This PDF file contains the front matter associated with SPIE Proceedings Volume 11779, including the Title Page, Copyright information, and Table of Contents.

In a multi-GeV laser-driven plasma accelerator the driving laser pulse must remain focused as it propagates through tens of centimetres of plasma of density 1017 cm-3. This distance is orders of magnitude greater than the Rayleigh range, and hence the laser pulse must be guided with low losses. Since many applications of laser-plasma accelerators will require that the pulse repetition rate is in the kilohertz range, methods for guiding relativistically-intense laser pulses at high repetition rates must be developed. We describe the development of hydrodynamic optical-field-ionized (HOFI) plasma channels and conditioned HOFI channels, which can meet all of these challenging requirements. We present experiments and numerical simulations that show that hydrodynamic expansion of optical-field-ionized plasma columns can generate channels at low plasma densities. We show that guiding a conditioning pulse in a HOFI channel leads to the formation of long, very low loss plasma channels via ionization of the collar of neutral gas which surrounds the HOFI channel. We describe proof-of-principle experiments in which we generated conditioned HOFI (CHOFI) waveguides with axial electron densities of ne0 ≈ 1×1017 cm−3 and a matched spot size of approximately 30 μm. We present hydrodynamic and particle-in-cell simulations which demonstrate that meter-scale, low-loss CHOFI waveguides could be generated with a total laser pulse energy of about 1 J per meter of channel.

Efficient forward electron acceleration by the direct laser acceleration (DLA) in the plasma channel was experimentally demonstrated using a 16 µm thick tape target. An electron beam with ∼0.05 rad divergence, 50-100 pC charge (for E<1.7 MeV), and temperature ∼ several MeV was observed on 1 TW laser system utilizing an additional controlled nanosecond prepulse. Using this beam, several near-threshold photonuclear reactions were studied and neutron flux of ∼ 10⁵ − 10⁶ s^-1 J^-1 was achieved. We also used neutron flux measurements to estimate electron beam charge, calculating conversion coefficients from GEANT4 simulations. Terahertz radiation emission from this type of interaction was also studied, exhibiting a two-maxima structure with the change of delay between main pulse and nanosecond prepulse.

A grid of equidistant electron stripes is generated during the collision of two laser pulses under a small angle in underdense plasma. Due to the oblique incidence, transverse standing wave in plasma is observed, in addition to the longitudinal traveling wave of the compound laser field. This standing wave results in the generation of plasma density grating. The ratio of the peak stripe density to background density can reach the value of 20:1. The grating period is determined by the interaction angle. Analytical theory of the compound electric fields is provided for plane waves. The grating formation is then verified via particle-in-cell simulations for short Gaussian laser pulses with typical experimental parameters. In addition, the interference pattern was also observed during experiments with Diocles laser. The results presented here are relevant for many laser-plasma applications, such as Raman scattering, inertial confinement fusion, plasma photonic crystals and laser wakefield acceleration.

Using analytical methods and computer simulations, we investigate physical processes which lead to the formation of ring-shaped electromagnetic and electron structures in laser-plasma interaction. We observe that as the intense laser pulse excites a nonlinear Langmuir wave in an underdense plasmas, a significant portion of the pulse is refracted outwards the propagation direction due to the interactions with thin, high-density electron walls surrounding the wave cavities. Because of the radial symmetry, the refracted light forms a distinct electromagnetic ring that encircles the driver pulse. The efficiency of the energy transfer to the electromagnetic ring is relatively high, so that the ring can generate its own Langmuir wave and trigger the electron self-injection, which results in a ring-shaped beam of high-energy electrons. The properties of the ring-shaped electromagnetic and electron beams depend on the parameters of the Langmuir wave cavity walls, thus they can be controlled by tuning the parameters of the laser and plasma. The ring structures could be applied as a drivers for acceleration of positively charged particles, or as a diagnostic to determine regimes and the overall efficiency of the laser-wakefield accelerator.

