2.1 High-repetition-rate target

The tape target consists of a motorized wheel that unwinds a thin film. A second wheel recovers the film and maintains the tautness to ensure the target flatness. The target itself is further described in Ref. [Reference Ehret, De Luis, Apiñaniz, Henares, Lera, Pérez-Hernández, Puyuelo-Valdes, Volpe and Gatti45]. Deviations of the target due to the winding movement were measured with a confocal chromatic sensor, ConfocalDT IFS2406-3. These fluctuations were found to be below 25 μm standard deviation in the laser propagation axis, well below the estimated Rayleigh length. The tape target is placed at the laser interaction point with an angle of 12° with respect to the laser forward axis. Two materials have been studied: aluminium and copper (10 and 7.6 μm thickness).

2.2 Particle detectors

A Thomson parabola spectrometer (TPS) coupled to a micro-channel plate (MCP) screen, placed at 12° with respect to the laser forward axis and pointed towards the interaction, was used to characterize the ion emission from the target. A TPS consists of a magnetic dipole followed by a pair of electric plates that allow one to measure the energy distributions of different ion species. The TPS was calibrated by measuring the magnetic field between the plates and then comparing with the expected particle trajectories. It was positioned at 72 cm from the tape target with a 200 μm diameter entrance pinhole. More specific information can be found in Ref. [Reference Salgado-López, Apiñaniz, Henares, Pérez-Hernández, De Luis, Volpe and Gatti46].

A time-of-flight (TOF) detector is also used to measure the maximum proton energies. It consists of two MCPs with sub-nanosecond rise time. The MCP detector is fitted with a 1.3 in. diameter input grid. The 2.5 in. diameter grid mounting plate presents a flat, field-free plane to the incoming ions. The detector was located at 190 cm from the interaction point in an inclined plane with respect to the equatorial plane at the angles 5° in the horizontal and 9° in the vertical direction. Signals are recorded with a 2 GHz oscilloscope. In addition, non-invasive diagnostics of the beam have been tested in order to seek data for future applications (e.g., irradiation), where neither TPS nor TOF detectors can be used to monitor the particle beam. Two Thermo Scientific ionization chamber (IC) detectors, models FHT190 and FHT192, are installed in the experimental area for radioprotection purposes. These detectors measure the photons generated as secondary radiation from primarily accelerated particles. Both FHT190 and FHT192 are suitable for measurements down to the natural radiation background with noble gas-nitrogen mixture of a volume of 4.25 L and a filling pressure of 7 bars. The system features high-precision ambient equivalent dose measurements in the range between 0.1 μSv/h and 1 Sv/h for photon radiation from 30 keV to 7 MeV energy range. The registered data from the IC is based on a collection of charges created by the direct IC within the gas. It only uses the discrete charges created by each interaction between the incident radiation and the gas. The FHT190 detector was installed close to the experimental chamber at 2.05 m from the interaction point and 60 degrees from the laser propagation axis, while the FHT192 IC was situated approximately at 10 m and 90 degrees from the laser propagation axis. The signal from the latter is moderated with a 58.8 cm standard concrete shielding. For the sake of comparison we will define the first detector (closest to the interaction) as the direct IC detector and the shielded one as the moderated IC detector. Both detectors are useful since the direct IC gives a precise idea of the production of radiation, while the moderated one is better for the optimization method due to its background noise reduction.

The electron ejection as well as the EMP amplitude is diagnosed by a target charging monitor (TCM), which is constructed based on the principles of an inductive current monitor^[ Reference Cikhardt, Krása, De Marco, Pfeifer, Velyhan, Krouský, Cikhardtová, Řezáč, Ullschmied, Skála, Kubeš and Kravárik^47, Reference Ehret, Cikhardt, Bradford, Vladisavlevici, Burian, De Luis, Henares, Martín, Apiñaniz, Lera, Pérez-Hernández, Santos and Gatti^48]. A TCM measures the time derivative of the current between the target and grounding that is guided through the device. Current pulses are excited by laser–plasma interaction, that is, the discharge pulse and pulsed return current EMP in the regime of relativistic laser interaction^[ Reference Ehret, Bailly-Grandvaux, Korneev, Apiñaniz, Brabetz, Morace, Bradford, d’Humières, Schaumann, Bagnoud, Malko, Matveevskii, Roth, Volpe, Woolsey and Santos^49]. For this work, return current EMPs are transported via RG142 coaxial cables and the circuit impedance is Z = 50 $\Omega$ . Cable lengths are measured with ns FWHM voltage pulses: the target and TCM are connected with a coaxial cable of 9.6 $\,\pm\,$ 0.2 ns length, and the TCM and grounding are connected with a coaxial cable of 13.6 $\,\pm\,$ 0.1 ns length. Induced signals are transported to a 2 GHz oscilloscope and acquisitions are corrected throughout for the frequency dependent attenuation of circuit elements. Circuit calibrations are done using an R&S ZNH 4 GHz vector network analyser. The effective bandwidth of the circuit is 2 GHz.

