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Laser-driven ion accelerators can deliver high-energy, high peak current beams from relativistic laser plasmas formed in solid-density materials [1, 2]. This innovative concept attracts a lot of attention for various multidisciplinary applications as a compact alternative to conventional accelerators [3]. However, achieving energy levels suitable for applications such as radiation therapy remains a challenge for laser-driven ion accelerators. Here, we report on experimental generation of plasma-accelerated proton beams with a spectrally separated high-energy component of up to 150MeV by irradiating solid-density plastic foil targets with ultrashort laser pulses from a repetitive Petawatt laser. Three-dimensional particle-in-cell simulations reveal that the observed beam parameters result from cascaded acceleration regimes that occur at the onset of relativistically induced transparency. The ultrashort pulse duration allows a rapid sequence of these regimes at highest intensity, enabling proton acceleration to unprecedented energy levels. Target transparency was identified to discriminate the high-performance domain of the acquired data set, making it a suitable feedback parameter for automated laser and target optimisation to enhance stability of plasma accelerators in the future. Ultimately, our results encourage further exploration and application of laser-driven plasmas as compact proton accelerators in the multi-100MeV range.

Due to the non-linear nature of relativistic laser induced plasma processes, the development of laser-plasma accelerators requires precise numerical modeling. Especially high intensity laser-solid interactions are sensitive to the temporal laser rising edge and the predictive capability of simulations suffers from incomplete information on the plasma state at the onset of the relativistic interaction. Experimental diagnostics utilizing ultra-fast optical backlighters can help to ease this challenge by providing temporally resolved inside into the plasma density evolution. We present the successful implementation of an off-harmonic optical probe laser setup to investigate the interaction of a high-intensity laser at $$5.4\times 10^{21}\,\hbox {W/cm}^{2}$$ 5.4 × 10 21 W/cm 2 peak intensity with a solid-density cylindrical cryogenic hydrogen jet target of $${5}\,{\upmu }\mathrm{m}$$ 5 μ m diameter as a target test bed. The temporal synchronization of pump and probe laser, spectral filtering and spectrally resolved data of the parasitic plasma self-emission are discussed. The probing technique mitigates detector saturation by self-emission and allowed to record a temporal scan of shadowgraphy data revealing details of the target ionization and expansion dynamics that were so far not accessible for the given laser intensity. Plasma expansion speeds of up to $$(2.3 \pm 0.4)\times 10^{7}\,\hbox {m/s}$$ ( 2.3 ± 0.4 ) × 10 7 m/s followed by full target transparency at $${1.4}\,{\mathrm{ps}}$$ 1.4 ps after the high intensity laser peak are observed. A three dimensional particle-in-cell simulation initiated with the diagnosed target pre-expansion at $${-0.2}\,{\mathrm{ps}}$$ - 0.2 ps and post processed by ray tracing simulations supports the experimental observations and demonstrates the capability of time resolved optical diagnostics to provide quantitative input and feedback to the numerical treatment within the time frame of the relativistic laser-plasma interaction.

After the rediscovery of the normal tissue sparing effect of high dose rate radiation, i.e. the so-called FLASH effect, by Favaudon et al. in 2014, research activities on this topic have been revived and are flourishing ever since. Yet, the exact biological mechanism as well as the required boundary conditions and radiation qualities to reach said sparing remain mostly unclear. We present a laser-based irradiation platform at the Draco PW facility that enables systematic studies into the FLASH regime using proton peak dose rates of up to 10^9 Gy/s. Besides the PW class laser acceleration source, a key component is a pulsed high-field beamline to transport and shape the laser driven proton bunches spectrally and spatially in order to generate homogeneous dose distributions tailored to match the irradiation sample. Making use of the diverse capabilities of the laser driven irradiation platform a pilot experiment of highest complexity has been conducted – a systematic in-vivo tumor irradiation in a specifically developed mouse model. A plethora of online particle diagnostics, including Time-of-Flight, bulk scintillators and screens as well as ionization chambers, in conjunction with diagnostics for retrospective absolute dosimetry (radiochromic films) allowed for an unprecedented level of precision in mean dose delivery (±10 %) and dose homogeneity (±5 %) for the challenging beam qualities of a laser accelerator. The tailored detector suite is complemented by predictive simulations. The talk addresses how our interdisciplinary team overcame all hurdles from animal model development, over enhancing the laser and laser acceleration stability, to dose delivery and online dose monitoring. Results on radiation induced tumor growth delay by laser driven as well as conventionally accelerated proton beams are critically discussed.