I will present recent results from 2 sets of laser plasma acceleration experiments spanning 4 orders of magnitude in plasma density. In near critical density hydrogen plasmas using 5 fs, < 3mJ laser pulses , we have demonstrated acceleration of few pC monoenergetic electron bunches up to 15 MeV at 1 kHz, at a record low beam divergence <10 mrad [1]. Mitigation of carrier envelope phase slip is key to this result. At the other extreme of plasma density, we have demonstrated 2 techniques [2,3] for generation of metre-scale low density plasma waveguides up to several hundred Rayleigh ranges in length, with recent preliminary results showing guiding of up to several hundred terawatts. [1] Laser-accelerated, low divergence 15 MeV quasi-monoenergetic electron bunches at 1 kHz, F. Salehi, M. Le, L. Railing, and H. M. Milchberg, submitted for publication [2] Optical Guiding in Meter-Scale Plasma Waveguides, B. Miao, L. Feder, J. E. Shrock, A. Goffin, and H. M. Milchberg, PHYSICAL REVIEW LETTERS 125, 074801 (2020) [3] Self-waveguiding of relativistic laser pulses in neutral gas channels, L. Feder, B. Miao, J. E. Shrock, A. Goffin, and H. M. Milchberg, PHYSICAL REVIEW RESEARCH 2, 043173 (2020)

The laser wakefield acceleration program at ELI-Beamlines benefits from the future availability of four unique high power laser systems that make possible the investigation of LWFA in a broad range of parameters, ranging from mJ to kJ in pulse energy. The experiments driven by the PW-class laser systems L3-HAPLS (Ti:Sapph, 30 J, 30 fs, 10 Hz) and L4-Aton (Nd:glass, 1.5 kJ, 150 fs, 0.01 Hz) are performed at the ELI-ELBA beamline, and aim at the counter-propagation of laser-accelerated GeV electron beams with high intensity laser pulses. These experiments are designed to study novel regimes of electromagnetic field interaction with matter and quantum vacuum. The flagship experiment of ELI-ELBA is the experimental measurement of synergic Cherenkov-Compton radiation, which will reveal the properties of the vacuum predicted by nonlinear quantum electrodynamics and will require the operation of L3-HAPLS and L4-Aton at full power. The LWFA experiments driven by the TW-class high rep-rate laser systems L1-Allegra (100 mJ, 15 fs, 1 kHz) and L2-Duha (>3J, 25 fs, 50 Hz) are oriented towards laser-driven FEL development and applications in the biomedical field, and to investigation of interaction of high power lasers with near critical density plasmas. In the presentation, the actual status of the ELI-ELBA GeV electron beamline will be presented, along with the schedule leading to the commissioning and full operations. The activity in the field of high repetition rate LWFA will be also presented, including recent theoretical and simulation results, and the description of the experiments planned. Finally, recent design work towards a laser-driven VHEE radiotherapy device will be presented.

We present experimental results of vacuum laser acceleration (VLA) of electrons using radially polarized laser pulses interacting with a plasma mirror. Tightly focused, radially polarized laser pulses have been proposed for electron acceleration because of their strong longitudinal electric field, making them ideal for VLA. However, experimental results have been limited until now because injecting electrons into the laser field has remained a considerable challenge. Here, we demonstrate experimentally that using a plasma mirror as an injector solves this problem and permits us to inject electrons at the ideal phase of the laser, resulting in the acceleration of electrons along the laser propagation direction while reducing the electron beam divergence compared to the linear polarization case. We obtain electron bunches with few-MeV energies and a 200-pC charge, thus demonstrating, for the first time, electron acceleration to relativistic energies using a radially polarized laser. High-harmonic generation from the plasma surface is also measured, and it provides additional insight into the injection of electrons into the laser field upon its reflection on the plasma mirror. Detailed comparisons between experimental results and full 3D simulations unravel the complex physics of electron injection and acceleration in this new regime: We find that electrons are injected into the radially polarized pulse in the form of two spatially separated bunches emitted from the p-polarized regions of the focus. Finally, we leverage on the insight brought by this study to propose and validate a more optimal experimental configuration that can lead to extremely peaked electron angular distributions and higher energy beams.

With the development of high-intensity and high-repetition-rate laser systems, it has become crucial to be able to measure and characterize the high-energy gamma radiation from laser-matter interaction in real-time. Therefore, a scintillator-based electromagnetic calorimeter aimed at high-energy electron and photon detection under high-repetition rate is being developed at the ELI Beamlines facility. Together with an ad hoc created unfolding technique, it is possible to reconstruct energies/temperatures of one or two thermal populations present in the radiation. A preliminary test of the device performed at the PALS experimental facility together with the corresponding signal unfolding is here presented.