3.1 Proton signal degradation at an HRR

As can be seen in Figure 2, all diagnostics show the same trend. In detail, the TPS, TOF detector and TCM agree in shot-to-shot details and the IC detectors show a coarse agreement in the general trends. HRR operation (1 Hz) produces an increase and then a degradation of the maximum proton signal measured in all detectors (around the 50th shot in the burst) from a single configuration of parameters with both aluminium and copper targets. The degradation is also clear in both IC detectors and the TCM. An increase of signal in the detectors was observed experimentally when the target was moved along the laser propagation axis (e.g., initial cut-off energies could be recovered in the TPS). We concluded that this degradation comes from a spatial drift of the focal spot when operating at 1 Hz. Thermal nature of this drift is under consideration as the optimal target position comes back to the initial value after some ‘cooling’ time or if the operation is made at single shot. This effect might come from the laser system amplifier or the laser gratings. The same effect was observed whether the amplification pumps were kept running at 1 Hz long before the operation or just during it, indicating that the drift may not be due to thermal lensing in the amplifiers. We are considering thermal effects in the compressor gratings as the most probable origin of the drift. Thermal effects in the laser gratings could also produce spatial chirp and degradation due to spatio-temporal coupling. The thermal effects in the gratings and the subsequent modifications of the laser beam properties need to be further investigated.

Figure 2 Proton maximum cut-off energy for (a) aluminium and (b) copper targets. Degradation of the signal can be observed in both figures and in all detectors. The maximum proton energy found for the aluminium target is around 11 MeV and decreases up to 6 MeV on average, while for the copper target it is around 13 MeV and decreases up to 7 MeV on average. The direct IC detects for both materials a maximum of about $2\times {10}^4$ μSv/h (not shown). In the case of the moderated IC, it measures 0.8 μSv/h (aluminium) and 1.0 μSv/h (copper). The TOF and TCM signals for copper were lost.

It is also important to remark that aluminium targets produce lower maximum proton energies but a more stable signal in the moderated IC detector, while copper targets generate a higher maximum proton energy but the signal in the moderated IC detector quickly decreases. This is probably because of the thickness difference between both targets.

3.2 Online optimization

Two ways were considered to mitigate the thermal effect. One option was to move the target position during operation along the laser axis (longitudinal) to retrieve the real focal spot position. This has the drawback of changing the reference position of the source. The second option was the modification of the collimation lens in the laser system to adapt the divergence of the laser beam. This lens is located between the last amplification stage and the compressor in a motorized stage. This has the drawback of changing the laser wavefront, the diameter of the laser, thus the minimum focal spot achievable, and perhaps adding astigmatism effects. In both cases, it was possible to perform online adaptation in order to retrieve the maximum signal (presumably at the minimum laser spot). Both movements are given in units of ${Z}_{\rm R}$ , where the reference zero was defined in the plane of the focal spot at low power and a single shot. Positive increments of ${Z}_{\rm R}$ correspond to movements increasing the distance from the off-axis parabolic mirror and vice versa. The effects of movement of the target position and a collimation lens for single shots are shown in Figure 3 using a copper target. In this case, a series of bursts of 10 shots were taken for each parameter. The region of the optimized maximum proton signal for single-shot operation can be clearly distinguished.

Figure 3 Measurements of the relative displacement from best focus of the Rayleigh length distance ${Z}_{\rm R}$ as a function of the maximum proton energies measured in single-shot laser interaction for target position movement (upper figure) and modification of the collimation lens (lower figure). The maximum return current EMP signal measured by the TCM is also shown for the same shots. Each point corresponds to an average of 10 shots with its standard deviation.