Laser-driven proton pulse provide unique properties in terms of pulse structure (ns) and instantaneous dose rate (10^9 Gy/s) but - inherently broadband and highly divergent - pose a challenge to established beamline concepts on the path to application-adapted irradiation field formation, particularly for three-dimensional cases. We present the successful implementation and characterisation of a highly efficient and tuneable dual pulsed solenoid beamline at the Draco PW facility[1] to generate volumetric dose distribution tailored to specific applications[2]. The vast experimental scope and already successfully performed studies range from systematic volumetric in-vivo tumour irradiations in a dedicated mouse model (with a stable mean dose delivery of ±10 % and a spatial dose homogeneity of ±5 % over a cylindrical volume of 5 mm diameter and height) to high-dose-rate irradiations in the FLASH regime (using proton peak dose rates of up to 10^9 Gy/s with about 20 Gy/shot homogeneously over a cylindrical sample volume of 4.5 mm diameter and 3 mm height) as well as particle diagnostics commissioning (with a multitude of spatial and spectral dose distributions). The beamline setup is complemented by a complex beam monitoring and dosimetry detector suite adapted to the ultra-high dose rate pulses and is in its unique synergy and redundancy capable of %-level precision dose delivery to samples as required for systematic irradiation studies. In addition to established radiochromic film dosimetry, the detector suite includes saturation-corrected (transmission) ionisation chambers [3] as well as screen and bulk scintillator setups, partly with tomographic reconstruction capabilities for 3D dose distribution retrieval. Moreover, non-invasive, single-shot-capable online time-of-flight-based spectral characterisation of filtered proton pulses has proven a powerful tool for beam monitoring as well as dosimetric purposes. In this presentation the complex and versatile dose delivery system of laser-driven protons at the Draco PW using pulsed solenoids will be discussed. Its characterisation, technological development and improvement as well as the dosimetry suite as a vital part of the precise dose delivery will be addressed, while the presentation by U. Schramm covers recent experimental activities in detail. [1] T. Ziegler, et al., Proton beam quality enhancement by spectral phase control of a PW-class laser system, https://arxiv.org/abs/2007.11499 (2020) [2] Brack, et al., Spectral and spatial shaping of laser-driven proton beams using a pulsed high-field magnet beamline, SciRep, 10:9118, (2020) [3] Gotz M, et al., A new model for volume recombination in plane‐parallel chambers in pulsed fields of high dose‐per‐pulse. Phys Med Biol., 62: 8634, (2017)

The interaction between intense 30 fs laser pulses and foam-coated 1.5 μm-thick Al foils in the relativistic regime (up to 5 × 1020 W cm−2) is studied to optimize the laser energy conversion into laser-accelerated protons. A significant enhancement is observed for foam targets in terms of proton cut-off energy (18.5 MeV) and number of protons above 4.7 MeV (4 × 109 protons/shot) with respect to uncoated foils (9.5 MeV, 1 × 109 protons/shot), together with a sixfold increase in the bremsstrahlung yield. This enhancement is attributed to increased laser absorption and electron generation in the foam meso- and nanostructure.

High power laser science and engineering 8, e22 (2020). doi:10.1017/hpl.2020.13, Published by Cambridge Univ. Press, Cambridge

Extreme field gradients intrinsic to relativistic laser plasma interactions enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. So far, only complex micro-engineering of the relativistic plasma accelerator itself and limited adoption of conventional beam optics provided access to global beam parameters that define propagation. We present a novel, counter-intuitive all-optical approach to imprint detailed spatial information from the driving laser pulse to the proton bunch. The concept was motivated by an effect initially observed in an experiment dedicated to laser-driven proton acceleration from a renewable micrometer sized cryogenic Hydrogen jet target at the 150 TW Draco laser at HZDR. A compact, recollimating single plasma mirror was used to enhance the temporal laser contrast, which could be monitored on a single-shot base by means of self-referenced spectral interferometry with extended time excursion (SRSI-ETE) at unprecedented dynamic and temporal range. Unexpectedly, the accelerated proton beam profile showed in this experiment prominent features of the collimated laser beam, such as the shadow of obstacles inserted deliberately in the beam. In a series of further experiments, the spatial profile of the energetic proton bunch was found to exhibit identical features as the fraction of the laser pulse passing around a target of limited size. The formation of quasi-static electric fields in the beam path by ionization of residual gas in the experimental chamber results in asynchronous information transfer between the laser pulse and the naturally delayed proton bunch. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a new level of proton beam manipulation.

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.