Laser plasma accelerators produce ultra-short, low emittance electron bunches that show potential for use in multistage colliders or for seeding free electron lasers. However, to optimize these novel accelerators for such applications, new diagnostics for micron-scale beams must be developed. In this paper we present single shot coherent optical transition radiation diagnostics that measure spatial and momentum distributions of microbunched high energy electron populations at the exit of a laser plasma accelerator. We show correspondence between the measured position and momentum of the electron beamlets as well as transverse distribution reconstructions of the coherent portion of the beam on a single shot at a variety of wavelengths. Finally, we propose a scheme for a full three-dimensional reconstruction of an electron bunch through coherent transition radiation analysis.

A full characterization of fast electrons is important for understanding laser-plasma physics. Conventionally, the total number, angular distribution and energy spectra of escaping electrons are diagnosed experimentally with image plates and electron spectrometers, which however are destructive for electrons themselves. In contrast, the terahertz (THz) radiation, generated by electrons crossing the target surface, enables a non-invasive in-situ diagnostic for electrons. A hybrid model involving the high-current electron emission and transient sheath dynamics at the target rear is proposed to account for the THz generation from laser-foil interactions. With this, the measured THz radiation have been employed in turn to quantitatively infer some properties (like the total charge and dynamics) of the escaping electrons and transient ion acceleration sheath, in good agreement with experimental measurements and theoretical calculations.

With the recent development of ultrashort laser pulse generation, many laser facilities around the world can routinely accelerate stable, high-energy electron bunches with a duration in the order of several femtoseconds in a very short distance. Due to the short bunch duration, they are suitable for various femtosecond and sub-picosecond applications. However, one of the less favorable properties of this acceleration is a relatively large electron bunch energy spread which causes the increase in the bunch duration when propagating a long distance in space. Hence, for utilizing them in such applications, they need to be compressed back down to the femtosecond duration. In this work we present a progress in design of the electron beam transport line preserving the femtosecond bunch duration. The transport line including the final focusing system consists of commonly used, standard types of electron optical devices – dipoles, quadrupoles and sextupoles. The design exploits setups in conventional radiofrequency accelerators for beams with low energy spread, including chromaticity correction. Our design focuses on the transport of electron bunches with relatively large energy spread, while maximizing the transport line acceptance for given beam parameters, as energy, relative energy spread and emittance.

We report on experimental investigations of proton acceleration from laser-irradiated solid foils with the DRACOPW laser, where highest proton cut-off energies were achieved for temporal pulse parameters that varied significantlyfrom those of an ideally Fourier transform limited (FTL) pulse. Controlled spectral phase modulation of the driverlaser by means of an acousto-optic programmable dispersive filter enabled us to manipulate the temporal shape ofthe last picoseconds around the main pulse and to study the effect on proton acceleration from thin foil targets. Theresults show that short and asymmetric pulses generated by positive third order dispersion values are favourable forproton acceleration and can lead to maximum energies of 60 MeV at 18 J laser energy for thin plastic foils, effectivelydoubling the maximum energy compared to ideally compressed FTL pulses. The talk will further prove the robustnessand applicability of this enhancement effect for the use of different target materials and thicknesses as well as laserenergy and temporal intensity contrast settings. Assuming appropriate control over the spectral phase of the laser andcomparable temporal contrast conditions, we believe that the presented method can be universally applied to improveproton acceleration performance using any other laser system, particularly important when operating in the PW regime.

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.

The spatio-temporal and polarisation properties of intense light is important in wide-ranging topics at the forefront of intense light-matter interactions, including laser-driven particle acceleration. In the context of experiments to optimize transparency-enhanced ion acceleration in expanding ultrathin foils, we investigate the polarisation and temporal properties of intense light measured at the rear of the target. An effective change in the angle of linear polarisation of the light results from a superposition of coherent radiation, generated by a directly accelerated bipolar electron distribution, and the light transmitted due to the onset of relativistic self-induced transparency. Simulations show that the generated light has a high-order transverse electromagnetic mode structure in both the first and second laser harmonics that can evolve on intra-pulse time-scales. The mode structure and polarisation state vary with the interaction parameters, opening up the possibility of developing this approach to achieve dynamic control of structured light fields at ultrahigh intensities [1]. We also report on frequency-resolved optical gating measurements of the light which demonstrate a novel and simple approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light. This is a key parameter for optimising ion acceleration in expanding ultrathin foils. Coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil producing spectral fringes. The fringe spacing enables the relative timing of the onset of transmission with respect to the transition radiation generation to be determined. This self-referencing approach to spectral interferometry provides a route to optically controlling and optimising ion acceleration from ultrathin foils undergoing transparency [2]. [1] M.J. Duff et al., Scientific Reports 10, 105 (2020) [2] S.D.R. Williamson et al., Phys. Rev. Applied 14, 034018 (2020)

