Fig. 1. Experimental setup of the ion experiment at FLASH2. The FEL beam hits the target from the right at a 22.5° angle. Ions are detected along the target normal with an open electron multiplier located 24 cm from the target.

Fig. 2. Signal of the 119EM electron multiplier induced by ions produced by interaction of FEL beam with Ni target (solid line) and TOF dependence of impact velocity of Ni^q+ ions on the first dynode affected by an EM bias of −3 kV, which was calculated for charge states q of 1–3 (dashed lines).

Fig. 3. Normalized fluence profile of the focused laser beam obtained by converting the measured imprints, i.e., the depth profile of the crater created on the surface of the PMMA target, to laser fluence. The black solid curve indicates an iso-fluence contour encircling an area equal to the effective area of 1027 µm², which corresponds to an effective beam radius of 36 µm.

Fig. 4. Signals from the 119EM electron multiplier induced by ions produced by interaction of the soft x-ray FLASH2 beam with Al, Fe, GaAs, Ni, PMMA, and Si targets. The TOF spectra were measured at 24 cm from the targets. The intensity on the targets was 2.8 × 10¹³ W/cm² and FP = 0.

Fig. 5. DOF spectra obtained by transformation of signals from the 119EM electron multiplier induced by ions produced by interaction of the soft x-ray FLASH2 beam with Al, Fe, GaAs, Ni, PMMA, and Si targets for a time of 1 µs that has elapsed since the laser–target interaction. The intensity on the targets was 2.8 × 10¹³ W/cm². The inset shows two fits of Eq. (3) to the DOF spectrum of Cu ions to estimate the ion sound velocity associated with slow (red line) and fast (blue line) ions.

Fig. 6. (a) Fitting of TOF spectrum of ions to partial currents P1–P6. (b) Gaussian parts G_EM of P1–P6 currents of the TOF signal function in (a). P1 and P2 peaks of beam-like ions (u_CM-P1 ≈ 2.3 × 10⁵ and u_CM-P2 ≈ 6.7 × 10⁴ m/s) and P3, …, P6 peaks of thermal ions emitted by Ni target irradiated with 1.4 × 10¹³ W/cm².

Fig. 7. Temperature per relative atomic mass kT/A_r for irradiated targets vs order of ion groups (partial peaks). Numbering of the revealed ion groups starts from the fastest one.

Fig. 8. Ablative imprints in (a) Si and (b) Cu created by 13.5-nm FLASH2 beam delivering energies of 100 and 50 µJ, respectively, on the targets.

Fig. 9. (a) Ablative imprints in GaAs and (b) TOF spectrum of ions emitted by the GaAs target irradiated with energy 100 µJ (2.7 × 10¹³ W/cm²).

Fig. 10. Peak energy per relative atomic mass E_peak/A_r of partial ion groups revealed from TOF spectra. E_peak corresponds to the peak current of ions forming the nth group. The inset shows the corresponding TOF spectra of ions produced on a Ni target exposed to laser energies of 100, 50, and 10 µJ.

Fig. 11. Calculated time course of the energy E_IC transferred by current j_IC flowing through the input impedance of the oscilloscope. The solid line is for “all ions,” for which j_IC = S_EM/1.25 × 10¹³ A, where S_EM is the EM signal (see Fig. 9). The other two lines show the hypothetical cases of generation of either fast or slow ions by a GaAs target irradiated with the 13.5-nm FLASH2 FEL.

Fig. 12. Dependence of the charges Q_fast and Q_slow carried by fast and slow ions, respectively, and their sum on the attenuation length of 13.5 nm laser radiation in different targets. Attenuation lengths were calculated for all the target materials used using the Henke method.³⁶