The proposed ‘fission-fusion’ reaction mechanism aims at investigating the rapid neutron-capture process, contributing to the formation of heavy elements, by using laser-accelerated thorium ions in a sandwich target configuration [1]. In a first step, the efficient acceleration of gold ions is investigated, as recently achieved in our measurement at the PHELIX laser with 500 fs long pulses [2]. In this experiment, for the first time, the laser-based acceleration of gold ions above 7 MeV/u was demonstrated. Additionally, individual gold charge states were resolved with unprecedent resolution. This allowed to investigate the role of collisional ionization using a developmental branch of the particle-in-cell simulation code EPOCH [3], showing a much better agreement of the simulated charge state distributions with the experimentally measured ones than when only considering field ionization. This work is continued at the Centre for Advanced Laser Applications (CALA), using the ATLAS 3000 laser (800 nm central wavelength, 25 fs pulse length). [1] D. Habs et al., Appl. Phys. B 103, 471-484 (2011) [2] F.H. Lindner et al., Sci. Rep. 12, 4784 (2022) [3] M. Afshari et al., Sci. Rep. 12, 18260 (2022)

Laser-plasma accelerated (LPA) proton bunches are now applied for research fields ranging from ultra-high-dose-rate radiobiology to material science. Yet, the capabilities to characterize the spectrally and angularly broad LPA bunches lag behind the rapidly evolving applications. The OCTOPOD translates the angularly resolved spectral characterization of LPA proton bunches into the spatially resolved detection of the volumetric dose distribution deposited in a liquid scintillator. Up to 24 multi-pinhole arrays record projections of the scintillation light distribution and allow for tomographic reconstruction of the volumetric dose deposition pattern, from which proton spectra may be retrieved. Applying the OCTOPOD at a cyclotron, we show the reliable retrieval of various spatial dose deposition patterns and detector sensitivity over a broad dose range. Moreover, the OCTOPOD was installed at an LPA proton source, providing real-time data on proton acceleration performance and attesting the system optimal performance in the harsh laser-plasma environment.

Target pre-heating has proven to be beneficial for laser-driven heavy-ion acceleration. As it allows to remove omnipresent carbohydrate surface contaminants, heating can affect the cutoff energy and increase the efficient acceleration of heavy ions as, e.g., required for the novel fission-fusion nuclear reaction scheme where kinetic energies of fissile species around 7 MeV/u are targeted. At the Centre for Advanced Laser Applications in Garching we use a 3W Nd:YAG cw laser to heat the (in our case gold) target foil in order to investigate the dependency of efficient gold ion acceleration on heating parameters. For real-time assessment of the surface temperature the thermal spectrum is measured with a NIR spectrometer to which Planck’s law is fitted.

Abstract In the past years, the interest in the laser-driven acceleration of heavy ions in the mass range of $$\text {A}\approx 200$$ A ≈ 200 has been increasing due to promising application ideas like the fission-fusion nuclear reaction mechanism, aiming at the production of neutron-rich isotopes relevant for the astrophysical r-process nucleosynthesis. In this paper, we report on the laser acceleration of gold ions to beyond 7 MeV/u, exceeding for the first time an important prerequisite for this nuclear reaction scheme. Moreover, the gold ion charge states have been detected with an unprecedented resolution, which enables the separation of individual charge states up to 4 MeV/u. The recorded charge-state distributions show a remarkable dependency on the target foil thickness and differ from simulations, lacking a straight-forward explanation by the established ionization models.

The development from single shot basic laser plasma interaction research toward experiments in which repetition rated laser-driven ion sources can be applied requires technological improvements. For example, in the case of radio-biological experiments, irradiation duration and reproducible controlled conditions are important for performing studies with a large number of samples. We present important technological advancements of recent years at the ATLAS 300 laser in Garching near Munich since our last radiation biology experiment. Improvements range from target positioning over proton transport and diagnostics to specimen handling. Exemplarily, we show the current capabilities by performing an application oriented experiment employing the zebrafish embryo model as a living vertebrate organism for laser-driven proton irradiation. The size, intensity, and energy of the laser-driven proton bunches resulted in evaluable partial body changes in the small (<1 mm) embryos, confirming the feasibility of the experimental system. The outcomes of this first study show both the appropriateness of the current capabilities and the required improvements of our laser-driven proton source for in vivo biological experiments, in particular the need for accurate, spatially resolved single bunch dosimetry and image guidance.