The maximum energy to which ions are accelerated in the interaction of a high power laser pulse with a thin foil target scales with the laser intensity, with a power-law that varies with the acceleration mechanism and laser pulse parameters. For fixed laser energy and pulse duration, maximizing the intensity by focusing to a smaller focal spot does not, however, necessarily result in higher-energy ions. For the case of relatively thick foil targets, it has been shown that self-generated magnetic fields and unfavourable changes to the temperature and divergence of the fast electron population injected into the target can result in lower-energy sheath-accelerated ions compared to that expected from intensity scaling laws. We report results from an investigation of the influence of laser focusing on ion acceleration in the ultrathin target regime, for which high energy protons have been achieved by our group [1]. We compare the interaction physics resulting from the use of f/3 and f/1 focusing geometries. Although f/1 focusing (achieved using a focusing plasma optic) produces a smaller nominal laser focal spot size and thus higher nominal peak intensity, more efficient ion acceleration to higher energies is achieved with the f/3 geometry for the case of expanding ultrathin foils undergoing relativistic self-induced transparency. Particle-in-cell simulations reveal that self-focusing in the expanding plasma produces a near-diffraction-limited focal spot, resulting in up to an order of magnitude higher focused intensity in the f/3 case. We also report on the extent to which this intensity enhancement is expected in the case of the short-pulse, ultrahigh-intensity regime that will soon be accessible using multi-petawatt lasers. The study is published in reference [2]. [1] A. Higginson et al., Nature Communications 9, 724 (2018) [2] T. P. Frazer et al., Phys. Rev. Research 2, 042015(R) (2020)

The availability of the ultra-high intensity (>10^21 W/cm2), PW (30J/30fs) L3-HAPLS laser at ELI-Beamlines allows entering advanced laser-driven ion acceleration regimes, at a repetition rate up to 10 Hz. A sub-aperture of the L3-HAPLS laser beam (1.5J/30fs) was recently used to accelerate protons with energies approaching the 10-MeV level using the relatively thick (10-40 µm) plastic and metallic foils. Ion diagnostics were optimized for a real-time feedback during the experiment through various detectors, such as ion collectors, single-crystal diamond and silicon carbide detectors, Thomson parabola spectrometer and gamma-ray scintillators, along with a set of complementary passive detectors such as radiochromic films (RCF) and solid-state nuclear track films (CR-39). Analysis of large data acquired during the experimental campaign, summary of the key results and optimal conditions for laser-driven ion acceleration will be presented and discussed.

Accessing novel ion acceleration mechanisms, such as Radiation Pressure Acceleration (RPA), is a promising route to generate high energy beams of both light and heavy ions [1]. In particular, the Light Sail (LS) regime predicts high efficiency, mono-energetic beams and can be accessed with currently available high power laser facilities with the use of ultra-thin foils and circular polarisation [2-4]. In recent experiments at the GEMINI laser facility (RAL, UK), target bulk (carbon) ions were favourably accelerated in the LS-RPA regime up to 33MeV/nucleon at an optimal carbon foil thickness of 15nm, whereas protons only reached energies of 18 MeV. This result, which differs from what is typically observed in laser-solid interactions, where protons are always accelerated more efficiently than heavier ions, is interpreted with the support of multi-dimensional Particle in Cell (PIC) simulations. While the 40fs pulse was temporally cleaned by a double plasma mirror arrangement to increase the laser contrast to 10-14 at the ns timescale, it is shown that the limited preceding laser fluence incident on the target on the ps scale causes target expansion, with protons, being lighter, escaping from the interaction region. This leaves a pre-dominantly carbon plasma which, for circular polarization, is accelerated by RPA, with proton energies determined instead by plasma expansion and sheath effects. It is shown through simulations that controlling the laser temporal profile and plasma mirror activation opens up a promising route for controlling which ion species is preferentially accelerated in the RPA regime. This has particular importance as <1PW systems are coming online currently where these accelerations will begin to inherently dominate, and the preceding laser intensity will need to be suitably controlled.

