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

Linear algebra with a didactical focus

How might you construct an introductory linear algebra course for first year mathematics students? What decisions would you have to make and what issues would you have to address? Barbara Jaworski, Stephanie Treffert-Thomas and Thomas Bartsch, as a small research team, set out to address these questions and others relating to a first year, first semester module in linear algebra. The authors are all members of the School of Science at Loughborough University, they all teach mathematics and do research into mathematics or mathematics education. Thomas Bartsch is a mathematician working in the Department of Mathematical Sciences; Barbara Jaworski and Stephanie Treffert-Thomas are mathematics educators working in the Mathematics Education Centre

A note on chances and limitations of psychometric AI

Human-level artificial intelligence (HAI) surely is a special research endeavor in more than one way: In the first place, the very nature of intelligence is not entirely clear; there are no criteria commonly agreed upon necessary or sufficient for the ascription of intelligence other than similarity to human performance (and even this criterion is open for a plethora of possible interpretations); there is a lack of clarity concerning how to properly investigate HAI and how to proceed after the very first steps of implementing an HAI system; etc. In this note I assess the ways in which the approach of Psychometric Artificial Intelligence [1] can (and cannot) be taken as a foundation for a scientific approach to HAI

Students as Partners in Complex Number Task Design

International audienceWe report on a collaborative project at university level involving students as partners in task design for a bridging mathematics module (known in the UK as a Foundation module) which is part of gaining access to first year degree studies. Three teacher-researchers met regularly with four student partners who developed a set of tasks on matrices and on complex numbers which were trialled with students on this Foundation module. We show the mediational processes by which the tasks developed from ‘static’ designs to more ‘dynamic’ designs using the software Autograph. Our analyses highlighted various tools in the mediation of the learning of mathematics, in the mediation of task development and in the mediation of the engagement of all team members in collaboration

Non-standard Problems in an Ordinary Differential Equations Course

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Irradiation of Materials with Short, Intense Ion pulses at NDCX-II

We present an overview of the performance of the Neutralized Drift Compression Experiment-II (NDCX-II) accelerator at Berkeley Lab, and report on recent target experiments on beam driven melting and transmission ion energy loss measurements with nanosecond and millimeter-scale ion beam pulses and thin tin foils. Bunches with around 10^11 ions, 1-mm radius, and 2-30 ns FWHM duration have been created with corresponding fluences in the range of 0.1 to 0.7 J/cm^2. To achieve these short pulse durations and mm-scale focal spot radii, the 1.1 MeV He+ ion beam is neutralized in a drift compression section, which removes the space charge defocusing effect during final compression and focusing. The beam space charge and drift compression techniques resemble necessary beam conditions and manipulations in heavy ion inertial fusion accelerators. Quantitative comparison of detailed particle-in-cell simulations with the experiment play an important role in optimizing accelerator performance.Comment: 15 pages, 7 figures. revised manuscript submitted to Laser and Particle Beam

Effects of memory training or task design? A commentary on 'neural evidence for the use of digit-image mnemonic in a superior memorist: an fMRI study'

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Explaining and inducing savant skills: privileged access to lower level, less-processed information

I argue that savant skills are latent in us all. My hypothesis is that savants have privileged access to lower level, less-processed information, before it is packaged into holistic concepts and meaningful labels. Owing to a failure in top-down inhibition, they can tap into information that exists in all of our brains, but is normally beyond conscious awareness. This suggests why savant skills might arise spontaneously in otherwise normal people, and why such skills might be artificially induced by low-frequency repetitive transcranial magnetic stimulation. It also suggests why autistic savants are atypically literal with a tendency to concentrate more on the parts than on the whole and why this offers advantages for particular classes of problem solving, such as those that necessitate breaking cognitive mindsets. A strategy of building from the parts to the whole could form the basis for the so-called autistic genius. Unlike the healthy mind, which has inbuilt expectations of the world (internal order), the autistic mind must simplify the world by adopting strict routines (external order)

Is Synaesthesia More Prevalent in Autism Spectrum Conditions? Only Where There Is Prodigious Talent

Savant syndrome is a condition where prodigious talent co-occurs with developmental difficulties such as autism spectrum conditions (ASC). To better understand savant skills, we previously proposed a link with synaesthesia: that savant syndrome may arise in ASC individuals who also happen to have synaesthesia. A second, unrelated claim is that people with autism may have higher rates of synaesthesia. Here we ask whether synaesthesia is indeed found more often in autism per se, or only in cases where autism co-occurs with savant skills. People with autism in previous studies when tested for synaesthesia were not differentiated into those with and without savant abilities. Here we tested three groups: people with autism who also have savant skills (n=40), people with autism without savant skills (n=34), and controls without autism (n=29). We used a validated test to diagnose grapheme–colour synaesthesia. Results show a significantly higher prevalence of synaesthesia in people with ASC, but only those who also have savant skills. This suggests that synaesthesia in autism is linked to those with savant abilities rather than autism per se. We discuss the role of synaesthesia in the development of prodigious talent.JEAH, JS and JW were supported by a grant from the Economic and Social Research Council [ESRC (http://www.esrc.ac.uk/) grant ES/K006215/1]. SBC was supported by the MRC and the Autism Research Trust during the period of this work

Design and Implementation of a Thomson Parabola for Fluence Dependent Energy-Loss Measurements at the Neutralized Drift Compression eXperiment

The interaction of ion beams with matter includes the investigation of the basic principles of ion stopping in heated materials. An unsolved question is the effect of different, especially higher, ion beam fluences on ion stopping in solid targets. This is relevant in applications such as in fusion sciences. To address this question, a Thomson parabola was built for the Neutralized Drift Compression eXperiment (NDCX-II) for ion energy-loss measurements at different ion beam fluences. The linear induction accelerator NDCX-II delivers 2 ns short, intense ion pulses, up to several tens of nC/pulse, or 10$^{10}$-10$^{11}$ ions, with a peak kinetic energy of ~1.1 MeV and a minimal spot size of 2 mm FWHM. For this particular accelerator the energy determination with conventional beam diagnostics, for example, time of flight measurements, is imprecise due to the non-trivial longitudinal phase space of the beam. In contrast, a Thomson parabola is well suited to reliably determine the beam energy distribution. The Thomson parabola differentiates charged particles by energy and charge-to-mass ratio, through deflection of charged particles by electric and magnetic fields. During first proof-of-principle experiments, we achieved to reproduce the average initial helium beam energy as predicted by computer simulations with a deviation of only 1.4 %. Successful energy-loss measurements with 1 {\mu}m thick Silicon Nitride foils show the suitability of the accelerator for such experiments. The initial ion energy was determined during a primary measurement without a target, while a second measurement, incorporating the target, was used to determine the transmitted energy. The energy-loss was then determined as the difference between the two energies