High Repetition-Rate Laser-Driven Particle Generation – Towards High Flux Fast Neutron Sources

High-flux, high repetition-rate pulsed neutron sources are of interest for probing studies such as neutron-induced damage processes in materials employed and considered for shielding purposes in fusion reactors. Simulating the effect an intense neutron flux has on such materials will ultimately guide designs for future fusion reactors. Laser-driven neutron sources employing petawatt laser systems show great potential to fulfill the need for such a neutron source. One of the most common approaches for neutron generation utilizing lasers as drivers is the pitcher–catcher geometry in which a directional ion beam is generated from a pitcher target and impinges on a catcher target producing neutrons through nuclear reactions within the catcher material. Despite the fact that neutron generation using such setups have only recently gained attention, it has so far shown the highest neutron yields using short-pulse lasers. To date, experiments predominantly studied neutron generation on a single shot basis, especially since the development of a high repetition-rate laser-driven neutron source faces a variety of challenges. In this thesis, the individual components of a successful high repetition-rate laser-driven neutron source were investigated and developed. The focus of this work was especially the development of a stable target system compatible with high repetition-rate laser operations, the development of a design for the catcher target, allowing for optimization of ion beam-catcher overlap, and an efficient ion and neutron beam detection platform. For this work, a robust version of the SLAC-developed converging liquid microjet target delivery systems was designed and fielded. This system was successfully implemented at two different laser facilities, surviving more than 1000 shots on target with no apparent damage to the nozzle or degradation of the liquid target. The liquid microjet system was implemented to study high repetition-rate deuteron acceleration from heavy water microjet targets at the ALEPH laser facility reaching average fluxes of 1×10^12 deuterons/sr/min at a repetition rate of 0.5 Hz. Stable deuteron acceleration over 60 shots was observed at varying laser energies on target, suggesting a more favorable scaling of the ion beam cut-off energy than currently established in the literature. A flexible, repetition-rate compatible neutron generation platform was designed around a stackable catcher target, which can be adjusted based on laser parameters and experimental conditions. This specific design aims at enhancing the generation of high-flux, directional neutron beams. A flexible detector setup simultaneously monitors the ion and neutron beam emission characteristics to study their individual shot-to-shot parameter changes and the correlations between them. Employing cryogenic or ambient-temperature liquid jet targets as a pitcher enables high-repetition-rate operation. This novel platform was successfully tested using cryogenic liquid deuterium jet targets at the Texas Petawatt laser facility demonstrating efficient generation of forward directed neutron beams with fluxes reaching 7.2×10^9 neutrons/sr within a narrow divergence angle of ±20◦. As such, this work lays the foundation for future high-repetition-rate experiments towards pulsed, high-flux, fast neutron sources for radiation-induced effect studies relevant for fusion science and applications that require neutron beams with short pulse duration for the probing of fast evolving processes complementary to X-rays

Towards High-Repetition-Rate Fast Neutron Sources Using Novel Enabling Technologies

High-flux, high-repetition-rate neutron sources are of interest in studying neutron-induced damage processes in materials relevant to fusion, ultimately guiding designs for future fusion reactors. Existing and upcoming petawatt laser systems show great potential to fulfill this need. Here, we present a platform for producing laser-driven neutron beams based on a high-repetition-rate cryogenic liquid jet target and an adaptable stacked lithium and beryllium converter. Selected ion and neutron diagnostics enable monitoring of the key parameters of both beams. A first single-shot proof-of-principle experiment successfully implemented the presented platform at the Texas Petawatt Laser facility, achieving efficient generation of a forward-directed neutron beam. This work lays the foundation for future high-repetition-rate experiments towards pulsed, high-flux, fast neutron sources for radiation-induced effect studies relevant for fusion science and applications that require neutron beams with short pulse duration

Correction: Treffert et al. Towards High-Repetition-Rate Fast Neutron Sources Using Novel Enabling Technologies. <i>Instruments</i> 2021, <i>5</i>, 38

In the original publication [...

Towards High-Repetition-Rate Fast Neutron Sources Using Novel Enabling Technologies

High-flux, high-repetition-rate neutron sources are of interest in studying neutron-induced damage processes in materials relevant to fusion, ultimately guiding designs for future fusion reactors. Existing and upcoming petawatt laser systems show great potential to fulfill this need. Here, we present a platform for producing laser-driven neutron beams based on a high-repetition-rate cryogenic liquid jet target and an adaptable stacked lithium and beryllium converter. Selected ion and neutron diagnostics enable monitoring of the key parameters of both beams. A first single-shot proof-of-principle experiment successfully implemented the presented platform at the Texas Petawatt Laser facility, achieving efficient generation of a forward-directed neutron beam. This work lays the foundation for future high-repetition-rate experiments towards pulsed, high-flux, fast neutron sources for radiation-induced effect studies relevant for fusion science and applications that require neutron beams with short pulse duration

Correction: Treffert et al. Towards High-Repetition-Rate Fast Neutron Sources Using Novel Enabling Technologies. Instruments 2021, 5, 38

In the original publication [1], there was a mistake in Figure 9 as published. The energy for the RCF layer shown in Figure 9a was erroneously stated to be 6.4 MeV. The corrected value for the RCF layer in Figure 9a is 23.7 MeV, as clarified in this erratum

Transient Laser-Induced Breakdown of Dielectrics in Ultrarelativistic Laser-Solid Interactions

For high-intensity laser-solid interactions, the absolute density and surface density gradients of the target at the arrival of the ultrarelativistic laser peak are critical parameters. Accurate modeling of the leading edge-driven target preexpansion is desired to strengthen the predictive power of associated computer simulations. The transition from an initial solid state to a plasma state, i.e., the breakdown of the solid, defines the starting point of the subsequent target preexpansion. In this work, we report on the time-resolved observation of transient laser-induced breakdown (LIB) during the leading edge of high-intensity petawatt-class laser pulses with peak intensities of up to 5.7×1021W/cm2 in interaction with dielectric cryogenic hydrogen jet targets. LIB occurs much earlier than what is typically expected following the concept of barrier suppression ionization. The observation is explained by comparing a characterization study of target-specific LIB thresholds with laser contrast measurements. The results demonstrate the relevance of the laser pulse duration dependence of LIB for high-intensity laser-solid interactions. We provide an effective approach to determine the onset of LIB and thereby the starting point of target preexpansion in other laser-target systems

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

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

Data for all figures of publication: " Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density". The folder structure is adapted to match the figures in the publication

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