Laser-driven ion sources have attracted significant interest due to their potential for developing compact, high-gradient ion accelerators and their suitability for a range of applications [1]. Many of these-including nuclear fuel breeding and neutron radiation material damage studies-require a stable, high-flux, multi-Hz proton source with energies above 10 MeV. Our research at SLAC indicates that liquid targets operated at high repetition rate offer a promising pathway towards meeting these requirements. We report experimental results on proton acceleration up to 29 MeV from sub-micron liquid water sheet targets [2] irradiated by high-intensity, ultrashort laser pulses at 3.3 Hz with energies between 1 and 3 J in a ~19,000-shot campaign conducted with the L3-HAPLS laser at ELI Beamlines. Target thickness, laser energy, and laser temporal shape scans were performed with the proton beam being characterized using time-of-flight, Thomson parabola, and ion imager diagnostics. This study resulted in reproducible, precise characterization over hundreds of datapoints at any laser and target condition. The measured proton spectra show strong sensitivity to the laser and target parameters indicating that we can access new acceleration regimes where we can manipulate the spectral shapes and cutoff energies of the laser-driven ion beam. These results show that the dominant acceleration mechanisms are highly sensitive to the target density evolution and that liquid water sheets provide a stable, high-repetition rate platform for exploring novel ion acceleration mechanisms and for advancing high-flux proton beam sources.

With the advent of high-repetition-rate, high-power laser systems, such as those at the ELI Beamlines facility, there is a growing need for a new generation of diagnostics for laser-generated particles and radiation. Scintillator-based detectors have gained significant interest in the scientific community, as they offer real-time, shot-to-shot measurements during experiments. This work introduces two advanced types of scintillator stack detectors developed for the ELIMAIA beamline to characterize laser-generated bremsstrahlung radiation (hard X-rays) and quasi-monoenergetic proton bunches. These detectors are promising candidates to replace previous-generation devices, such as single-photon counters or radiochromic films, which are no longer suitable for high-power, high-repetition-rate laser-plasma experiments due to issues like saturation or the need for time-consuming post-processing. The X-ray Scintillator Stack Calorimeter (SCIS-CAL) and the proton stack (p+Stack) have been extensively tested in various experiments, effectively measuring and analyzing shot-to-shot fluctuations in radiation and particle properties. Furthermore, these detectors have provided new insights into laser-plasma interaction processes, such as correlation studies between bremsstrahlung radiation parameters and proton energies and bremsstrahlung temperature scaling with laser intensity.

An experimental platform for laser-driven ion (sub-MeV) acceleration and potential applications was commissioned at the HiLASE laser facility. The auxiliary beam of the Bivoj laser system operating at a GW level peak power (~10 J in 5–10 ns) and 1–10 Hz repetition rate enabled a stable production of high-current ion beams of multiple species (Al, Ti, Fe, Si, Cu, and Sn). The produced laser–plasma ion sources were fully characterized against the laser intensity on the target (1013–1015 W/cm2) by varying the laser energy, focal spot size, and pulse duration. The versatility and tuneability of such high-repetition-rate laser–plasma ion sources are of potential interest for user applications. Such a statistically accurate study was facilitated by the large amount of data acquired at the high repetition rate (1–10 Hz) provided by the Bivoj laser system.

Real-time evaluation of laser-driven byproducts is crucial for state-of-the-art facilities operating at high repetition rates. This work presents real-time measurements of hard X-rays (bremsstrahlung radiation) generated from the interaction of high-intensity laser pulses with solid targets in the target normal sheath acceleration regime using a scintillator stack detector. The detector offers insights into the effectiveness of laser-plasma interaction through measured fluctuations in bremsstrahlung radiation temperature and scintillation light yield on a shot-to-shot basis. Moreover, a strong correlation of the bremsstrahlung measurements (i.e., temperature and yield) with the cutoff energy of laser-driven protons was observed. The scintillator stack detector serves not only as a diagnostic for online monitoring of the laser-plasma interaction but also as a promising tool for estimating proton energy fluctuations in a non-disruptive manner, which is particularly important when direct proton source characterization is impractical, for example, during experiments aimed at irradiating user samples with the accelerated proton beam.

Tight focusing with very small f-numbers is necessary to achieve the highest at-focus irradiances. However, tight focusing imposes strong demands on precise target positioning in-focus to achieve the highest on-target irradiance. We describe several near-infrared, visible, ultraviolet and soft and hard X-ray diagnostics employed in a similar to 10(22) W/cm(2) laser-plasma experiment. We used nearly 10 J total energy femtosecond laser pulses focused into an approximately 1.3-mu m focal spot on 5-20 mu m thick stainless-steel targets. We discuss the applicability of these diagnostics to determine the best in-focus target position with approximately 5 mu m accuracy (i.e., around half of the short Rayleigh length) and show that several diagnostics (in particular, 3 omega reflection and on-axis hard X-rays) can ensure this accuracy. We demonstrated target positioning within several micrometers from the focus, ensuring over 80% of the ideal peak laser intensity on-target. Our approach is relatively fast (it requires 10-20 laser shots) and does not rely on the coincidence of low-power and high-power focal planes.