Figure 4 Optimization with (a) target position and (b) laser collimation lens during operation in the copper target. Shot numbers (#) are indicated by red lines. In case (a), the target was moved 3 ${Z}_{\rm R}$ (#30), 5 ${Z}_{\rm R}$ (#60), 8 ${Z}_{\rm R}$ (#90) from the original position to optimize the signal. In (b), the target was moved prior to the run to 8 ${Z}_{\rm R}$ and then the laser focal spot was moved to 9 ${Z}_{\rm R}$ (#86) and then to 11 ${Z}_{\rm R}$ (#115), clearly exceeding the optimum interaction plane. Afterwards the focal spot is moved back to 8 ${Z}_{\rm R}$ (#153).

It is important to note that motor movement during optimization runs is performed online between shots (thus a 1000-shot run takes 1000 s). Figure 4 shows the relevance of the correction procedure in both cases of signal optimization during an online burst. In Figure 4(a) the target was moved 3 ${Z}_{\rm R}$ , 5 ${Z}_{\rm R}$ , 8 ${Z}_{\rm R}$ from the original position to retrieve the signal. In the last configuration it is possible to conclude that the spot was still moving as the proton signal slowly increased. Then in Figure 4(b) the collimation lens was used to modify the laser spot position. In this case the target was moved before the run to the 8 ${Z}_{\rm R}$ position with respect to the zero position. It is possible to see that moving from 9 ${Z}_{\rm R}$ to 11 ${Z}_{\rm R}$ decreased the proton signal (meaning the laser spot overtakes the target) and then moving back to 8 ${Z}_{\rm R}$ increased the signal again. Note that the displacement of the focal plane by 8Z _R is consistent for both approaches. The magnitude of the displacement mandates a correction not only for ion generation, but also for any HRR experiment. It is important to note that the changes in the proton cut-off energies are well reflected in signals of the moderated IC detector and the TCM, a behaviour correlated with the hot electron temperature, which has already been reported in Ref. [Reference Ehret, Cikhardt, Bradford, Vladisavlevici, Burian, De Luis, Henares, Martín, Apiñaniz, Lera, Pérez-Hernández, Santos and Gatti48]. It is also interesting to observe that a change in the target position does not affect differently the signal measured by the TPS and TOF detectors (meaning the divergence of the particle beam remains similar), while modifying the collimation lens also affects the divergence of the beam since there is a change in the properties of the laser propagation (i.e., wavefront quality, focal spot size, etc.). In Figure 4(b) in the range of shots #85–#115, the signal from TPS increases while decreasing for the TOF detector. This means that more energetic protons are concentrated in the 0 deflection and the divergence of the proton beam is lower. The modification of the collimation lens could be a valuable avenue to control the divergence and collimation of the proton beam, to be studied in future experiments.

3.3 Proton signal stabilization

The ab initio adjustment of the target position to the position where the highest laser intensities are expected leads to a stable operation. The predictability of this position is significant, because it allows one to define the corrected position as the source position, for example, for beamlines. In Figures 5(a) and 5(b) the target was moved 9 ${Z}_{\rm R}$ prior to the run to compensate for the gradual defocusing and then small adjustments were done with the collimation lens. It is important to remark that 750 shots at 1 Hz were achieved with the copper target in a stable configuration (Figure 5(b)) of about 9 MeV proton maximum energies. Both the TPS and TOF detector detect the same energies and the TCM can follow the increase and stabilization of the signal. IC detectors show a signal increase, decrease and then stabilization.

Figure 5 Stabilization of maximum proton signal for aluminium and copper targets. In both cases the target was moved prior to the run to 9 ${Z}_{\rm R}$ . The maximum proton energy found for aluminium target is around 10 MeV at the end of the run, while for the copper target it is around 9 MeV. The energy jumps in (b) cannot be explained and both the IC detectors and TCM do not show sharp differences. The direct IC detector measures a maximum of about $2\times {10}^4$ μSv/h (aluminium) and $1\times {10}^4$ μSv/h (copper) (not shown) and the moderated one 1 μSv/h (aluminium) and 0.5 μSv/h (copper). The maximum return current EMP detected stabilizes around 0.6 kA at the end of the run.

In addition, a long run of 1000 shots at 1 Hz was performed to study the reliability of the system (Figure 6). To our knowledge this is the first time that a long run of proton production from metal targets was achieved using a 0.8 PW laser system at 1 Hz. The target was moved prior to the run to 9 ${Z}_{\rm R}$ . In order to prove the effect of the collimation lens, it was moved to 5 ${Z}_{\rm R}$ and the signal decreased while it recovered when moving back. In this case, the TPS and TOF detector show a more collimated proton beam since the signal from the TOF detector is smaller on average. This long run proves the stability and reliability of this target for many applications. It is interesting to observe how the TCM perfectly follows the modifications of the parameters and further stabilization. On the same path, the IC detectors measure an increase of signal at the beginning and the stabilization after parameter optimization.

