Dosimetry for radiobiologicalin vivoexperiments at laser plasma-based proton accelerators.

Objective.Laser plasma-based accelerators (LPAs) of protons can contribute to research of ultra-high dose rate radiobiology as they provide pulse dose rates unprecedented at medical proton sources. Yet, LPAs pose challenges regarding precise and accurate dosimetry due to the high pulse dose rates, but also due to the sources' lower spectral stability and pulsed operation mode. Forin vivomodels, further challenges arise from the necessary small field dosimetry for volumetric dose distributions. For these novel source parameters and intended applications, a dosimetric standard needs to be established.Approach.In this work, we present a dosimetry and beam monitoring framework forin vivoirradiations of small target volumes with LPA protons, solving aforementioned challenges. The volumetric dose distribution in a sample (mean dose value and lateral/depth dose inhomogeneity) is provided by combining two independent dose measurements using radiochromic films (dose rate-independent) and ionization chambers (dose rate-dependent), respectively. The unique feature of the dosimetric setup is beam monitoring with a transmission time-of-flight spectrometer to quantify spectral fluctuations of the irradiating proton pulses. The resulting changes in the depth dose profile during irradiation of anin vivosample are hence accessible and enable pulse-resolved depth dose correction for each dose measurement.Main results.A first successful small animal pilot study using an LPA proton source serves as a testcase for the presented dosimetry approach and proves its performance in a realistic setting.Significance.With several facilities worldwide either setting up or already using LPA infrastructure for radiobiological studies with protons, the importance of LPA-adapted dosimetric frameworks as presented in this work is clearly underlined.

Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density.

Laser plasma-based particle accelerators attract great interest in fields where conventional accelerators reach limits based on size, cost or beam parameters. Despite the fact that particle in cell simulations have predicted several advantageous ion acceleration schemes, laser accelerators have not yet reached their full potential in producing simultaneous high-radiation doses at high particle energies. The most stringent limitation is the lack of a suitable high-repetition rate target that also provides a high degree of control of the plasma conditions required to access these advanced regimes. Here, we demonstrate that the interaction of petawatt-class laser pulses with a pre-formed micrometer-sized cryogenic hydrogen jet plasma overcomes these limitations enabling tailored density scans from the solid to the underdense regime. Our proof-of-concept experiment demonstrates that the near-critical plasma density profile produces proton energies of up to 80 MeV. Based on hydrodynamic and three-dimensional particle in cell simulations, transition between different acceleration schemes are shown, suggesting enhanced proton acceleration at the relativistic transparency front for the optimal case.

Enhanced ion acceleration from transparency-driven foils demonstrated at two ultraintense laser facilities.

Laser-driven ion sources are a rapidly developing technology producing high energy, high peak current beams. Their suitability for applications, such as compact medical accelerators, motivates development of robust acceleration schemes using widely available repetitive ultraintense femtosecond lasers. These applications not only require high beam energy, but also place demanding requirements on the source stability and controllability. This can be seriously affected by the laser temporal contrast, precluding the replication of ion acceleration performance on independent laser systems with otherwise similar parameters. Here, we present the experimental generation of >60 MeV protons and >30 MeV u-1 carbon ions from sub-micrometre thickness Formvar foils irradiated with laser intensities >1021 Wcm2. Ions are accelerated by an extreme localised space charge field ≳30 TVm-1, over a million times higher than used in conventional accelerators. The field is formed by a rapid expulsion of electrons from the target bulk due to relativistically induced transparency, in which relativistic corrections to the refractive index enables laser transmission through normally opaque plasma. We replicate the mechanism on two different laser facilities and show that the optimum target thickness decreases with improved laser contrast due to reduced pre-expansion. Our demonstration that energetic ions can be accelerated by this mechanism at different contrast levels relaxes laser requirements and indicates interaction parameters for realising application-specific beam delivery.

Time-of-flight spectroscopy for laser-driven proton beam monitoring.

Application experiments with laser plasma-based accelerators (LPA) for protons have to cope with the inherent fluctuations of the proton source. This creates a demand for non-destructive and online spectral characterization of the proton pulses, which are for application experiments mostly spectrally filtered and transported by a beamline. Here, we present a scintillator-based time-of-flight (ToF) beam monitoring system (BMS) for the recording of single-pulse proton energy spectra. The setup's capabilities are showcased by characterizing the spectral stability for the transport of LPA protons for two beamline application cases. For the two beamline settings monitored, data of 122 and 144 proton pulses collected over multiple days were evaluated, respectively. A relative energy uncertainty of 5.5% (1[Formula: see text]) is reached for the ToF BMS, allowing for a Monte-Carlo based prediction of depth dose distributions, also used for the calibration of the device. Finally, online spectral monitoring combined with the prediction of the corresponding depth dose distribution in the irradiated samples is demonstrated to enhance applicability of plasma sources in dose-critical scenarios.

Off-harmonic optical probing of high intensity laser plasma expansion dynamics in solid density hydrogen jets.

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 [Formula: see text] peak intensity with a solid-density cylindrical cryogenic hydrogen jet target of [Formula: see text] 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 [Formula: see text] followed by full target transparency at [Formula: see text] after the high intensity laser peak are observed. A three dimensional particle-in-cell simulation initiated with the diagnosed target pre-expansion at [Formula: see text] 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.