Gamma rays consist of high-energy photons that selectively interact with nuclei, induce and mediate nuclear reactions and elementary particle interactions, and exceed x-rays in penetrating power and thus are indispensable for analysis and modification of dense or compressed object interior. Yet, the available gamma sources lack power and brightness which, if available, would revolutionize science and technology. The predicted laser-driven Gamma Flash (GF) would be the highest-power and the brightest terrestrial gamma source with a 30-40 scattering in a laser-solid interaction at irradiance typically above 10^23W/cm^2. GF is one of the motivating goals for the most advanced laser facilities. However, till now GF remains overshadowed by simultaneously generated low-brightness Bremsstrahlung. Here we experimentally differentiate these two mechanisms and demonstrate a GF dominant regime producing several times the number of Bremsstrahlung photons. We found steep GF yield growth with the laser power and irradiance. Simulations revealed a Terawatt GF with nanometre source and an attosecond pulse train with a record brightness of 10^22photons/mm^2mrad^2s0.1%BW at up to tens of MeV. The small source size and high brightness paves the way towards spatially coherent gamma rays. At high photon energies, our regime is comparable in brightness to astrophysical Gamma Ray Bursts. We anticipate that the gamma ray source based on our findings will facilitate a breakthrough in research on future inertial fusion energy by enabling high-spatial-resolution time-resolved radiography of fuel mixing instabilities in extremely compressed targets. Such a new compact bright ultrafast gamma source could facilitate significant advances in time-resolved nuclear physics, homeland security, and nuclear waste management and non-proliferation.

The nuclear fusion channel of the p- 11 B reaction producing α particles with multi-MeV kinetic energies was induced by a sub-nanosecond laser pulse focused onto 10 μm thick boron-doped thin targets at intensities of ∼ 10 16 W/cm 2 . A full characterization in terms of α particle flux and angular distribution was performed thanks to the simultaneous use of several diagnostics (time-of-flight detectors, nuclear track detectors, and Thomson Parabola spectrometers), which enabled to measure key features of particles produced both in the backward (target front side) and forward (target rear side) directions. Maximum α particle flux and cut-off energy were recorded at small detection angles with respect to the target normal in the backward direction. The maximum kinetic energy shown by the α -particles produced in the nuclear fusion reaction was ascribable to a post-accelerating transient electric field present in the laser-generated plasma, in agreement with our previous preliminary results.

We report on the commissioning of the ELIMAIA beamline laser-plasma Ion Accelerator carried out at relativistic intensities (~1021 W/cm2) with the high repetition-rate, high peak-power L3-HAPLS (>10J in 30 fs) laser available at the ELI Beamlines user facility. Targets of different composition and thickness were used to optimize the performance of the Ion Accelerator. In the best conditions, we were able to reach proton cutoff energies around 30 MeV and fluxes above 1011/sr. Moreover, we have demonstrated an excellent reliability and shot-to-shot stability (1-2% in energy) of the Ion Accelerator up to a repetition rate of 0.5 Hz for several hundreds of consecutive shots, along with on-shot target positioning and data acquisition and analysis systems. These results demonstrate the robustness of the developed technology available for users at the ELIMAIA beamline, thus paving the way towards its future use for fundamental and applied research, including biomedical ones.

Solid-state nuclear track detectors (CR-39 type) are frequently used for the detection of ions accelerated by laser-plasma interaction because they are sensitive to each single particle. To the present day, CR-39 detectors are the main diagnostics in experiments focused on laser-driven proton-boron (p(11)B) fusion reactions to detect alpha particles, which are the main products of such a nuclear reaction, and to reconstruct their energy distribution. However, the acceleration of multispecies ions in the laser-generated plasma makes this spectroscopic method complex and often does not allow to unambiguously discriminate the alpha particles generated from p(11)B fusion events from the laser-driven ions. In this experimental work, performed at the PALS laser facility (600 J, 300 ps, laser intensity 10(16) W/cm(2)), CR-39 detectors were used as main detectors for the angular distribution of the produced alpha particles during a p(11)B fusion dedicated experimental campaign. Additionally, a CR-39 detector was set inside a Thomson Parabola (TP) spectrometer with the aim to calibrate the CR-39 response for low energetic laser-driven ions originating from the plasma in the given experimental conditions. The detected ion energies were ranging from hundreds of keV to a few MeV, and the ion track diameters were measured for etching times up to 9 hours. The goal of the test was the evaluation of the detectors' ability to discriminate the alpha particles from the aforementioned ions. Within this study, the calibration curves for protons and silicon low energy ions are accomplished, the overlapping of the proton tracks and alpha particles is verified, and a methodology to avoid this problem is realized.