The 'Centre for Advanced Laser Applications' (CALA) is a new research institute for laser-based acceleration of electron beams for brilliant x-ray generation, laser-driven sub-nanosecond bunches of protons and heavy ions for biomedical applications like imaging and tumour therapy as well as for nuclear and high-field physics. The radiation sources emerging from experiments using the up to 2.5 petawatt laser pulses with 25 femtosecond duration will be mixed particle-species of high intensity, high energy and pulsed, thus posing new challenges compared to conventional radiation protection. Such worldwide pioneering laser experiments result in source characteristics that require careful a-priori radiation safety simulations. The FLUKA Monte-Carlo code was used to model the five CALA experimental caves, including the corridors, halls and air spaces surrounding the caves. Beams of electrons (<5 GeV<i), protons (<200 MeV<i),C-12 (<400MeV/u<i) and(197)Au (<10MeV/u<i) ions were simulated using spectra, divergences and bunch-charges based on expectations from recent scientific progress. Simulated dose rates locally can exceed 1.5 kSv h(-1)inside beam dumps. Vacuum pipes in the cave walls for laser transport and extraction channels for the generated x-rays result in small dose leakage to neighboring areas. Secondary neutrons contribute to most of the prompt dose rate outside caves into which the beam is delivered. This secondary radiation component causes non-negligible dose rates to occur behind walls to which large fluences of secondary particles are directed. By employing adequate beam dumps matched to beam-divergence, magnets, passive shielding and laser pulse repetition limits, average dose rates in- and outside the experimental building stay below design specifications (<0.5 mu Sv h(-1)) for unclassified areas,<2.5 mu Sv h(-1) for supervised areas,<7.5 mu Sv h(-1) aximum local dose rate) and regulatory limits (<1mSv a(-1) for unclassified areas).

The shape of a wave carries all information about the spatial and temporal structure of its source, given that the medium and its properties are known. Most modern imaging methods seek to utilize this nature of waves originating from Huygens’ principle. We discuss the retrieval of the complete kinetic energy distribution from the acoustic trace that is recorded when a short ion bunch deposits its energy in water. This novel method, which we refer to as Ion-Bunch Energy Acoustic Tracing (I-BEAT), is a refinement of the ionoacoustic approach. With its capability of completely monitoring a single, focused proton bunch with prompt readout and high repetition rate, I-BEAT is a promising approach to meet future requirements of experiments and applications in the field of laser-based ion acceleration. We demonstrate its functionality at two laser-driven ion sources for quantitative online determination of the kinetic energy distribution in the focus of single proton bunches.

The fission-fusion reaction mechanism was proposed in order to generate extremely neutron-rich nuclei close to the waiting point N = 126 of the rapid neutron capture nucleosynthesis process (r-process). The production of such isotopes and the measurement of their nuclear properties would fundamentally help to increase the understanding of the nucleosynthesis of the heaviest elements in the universe. Major prerequisite for the realization of this new reaction scheme is the development of laser-based acceleration of ultra-dense heavy ion bunches in the mass range of A = 200 and above. In this paper, we review the status of laser-driven heavy ion acceleration in the light of the fission-fusion reaction mechanism. We present results from our latest experiment on heavy ion acceleration, including a new milestone with laser-accelerated heavy ion energies exceeding 5 MeV/u.

Laser-based ion acceleration is driven by electrical fields emerging when target electrons absorb laser energy and consecutively leave the target material. A direct correlation between these electrons and the accelerated ions is thus to be expected and predicted by theoretical models. We report on a modified wide-angle spectrometer, allowing the simultaneous characterization of angularly resolved energy distributions of both ions and electrons. Equipped with online pixel detectors, the RadEye1 detectors, the investigation of this correlation gets attainable on a single shot basis. In addition to first insights, we present a novel approach for reliably extracting the primary electron energy distribution from the interfering secondary radiation background. This proves vitally important for quantitative extraction of average electron energies (temperatures) and emitted total charge.

We report on a scintillator-based online detection system for the spectral characterization of polychromatic proton bunches. Using up to nine stacked layers of radiation hard polysiloxane scintillators, coupled to and readout edge-on by a large area pixelated CMOS detector, impinging polychromatic proton bunches were characterized. The energy spectra were reconstructed using calibration data and simulated using Monte-Carlo simulations. Despite the scintillator stack showed some problems like thickness inhomogeneities and unequal layer coupling, the prototype allows to obtain a first estimate of the energy spectrum of proton beams.

