Transverse emittance growth of proton sources from laser-irradiated sub-μm-thin planar targets

Proton bunches with maximum energies between 12 and 22 MeV were emitted from submicrometer-thin plastic foils upon irradiation by laser pulses with peak intensity of 4×10^{20}W/cm^{2}. The images of the protons by a magnetic quadrupole doublet on a screen remained consistently larger by a factor of 10 compared to expectations drawn from the ultralow transverse emittance values reported for thick foil targets. Analytic estimates and particle-in-cell simulations attribute this drastically increased emittance to formerly excluded Coulomb collisions between charged particles. The presence of carbon ions and significant transparency likely play a decisive role. This observation is highly relevant because such thin, partially transparent foils are considered ideal for optimizing maximum proton energies.

A feasibility study of zebrafish embryo irradiation with laser-accelerated protons

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.

Time-of-flight spectrometry of ultra-short, polyenergetic proton bunches

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.

A novel approach to electron data background treatment in an online wide-angle spectrometer for laser-accelerated ion and electron bunches

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.

Considerations on employing a PMQ-doublet for narrow and broad proton energy distributions

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.