The Centre for Advanced Laser Applications (CALA) in Garching near Munich features the ATLAS 3000 laser system, which can deliver up to 3 PW within a pulse length of 20 fs. It is the driver for the Laser-driven ION (LION) beamline, which aims to accelerate protons and carbons for applications. The laser beam delivery comprises also a full aperture deformable mirror (DM) after the compressor. A 20 degrees off-axis parabolic mirror with a focal length of 1.5 m focusses the 28 cm diameter laser-beam down to a micrometere-sized spot, where a vacuum-compatible wave-front sensor is used for the DM feed-back loop focus optimization. The nano-Foil Target Positioning System (nFTPS) can replace targets with a repetition rate of up to 0.5 Hz and store up to 19 different target foils. A dipole magnet in a wide-angle spectrometer configuration deflects ions onto a CMOS detector for an online read-out. Commissioning started mid 2019 with regular proton acceleration using nm-thin plastic foils as targets. Since then proton cut-off energies above 20 MeV have been regularly achieved. The amount of light traveling backwards from the experiment into the laser is constantly monitored and 5 J on target have been determined as the current limit to prevent damage in the laser. Protons with a kinetic energy of 12 MeV are stably accelerated with the given laser parameters and are suitable for transport with permanent magnet quadrupoles towards our application platform. We have performed parameter scans varying target thicknesses and laser-pulse shape to optimize for highest and most stable proton numbers at 12 MeV kinetic energy, and investigated shot-to-shot particle number stability for the best parameters.

Laser-driven acceleration of ions from near-critical density plasma layer, initially inhomogeneous in density in one spatial dimension (along laser propagation direction), was investigated in multidimensional particle-in-cell simulations. Tracking of high-energy accelerated ions in these simulations reveals the evolution of accelerating fields affecting the particles. While the acceleration of ions occurs in a short time interval when a steep (infinite) density gradient was introduced at the back side of the plasma layer, three phases of ion acceleration can be clearly observed when a smooth (Gaussian) density profile was assumed. These phases are attributed to the accelerating field generated by electron bunches carried by the laser wave, by expanding transverse magnetic field, and by the apex of electron filament behind the laser wave, respectively. The accelerating field affecting the most energetic ions has unexpected local maxima about 50 fs after the moment when ultrashort (30 fs) laser pulse completely left the plasma with Gaussian density profile due to this electron filament apex created behind the transmitted laser pulse. Full 3D simulation confirms the observations in 2D simulations in terms of ion acceleration mechanisms. However, it shows a substantial reduction of maximum achievable ion energies and a larger angular spread of accelerated ions compared with 2D approach, which demonstrates the necessity of using computationally demanding full 3D geometry for similar numerical studies.