Often, the interpretation of experiments concerning the manipulation of the energy distribution of laser-accelerated ion bunches is complicated by the multitude of competing dynamic processes simultaneously contributing to recorded ion signals. Here we demonstrate experimentally the acceleration of a clean proton bunch. This was achieved with a microscopic and three-dimensionally confined near critical density plasma, which evolves from a 1 µm diameter plastic sphere, which is levitated and positioned with micrometer precision in the focus of a Petawatt laser pulse. The emitted proton bunch is reproducibly observed with central energies between 20 and 40 MeV and narrow energy spread (down to 25%) showing almost no low-energetic background. Together with three-dimensional particle-in-cell simulations we track the complete acceleration process, evidencing the transition from organized acceleration to Coulomb repulsion. This reveals limitations of current high power lasers and viable paths to optimize laser-driven ion sources.

A common approach for spectrum determination of polyenergetic proton bunches from laser-ion acceleration experiments is based on the time-of-flight (TOF) method. However, spectra obtained using this method are typically given in relative units or are estimated based on some prior assumptions on the energy distribution of the accelerated ions. In this work, we present a new approach using the TOF method that allows for an absolute energy spectrum reconstruction from a current signal acquired with a sub-nanosecond fast and 10 µm thin silicon detector. The reconstruction is based on solving a linear least-squares problem, taking into account the response function of the detection system. The general principle of signal generation and spectrum reconstruction by setting up an appropriate system response matrix is presented. Proof-of-principle experiments at a 12 MV Tandem accelerator using different nanosecond-short (quasi-)monoenergetic and polyenergetic proton bunches at energies up to 20 MeV were successfully performed. Within the experimental uncertainties of 2.4% and 12.1% for energy and particle number, respectively, reconstructed energy distributions were found in excellent agreement with the spectra calculated using Monte Carlo simulations and measured by a magnetic spectrometer. This TOF method can hence be used for absolute online spectrometry of laser-accelerated particle bunches.

We introduce a target concept for laser-driven ion acceleration with ultrashort, highly intense laser pulses that includes an integrated double plasma mirror for contrast enhancement. It comprises three nanometer thin plastic foils, embedded in a small metal structure, which ensures precise mounting. The geometry allows to apply a double plasma mirror directly in front of a target foil within the converging beam, enabling moderate-energy (∼1 J) laser systems to reach the required fluence of several hundred J/cm2 on the plasma mirrors. During an experimental campaign, performed at the Laboratory for Extreme Photonics in Munich, we observed proton energies increased by a factor of three using this new target, which is attributed to an enhanced laser contrast after the integrated double plasma mirrors.

Laser-driven acceleration of particle beams is an emerging modality under research for biomedical applications. The spatially resolved diagnostics of laser-accelerated proton bunches is crucial for their application. The RadEye detector, featuring up to 10 cm x 5 cm area of online complementary metal-oxide-semiconductor (CMOS) detector made of 48 μm pixels, is established for x-ray, proton and ion beam diagnostics. We exploit the usually undesired ‘Image lag’ phenomenon of incomplete pixel reset to generate 2D-images with a larger dynamic range than the single frame range of 12-bit. Using 532 nm laser pulses and computer simulations for single-slit diffraction, calibration factors to stack multiple readouts were successfully derived to quantitatively reconstruct spatial information about an optical beam and hence extend the dynamic range of the detector compared to a single frame. The final goal is focus quantification for a permanent magnet quadrupole system for protons and terawatt (TW-class) laser focus diagnostics.

Irradiating thin foils with very intense laser pulses results in the emission of swift ion bunches. These laser-accelerated ions exhibit properties that substantially differ from conventionally accelerated particles. A large number of a variety of ion species distributed over a broad energy and angular spectrum are confined to very short bunch durations and are sought to enable novel applications [1]. For many years these properties have been studied and optimized mainly by investigating the interaction process of intense lasers pulses with plasmas in exemplary experiments [2].

Summary form only given. With the continuous advances in the development of high-intensity lasers, laser-driven ion acceleration may become a compact alternative to produce ions with energies suitable for medical applications [1]. However, the unique properties of laser-accelerated ions, such as the mixed radiation field, a broad energy spectrum of up to 100% energy spread and ion fluxes exceeding 10 7 protons/cm 2 /ns are a challenge for electronic detectors. Due to relatively large shot-to-shot fluctuations in the energy spectrum, a non-destructive characterization of each ion bunch is necessary for reliable biomedical applications. Our approach to obtain the spectra exploits the time-of-flight (TOF) difference of ions with different kinetic energies, which requires fast and sensitive detectors. We used a novel silicon microdosimeter (“bridge” microdosimeter), developed at the Centre for Medical Radiation Physics at the University of Wollongong for our experiments. The detector consists of an array of 3D sensitive volumes (30 x 30 μm2), connected in parallel via thin silicon bridges [2]. The sensitive thickness of each pixel is 10 μm. In order to test the setup under reproducible conditions, first TOF-measurements were performed at the Maier-Leibnitz-Laboratorium Tandem van-de-Graaf accelerator. By using a chopper and buncher system, 20 MeV proton pulses with a bunch duration of less than 2 ns and a high flux could be obtained. Two dedicated energy-degrader filters were designed to obtain poly-energetic spectra from the mono-energetic proton beam. Moreover, we measured the TOF-spectrum of laser-accelerated ion bunches, where a 35 fs laser pulse of the ATLAS-300, a 300 TW Ti:Sa laser system, was focussed onto thin plastic and gold foils. In both cases, the TOF-signal was acquired using a 4 GHz oscilloscope. The proton energy spectrum was deconvolved from the measured TOF-signal using an iterative deconvolution algorithm.