Ion acceleration resulting from the interaction of 11 fs laser pulses of ∼ 35 mJ energy with ultrahigh contrast (<10 −10 ) and 10 19 W cm −2 peak intensity with foil targets made of various materials and thicknesses at normal (0°) and 45° laser incidence is investigated. The maximum energy of the protons reached ∼1.4 MeV accelerated in the laser propagation direction and ∼1.2 MeV in the opposite direction from a formvar target. The energy conversion efficiency from the laser to the proton beam is estimated to be as high as ∼1.4% at 45° laser incidence using a 51 nm thick Al target. The high laser contrast indicates the predominance of vacuum heating via Brunel’s effect as an absorption mechanism involving a tiny pre-plasma at the target front. The experimental results are in reasonable agreement with theoretical estimates, where proton acceleration from the target front side in the backward direction is well explained by the Coulomb explosion of a charged cavity formed in a tiny pre-plasma, while forward proton acceleration is likely to be a two-step process: protons are first accelerated in the target front-side cavity and then further boosted in energy through the target back side via the target normal sheath acceleration (TNSA) mechanism.

We review a number of instruments employed in a high-intensity J-KAREN-P laser-solid interaction experiment and discuss the applicability of the diagnostics to the best target position determination with a ~10 μm accuracy, while the focal spot size was ~1 μm and peak intensity was up to 7×1021 W/cm2. We discuss both front- and back-side diagnostics, some of them operated in the infrared, visible and ultraviolet ranges, while others in the extreme ultraviolet, soft X-ray and gamma-ray ranges. We found that the applicability of some of the instruments to the best at-focus target position determination depends on the thickness of the target.

Tight focusing with very small f-numbers is necessary to achieve highest at-focus irradiances. However, tight focusing also means short Rayleigh length, which imposes strong demands on the precise positioning of the target at the best focus to achieve the highest on-target irradiance. We describe several near-infrared, visible, ultraviolet, soft and hard X-ray diagnostics employed in the ~10^22 W/cm2 laser-plasma experiment at the J-KAREN-P laser facility in KPSI, Japan. The experiment requires a tight focusing of ~10 J femtosecond infrared laser pulses into ~1.3-µm-diameter focal spots on stainless steel (SUS) solid targets of different thicknesses (5–20 µm). We discuss the applicability of these diagnostics to determine the best in-focus position of the target with <10 µm accuracy (i. e., within the short Rayleigh length) in high-power laser-matter experiments, and suggest which diagnostics should and which ones should not be used for this purpose. It was demonstrated that the target could be positioned to within few µm out of the best laser focus, ensuring over 80% of the ideal peak intensity.

We have developed a compact liquid target setup that produces a continuous ø50 µm cylindrical water jet, capable of operating at high vacuum. It has been tested with a commercial ultrashort-pulse laser in a series of proof-of-principle laser-driven ion acceleration and x-ray generation experiments at repetition rates up to 1 kHz. In optimized conditions, measurements by the time-of-flight (TOF) method have demonstrated a proton signal cut-off energy of 179±9 keV. The laser-generated x-ray emission was characterized in the range 2-36 keV and used as excitation for x-ray fluorescence spectroscopy (XRF) measurements.

We report on the technological commissioning of the Laser–Plasma Ion Accelerator section of the ELIMAIA user beamline at the ELI Beamlines facility in the Czech Republic. The high-peak, high-average power L3-HAPLS laser system was used with an energy of ~10 J and pulse duration of ~30 fs on target, both in single-pulse and high repetition-rate (~0.5 Hz) mode. The laser pulse was tightly focused to reach ultrahigh intensity on target (~1021 W/cm2) and sustain such laser–plasma interaction regime during high repetition-rate operations. The laser beam, ion beam, and laser–plasma emission were monitored on a shot-to-shot basis, and online data analysis at 0.5 Hz was demonstrated through the full set of used diagnostics (e.g., far and near field, laser temporal diagnostics, X- and gamma-ray detectors, Thomson Parabola ion spectrometer, time-of-flight ion detectors, plasma imaging, etc.). The capability and reliability of the ELIMAIA Ion Accelerator was successfully demonstrated at a repetition rate of 0.5 Hz for several hundreds of consecutive laser shots.