Radiographic imaging is an omnipresent tool in basic research and applications in industry, material science and medical diagnostics. Often, the information contained in more than one modality can be valuable, but difficult to access simultaneously. This talk reviews developments in laser-plasma-accelerators for protons, electrons and x-rays from solid and gas targets for multimodal imaging. Laser-driven ion acceleration and x-ray generation have been investigated using tungsten micro-needle-targets at the Texas Petawatt laser [1]. The experiments and supporting numerical simulations reveal peaked proton spectra around 10 MeV with significant particle count and a strong keV level x-ray source. The source size for both has been measured to be in the few-µm range. Both sources were eventually applied to simultaneous radiographic imaging of biological and technological samples. In recent experiments at BELLA Center’s high repetition rate 100 TW dual-arm laser, steps were taken towards bi-modal x-ray and electron imaging of dynamic events such as hydrodynamic shocks, in which often both density and electro-magnetic fields are important quantities to measure. Here, a shock was driven by a 1 Joule, 200 ps laser focused in a 30 µm wide water jet. A laser wakefield accelerator was driven by a second 2 Joule, 40 fs laser in a gas-jet target, providing both 150 MeV electrons and broadband betatron x-rays up to ˜10 keV for projection imaging. This research aims to leverage unique properties readily available in laser plasma accelerators for applications. Specifically, the emission of pulsed, bright, multimodal bursts of radiation can open new ways in biological imaging (e.g., with ns-synchronized ions and x-rays) and in high-resolution diagnostics for high-energy density science (e.g., with fs-synchronized electrons and x-rays). [1] T. M. Ostermayr et al., “Laser-driven x-ray and proton micro-source and application to simultaneous single-shot bi-modal radiographic imaging,” Nat. Commun., vol. 11, no. 1, pp. 1–9, Dec. 2020. This work was supported by the DFG via the Cluster of Excellence Munich-Centre for Advanced Photonics (MAP) and Transregio SFB TR18. This work has been carried out within the framework of the EUROfusion Consortium and has received funding, through the ToIFE, from the European Union’s Horizon 2020 research and innovation program under grant agreement number 633053. The authors acknowledge funding by the Air Force Office of Scientific Research (AFOSR)(FA9550-14-1-0045, FA9550-17-1-0264). Work supported by DOE FES under grant DE-SC0020237. Work supported by US DOE NNSA DNN R&D, by Sc. HEP, by the Exascale Computing Project and by FES LaserNetUS under DOE Contract DE-AC02-05CH11231.

In this work we investigated the use of a plasma shutter in the form of a thin foil for laser-driven ion acceleration enhancement. It is shown with the help of 3D particle-in-cell simulations that the laser pulse intensity can be increased and its profile steepened after burning through the plasma shutter. The enhanced intensity profile has a positive effect on the subsequent ion acceleration from the main foil, significantly increasing the maximal ion energy. The pre-expansion of the plasma shutter caused by prepulses is investigated using 2D hydrodynamic simulations. A scheme using a double plasma shutter configuration (the first one filtering out the prepulses and the second one shaping the main pulse) is proposed.

Many applications of laser-accelerated ions require a delivery of the particles to locations remote from the plasma source. Due to the large bunch divergence, achieving experimentally relevant particle fluences at more than a few tens of cm distance requires a dedicated bunch transport system. The most compact solution is a doublet of permanent magnet quadrupoles. Since these quadrupoles have a fixed magnetic field gradient, their focusing properties depend only on their geometric positions and relative rotations which therefore require careful alignment. We performed comprehensive ion optical simulations to characterize gradients and fields of the individual magnets and to identify the sensitivity of the focus shape to various positioning parameters, especially relative distances and rotation. The simulations also allowed devising radiation and laser safety measures for the quadrupoles. Based on these results and thanks to a very stable and reproducible proton source, we could optimize the obtained focus experimentally to a symmetric star like shape. This optimization yielded a proton spot in which the central area of highest fluence had a minimum diameter of approximately 2 mm FWHM, as revealed with spatially resolved scintillator and radiochromic film measurements. Furthermore, we developed a low-material-budget ionization chamber to monitor bunch charge and performed first tests with the aim to quantify the proton focus dosimetrically. The implemented controls and monitoring tools allow now for planning of first application experiments with (sub-) mm proton bunches of (sub-) ns duration.

The properties of laser-accelerated ion bunches are demanding and require development of suitable beam diagnostics. In particular, the short and intense particle bunches with a broad energy spectrum emitted in conjunction with a strong electromagnetic pulse (EMP) are challenging for conventional and well established monitoring systems. An approach based on measuring the acoustic signals of particles depositing their energy in water, referred to as ionoacoustics was recently developed into Ion-Bunch Energy Acoustic Tracing (I-BEAT). I-BEAT allows online detection of single proton bunches while being cost effective and EMP resistant. A simple water phantom equipped with only one ultrasound transducer positioned on the ion axis allows for reconstructing a rather complex energy spectrum that is typical for (manipulated) laser-accelerated ion bunches. To deduce the lateral bunch properties, additional transducers can be added, for example perpendicular to the ion beam axis. This established setup has been adapted for use closely behind the laser target and tested at the PHELIX laser at GSI. The capability of the system to retrieve information about the broad proton spectrum close to the source despite the harsh conditions has been demonstrated. Future improvements are required, most importantly the increase of dynamic range. Nevertheless, I-BEAT holds promise to evolve into an online diagnostic tool particularly suited for laser-driven source development and optimization at high repetition rates.