We report on a target system supporting automated positioning of nano-targets with a precision resolution of $4~\unicode[STIX]{x03BC}\text{m}$ in three dimensions. It relies on a confocal distance sensor and a microscope. The system has been commissioned to position nanometer targets with 1 Hz repetition rate. Integrating our prototype into the table-top ATLAS 300 TW-laser system at the Laboratory for Extreme Photonics in Garching, we demonstrate the operation of a 0.5 Hz laser-driven proton source with a shot-to-shot variation of the maximum energy about 27% for a level of confidence of 0.95. The reason of laser shooting experiments operated at 0.5 Hz rather than 1 Hz is because the synchronization between the nano-foil target positioning system and the laser trigger needs to improve.

We simulated a doublet of permanent magnet quadrupoles (PMQs) to estimate the sensitivity on positioning precision and its impact on the spectral properties of transported protons. The study guided the construction and testing of a focusing setup for laser-accelerated proton bunches with energies between 6 and 10 MeV. Our results shed light on possible applications that may arise from broad input particle spectra.

Laser-driven acceleration of ions has inspired novel applications, that can benefit from ion bunch properties different from conventionally (non-laser based) accelerated particle beams. Those differences range from extremely short bunch durations, broad energy spectra, large divergence angles and small source sizes to ultra-high ion bunch densities. So far, the main focus of research has been concentrating on the physics of the interaction of intense laser pulses with plasmas and the related mechanisms of ion acceleration. Now, the new Centre for Advanced Laser Applications (CALA) near Munich aims at pushing these ion bunches towards applications, including radiation therapy of tumors and the development of heavy ion bunches with solid-state-like density. These are needed for novel reaction mechanisms (‘fission-fusion’) to study the origin of heavy elements in the universe and to prepare for related studies at the upcoming EU-funded high-power laser facility ELI – Nuclear Physics in Bucharest.

Purpose:To measure and simulate well‐defined electron spectra using a linear accelerator and a permanent‐magnetic wide‐angle spectrometer to test the performance of a novel reconstruction algorithm for retrieval of unknown electron‐sources, in view of application to diagnostics of laser‐driven particle acceleration.Methods:Six electron energies (6, 9, 12, 15, 18 and 21 MeV, 40cm × 40cm field‐size) delivered by a Siemens Oncor linear accelerator were recorded using a permanent‐magnetic wide‐angle electron spectrometer (150mT) with a one dimensional slit (0.2mm × 5cm). Two dimensional maps representing beam‐energy and entrance‐position along the slit were measured using different scintillating screens, read by an online CMOS detector of high resolution (0.048mm × 0.048mm pixels) and large field of view (5cm × 10cm). Measured energy‐slit position maps were compared to forward FLUKA simulations of electron transport through the spectrometer, starting from IAEA phase‐spaces of the accelerator. The latter ones were validated against measured depth‐dose and lateral profiles in water. Agreement of forward simulation and measurement was quantified in terms of position and shape of the signal distribution on the detector.Results:Measured depth‐dose distributions and lateral profiles in the water phantom showed good agreement with forward simulations of IAEA phase‐spaces, thus supporting usage of this simulation source in the study. Measured energy‐slit position maps and those obtained by forward Monte‐Carlo simulations showed satisfactory agreement in shape and position.Conclusion:Well‐defined electron beams of known energy and shape will provide an ideal scenario to study the performance of a novel reconstruction algorithm using measured and simulated signal. Future work will increase the stability and convergence of the reconstruction‐algorithm for unknown electron sources, towards final application to the electrons which drive the interaction of TW‐class laser pulses with nanometer thin target foils to accelerate protons and ions to multi‐MeV kinetic energy.Cluster of Excellence of the German Research Foundation (DFG) “Munich‐Centre for Advanced Photonics”