Fig. 1. TNSA ion beams are produced in the interaction of the VEGA-3 laser (toward the target normal direction) and a tape target system, and are characterized with a Thomson parabola ion spectrometer (TPS). In parallel, the auto-generated return current is measured with an inductive target charging monitor (TCM).

Fig. 2. Return current measured with the TCM for a sequence of 20 shots at 1 Hz on 7 μm-thick copper tape with full laser power, best compression, and best focus position of the off-axis parabola.

Fig. 3. Correlations of peak current ∂_tQ^Peak with total charge Q^Total (top) and peak charge Q^Peak for all datasets presented in this work, i.e., those obtained by varying the laser pulse duration and the target position with respect to the best focus position while maintaining the laser pulse energy constant.

Fig. 4. Motion of the target in the longitudinal direction [in terms of theoretical Rayleigh length z_R = (464 ± 145) μm for a (12.8 ± 1.9) μm focal spot] changes the peak intensity on target (red) and therefore affects the maximum proton cutoff energy (blue) and the maximum return current into the target (magenta). Each point is the average of 20 shots on 7 μm-thick copper tape. The laser power of the input beam (green) for this series of shots is reasonably constant, with slightly higher power values when positive displacements are compared with negative displacements. Positions ±8λF²/z_R of the respective first occurrence of strongly reduced axial intensity due to the interference pattern are indicated as dashed lines. Negative displacement corresponds to arrival of the laser pulse at the tape surface before the laser has reached best focus, and the best focus position of the OAP at zero displacement is determined by low-energy alignment.

Fig. 5. Correlations of proton-spectrum cutoff energy with the peak current ∂_tQ^Peak (top) and with the total charge Q^Total (bottom) for a focus scan in which the target was moved in the longitudinal direction. Each point is the average of 20 shots on 7 μm-thick copper tape at full power, with the tape being moved relative to the best focus position of the OAP. The dashed lines represent segmented linear fits.

Fig. 6. Mapped time-resolved return current for a focus scan in which the target was moved relative to the best-focus position in the longitudinal direction [in terms of the theoretical Rayleigh length z_R = (464 ± 145) μm]. Shots were on 7 μm-thick copper tape at full power, with the best low-power focus position of the OAP at zero displacement.

Fig. 7. Peak current measured by the TCM and the proton-spectrum cutoff energy for a Dazzler scan of group delay dispersion (GDD) with zero-position at best compression in the laser bay as found before the shot sequence (bottom), compared with calculated peak intensity and measured laser pulse duration (top). The measured pulse duration is fitted with respect to GDD and corrected for the additional GDD due to windows and filters. Local minima of cutoff energies and peak currents at (712 ± 21) fs² coincide with best laser pulse compression. Each point is the average of ten shots on 10 μm-thick aluminum tape at full energy and best focus position of the OAP.

Fig. 8. Correlation of proton-spectrum cutoff energy with peak current ∂_tQ^Peak (top) and total charge Q^Total (bottom) for a Dazzler scan. The pulse duration is modified via the GDD. Each point is the average of ten shots on 10 μm-thick aluminum tape at full energy and best focus position of the OAP. The dashed lines represent linear fits.

Fig. 9. Mapped time-resolved return current for a laser pulse duration scan using a Dazzler for modifying the GDD, with the zero-position at best compression in the laser bay as found before the sequence. The time-base at the TCM is relative to laser arrival at ≈0 ns. The current peaks are approximately symmetric about a GDD of 741 fs² (dashed horizontal line). Shots were on 10 μm-thick aluminum tape at full energy and best focus position of the OAP.

Fig. 10. Measurement of ion temperature by fitting of experimental data with a 3D Maxwellian function A/Ti3/2Eexp(−E/Ti) in the range 0.5–7.0 MeV (black boxes). Correlation of the Dazzler scan value with the population factor A (solid blue curve), integrated number of ions in the range 0.3–9 MeV (dashed blue curve), and cutoff energy (solid red curve) are demonstrated. The position of best compression is indicated by the green vertical dashed line. Each point is the average of ten shots on 10 μm-thick aluminum tape at full energy and best focus position of the OAP.

Fig. 11. Measurement of ion temperature and population factor by fitting of experimental data in the same way as in Fig. 10, but in the range 0.3–2.5 MeV (black boxes) vs focal displacement. The correlation with the ion number integrated over 0.3–8 MeV (in ions·sr⁻¹) and the cutoff energy is also shown. Each point is the average of at least ten shots on 7 μm-thick copper tape at full energy and a Dazzler displacement of 400 fs².