UHDpulse - Metrology for Advanced Radiotherapy using beams with Ultra-High Pulse Dose Rates is a European project aimed at developing novel dosimetry standards, as well as improving existing ones, for FLASH radiotherapy, very high energy electrons radiotherapy, and laser-driven medical accelerators. Within the scope of this project, Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) detectors are used to measure stray radiation fields. Experiments performed with conventional pulsed particle-beams allow to characterize the dosimeters in known and controllable radiation fields. In turn, this allows to develop models and predict their behavior in complex radiation fields, such as those at laser-driven and FLASH facilities. TL and OSL detectors were irradiated at the Microtron MT25 electron accelerator in Prague, Czech Republic. GAFChromicTM films and plastic nuclear track detectors were used to study the beam profile and the neutron background respectively. The responses of the different detector to the pulsed mixed radiation fields of the Microtron MT25 are compared among each other and presented in this paper.

We report on recent progress in deploying a continuous solid hydrogen ribbon as a debris-free and renewable laser-driven source of pure proton beams generated by a 30-fs laser with ∼1-J laser energy focused on target at relativistic intensities of ∼1019 W/cm2 and repetition rate of 0.1 Hz. The stability of the ribbon position versus the laser interaction point and maximum repetition rate was tested up to 3.3 Hz. The acceleration of protons with cut-off energies up to 1.5 MeV is demonstrated using a 100-μm thick hydrogen ribbon as proof-of-principle capability of the relatively thick target delivery system. The laser-target geometry presented demonstrates an experimental technique that can potentially enables the operation of a laser–plasma source at Hz-level repetition rate.

Experiments performed with conventional radiation beams allow to characterize the dosimeters in known and controllable radiation fields. Introduction UHDpulse Metrology for Advanced Radiotherapy using Particle beams with UltraHigh Pulse Dose Rates is a joint European research project aimed at developing novel dosimetry standards, as well as improving existing ones, for FLASH-RT, VHEE RT, and laser-driven medical accelerators. A dedicated work package focuses on the development of traceable and validated methods for characterizing stray radiation fields which may deliver parasitic dose to healthy tissue and organs outside the targeted area. Secondary radiation fields have the same pulsed time structure of the primary beams. Additionally, they are composed of different types of particles with different energies. The various field components need to be identified, thus adding to the metrological difficulties. The use of thermoluminescence (TL) and optically simulated luminescence (OSL) detectors in this context is being investigated. In Jan. 2020, a dedicated data-taking campaign was performed at the Microtron MT25 a cyclic electron accelerator in Prague, Czech Republic. Characterization of OSL and TL Dosimeters with Data Collected at the MT25 Cyclic Electron Accelerator . A. Cimmino1, I. Ambrožová2, , S. Motta1,3, R.Versaci1, D. Chvátil4, V. Olšanský4, V. Olšovcová1, A. Velyhan1, V. Stránský1, R. Truneček1, J. Šolc5

The Microtron MT25 is a cyclic electron accelerator with a Kapitza resonator, maximum beam energy of 25 MeV, standard repetition frequency of 423 Hz, pulse length of 3.5 μs and mean current of 30 μA. Studies at conventional particle accelerators allow to understand the response of dosemeters in known and controllable radiation fields. Subsequently, it is possible to develop models and predict their behavior in complex radiation fields, such as those generated at laser and FLASH facilities. Therefore, response of thermally and optically stimulated luminescence detectors outside of the beam was studied at the Microtron MT25. The detectors were placed on a Plexiglas phantom inside a lead and iron bunker to shield-off background radiation. In addition, GAFChromic™ films and track detectors were used. Two irradiations were performed: with and without an 8-cm thick polyethylene moderator. This paper presents a comparison of the responses of the different detection systems.

A series of thin films made of aligned carbon nanotubes (CNTs) embedded in a polyimide substrate was designed, fabricated and used for the first time to accelerate protons and C ions by interaction with a sub-nanosecond, high power laser beam (600 J energy and 300 ps pulse width) with peak intensity of about 3 × 1016 W/cm2 on target. Each target was 5 µm thick, and the composite material contained CNTs aligned in different directions in the substrate. The results obtained from the analysis of a Thomson Parabola spectrometer, and of the spots imprinted by ions on a series of PM355 nuclear track detectors, indicate high energies (up to 3 MeV for protons and 9 MeV for C ions) and a marked influence of the CNTs’ orientation on the produced proton beam current. An increase of the proton fluxes, more than two orders of magnitude, was recorded with the targets containing CNTs aligned parallel to the target normal, in comparison to the other targets. The presented experimental results demonstrate that the laser-driven proton beam flux can be increased using ad hoc designed targets (with embedded and aligned nanotubes) and sub-nanosecond laser pulses with moderate intensities and poor temporal contrast, thus in an acceleration regime very far from those typically investigated experimentally using relativistic intensities (>5 × 1018 W/cm2) and short laser pulses (10 fs to 10 ps).