Figure 6 One-thousand shot run of 0.8 PW at 1 Hz. The target was moved prior to the run to 9 ${Z}_{\rm R}$ ; then the laser focal spot was moved to 5 ${Z}_{\rm R}$ (#423), 7 ${Z}_{\rm R}$ (#475) and back to 9 ${Z}_{\rm R}$ (#502). The signal is stable at 10 MeV (TPS) and 6 MeV (TOF detector). The direct IC detector measures a maximum of about $2.5\times {10}^4$ μSv/h (not shown) and the moderated one 0.8 μSv/h. The TCM measures an almost stable 0.75 kA return current EMP.

3.4 Diagnostics comparison

A new paradigm in the detection of particles generated by laser–plasma interaction is needed to accomplish effective HRR operation. Here the figure of merit will be used to demonstrate the simultaneous detection of different radiations (and the detection of the same radiations by different means). For this reason, we will compare different diagnostics used during the experiment (TPS, TOF detector, TCM and IC detectors in both direct and moderated modes) in order to quantify their agreement to detect changes in the interaction due to parametric modifications during optimization, and their reliability to measure the stabilization of the source.

In this study, the TPS and TOF detector have a similar detection range from 0.4 to 25 MeV maximum proton energy. In general, the TPS and TOF detector follow similar trends (with the exception of the use of the collimation lens) and a range of values were obtained from 5 up to 16 MeV. In the case of instrumentation for applications, the TPS has the drawback of measuring the direct proton beam, while the TOF detector has the advantage that it can be mounted in an angle sufficiently far away from the propagation axis to use the direct proton beam and still obtain information; however care has to be taken when modifying target or laser parameters.

TCM maximum amplitude measurements range from 0.290 to 1.03 kA. The TCM provided high sensitivity to changes and valuable information about the laser–plasma interaction, and the quality of the expelled electrons (thus proton acceleration). Still more information can be extracted from this detector (e.g., integrated return current signal, oscillations) and it will be further studied.

The IC detectors have a minimum detection threshold of 0.1 μSv/h, which comes from background radiation. The direct IC detector measured up to 38 mSv/h in some punctual cases, while the moderated IC detector measured up to 1.33 μSv/h. The direct IC detector provided peaked distributions with information of the total production of radiation; however, the shot-to-shot oscillations of this signal make it unusable for optimization or evaluation of stabilization. For this reason, it was found that the moderated IC detector was the tool to be used to control changes, stability and reliability of the source. However, it was observed that, due to its nature, the moderated IC detector has some seconds delay with respect to other detectors (clearly observable in Figure 3(a) around shots #30 and #60, and in Figure 4(b) around shot #115). This effect will be further analysed in a following publication.

Three cases will be studied in detail concerning the stabilization of the proton source: 170 shots in Al (Figure 5(a)), 735 shots in Cu (Figure 5(b)) and 1000 shots in Cu (Figure 6). For the sake of comparison all averages will be evaluated by the consecutive last 100 shots for each stabilization case as a figure of merit (Table 1).

Table 1 Cases studied with the figure of merit of the last 100 shots to evaluate the stabilization of the source.

All cases in this study obtained similar maximum proton energies (focusing on the stable part) in the TPS and TOF detectors with the exception of the use of the collimation lens parameter (e.g., in Figure 6), which was considered as an effective reduction of the proton beam divergence. It was expected that the IC detectors might be also sensitive to beam divergence effects; however, it is not possible to make a clear conclusion and this will be the topic of future experiments. A similar maximum return current EMP was measured by the TCM in all cases, and this detector proved to be very stable and precise when detecting changes in the interaction efficiency (e.g., in Figure 6 around shot #450). However, there is a behaviour change when both IC detectors were analysed. The direct and moderated IC signals detected are higher in Figure 5(a) compared to lower signals obtained in the other two. This could come from the nature of the target (aluminium) and needs further study with more data. As a conclusion, different diagnostics were compared, observing promising behaviour to measure changes simultaneously. This will be fundamental in order to characterize the performance of a laser–plasma source at HRR.