We show efficient laser driven proton acceleration up to 14MeV from a 50 $\mu$m thick cryogenic hydrogen ribbon. Pulses of the short pulse laser ELFIE at LULI with a pulse length of $\approx$ 350 fs at an energy of 8 J per pulse are directed onto the target. The results are compared to proton spectra from metal and plastic foils with different thicknesses and show a similar good performance both in maximum energy as well as in proton number. Thus, this target type is a promising candidate for experiments with high repetition rate laser systems., Comment: 9 pages

Author(s): Brack, Florian-Emanuel; Kroll, Florian; Gaus, Lennart; Bernert, Constantin; Beyreuther, Elke; Cowan, Thomas E; Karsch, Leonhard; Kraft, Stephan; Kunz-Schughart, Leoni A; Lessmann, Elisabeth; Metzkes-Ng, Josefine; Obst-Huebl, Lieselotte; Pawelke, Jorg; Rehwald, Martin; Schlenvoigt, Hans-Peter; Schramm, Ulrich; Sobiella, Manfred; Szabo, Emilia Rita; Ziegler, Tim; Zeil, Karl | Abstract: An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Probing the rapid dynamics of plasma evolution in laser-driven plasma interactions provides deeper understanding of experiments in the context of laser-driven ion acceleration and facilitates the interplay with complementing numerical investigations. Besides the microscopic scales involved, strong plasma (self-)emission, predominantly around the harmonics of the driver laser, often complicates the data analysis. We present the concept and the implementation of a stand-alone probe laser system that is temporally synchronized to the driver laser, providing probing wavelengths beyond the harmonics of the driver laser. The capability of this system is shown during a full-scale laser proton acceleration experiment using renewable cryogenic hydrogen jet targets. For further improvements, we studied the influence of probe color, observation angle of the probe and temporal contrast of the driver laser on the probe image quality.

For laser-driven ion acceleration from thin foils (~10 µm- 100 nm) in the target normal sheath acceleration (TNSA) regime, the hydro-carbon contaminant layer at the target surface generally serves as the ion source and hence determines the accelerated ion species, i.e. mainly protons, carbon and oxygen ions. The specific characteristics of the source layer - thickness and relevant lateral extent - as well as its manipulation have both been investigated since the first experiments on laser-driven ion acceleration using a variety of techniques from direct source imaging to knife-edge or mesh imaging. In this publication, we present an experimental study in which laser ablation in two fluence regimes (low: F~0.6 J/cm², high: F~4 J/cm²) was applied to characterize and manipulate the hydro-carbon source layer. The high-fluence ablation in combination with a timed laser pulse for particle acceleration allowed for an estimation of the relevant source layer thickness for proton acceleration. Moreover, from these data and independently from the low-fluence regime, the lateral extent of the ion source layer became accessible.

The complex physics of the interaction between short-pulse ultrahigh-intensity lasers and solids is so far difficult to access experimentally, and the development of compact laser-based next-generation secondary radiation sources, e.g., for tumor therapy, laboratory astrophysics, and fusion, is hindered by the lack of diagnostic capabilities to probe the complex electron dynamics and competing instabilities. At present, the fundamental plasma dynamics that occur at the nanometer and femtosecond scales during the laser-solid interaction can only be elucidated by simulations. Here we show experimentally that small-angle x-ray scattering of femtosecond x-ray free-electron laser pulses facilitates new capabilities for direct in situ characterization of intense short-pulse laser-plasma interactions at solid density that allows simultaneous nanometer spatial and femtosecond temporal resolution, directly verifying numerical simulations of the electron density dynamics during the short-pulse high-intensity laser irradiation of a solid density target. For laser-driven grating targets, we measure the solid density plasma expansion and observe the generation of a transient grating structure in front of the preinscribed grating, due to plasma expansion. The density maxima are interleaved, forming a double frequency grating in x-ray free-electron laser projection for a short time, which is a hitherto unknown effect. We expect that our results will pave the way for novel time-resolved studies, guiding the development of future laser-driven particle and photon sources from solid targets.

Recent oncological studies identified beneficial properties of radiation applied at ultrahigh dose rates, several orders of magnitude higher than the clinical standard of the order of Gy min–1. Sources capable of providing these ultrahigh dose rates are under investigation. Here we show that a stable, compact laser-driven proton source with energies greater than 60 MeV enables radiobiological in vivo studies. We performed a pilot irradiation study on human tumours in a mouse model, showing the concerted preparation of mice and laser accelerator, dose-controlled, tumour-conform irradiation using a laser-driven as well as a clinical reference proton source, and the radiobiological evaluation of irradiated and unirradiated mice for radiation-induced tumour growth delay. The prescribed homogeneous dose of 4 Gy was precisely delivered at the laser-driven source. The results demonstrate a complete laser-driven proton research platform for diverse user-specific small animal models, able to deliver tunable single-shot doses up to around 20 Gy to millimetre-scale volumes on nanosecond timescales, equivalent to around 109 Gy s–1, spatially homogenized and tailored to the sample. The platform provides a unique infrastructure for translational research with protons at ultrahigh dose rates.

Nature Communications 9(1), 5292 (2018). doi:10.1038/s41467-018-07756-z, Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. Here we present a readily applicable all-optical approach to imprint detailed spatial information from the driving laser pulse onto the proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation., Published by Nature Publishing Group UK, [London]