nm in different diffraction orders from m = 1 to m = 8. Strong characteristic lines Fe K_α (λ = 0.194 nm) and Cr K_α (λ = 0.229 nm) were observed in m = 8 and m = 7 diffraction orders, respectively. The continuous signal corresponds to bremsstrahlung. A narrow strip of a 25-μm C₃H₆ filter allows for observing a narrow portion of the spectrum, suppressing emission from lower diffraction orders (m = 1, m = 2). (b), (c) FSSR-F data for 15-μm SUS targets, integrated within an area without (b) and with (c) the 25-μm-thick C₃H₆ filter. (d), (e) FSSR-R integrated bremsstrahlung signal for 15-μm (d) and 5-μm (e) SUS targets. (f) FSSR-R Fe K_α integrated signals for 15- and 5-μm SUS targets. The black error bars correspond to statistical shot-to-shot signal variations, while the smaller colored error bars are due to CCD camera noise. Lorentzian fits are shown, where applicable.

Fig. 7. (a) 3FF spectrum with harmonics. The upper and lower parts correspond to the shallow- and high-deviation-angle mirrors, respectively. Dashed lines show the Al filter cutoff (λ = 17 nm) in the first and second diffraction orders. (b), (c) Integrated 3FF signal versus target position for 10- and 15-μm SUS targets, respectively. The error bars correspond to shot-to-shot signal variations.