arXiv subjects

A. Davis

Publications and source records attributed to A. Davis.

10 recordsLinked to original sources

arXiv2024

Mechanical Model for a Full Fusion Tokamak Enabled by Supercomputing

Determining stress and strain in a component of a fusion power plant involves defining boundary conditions for the mechanical equilibrium equations, implying the availability of a full reactor model for defining those conditions. To address this fundamental challenge of reactor design, a finite element method (FEM) model for the Mega-Ampere Spherical Tokamak Upgrade (MAST-U) fusion tokamak, operating at the Culham Campus of UKAEA, has been developed and applied to assess mechanical deformations, strain, and stress in the full tokamak structure, taken as a proxy for a fusion power plant. The model, handling 127 million finite elements using about 800 processors in parallel, illustrates the level of fidelity of structural simulations of a complex nuclear device made possible by the modern supercomputing systems. The model predicts gravitational and atmospheric pressure-induced deformations in broad agreement with observations, and enables computing the spectrum of acoustic vibrations of a tokamak, arising from mechanical disturbances like an earthquake or a plasma disruption. We introduce the notion of the density of stress to characterise the distribution of stress in the entire solid body of the tokamak, and to predict the magnitude and locations of stress concentrations. The model enables defining computational requirements for simulating a whole operating fusion power plant, and provides a digital foundation for the assessment of reactor performance as well as for specifying the relevant materials testing programme.

physics.app-ph↗

arXiv2024

The LHCb VELO Upgrade Module Construction

The LHCb detector has undergone a major upgrade for LHC Run 3. This Upgrade I detector facilitates operation at higher luminosity and utilises full-detector information at the LHC collision rate, critically including the use of vertex information. A new vertex locator system, the VELO Upgrade, has been constructed. The core element of the new VELO are the double-sided pixelated hybrid silicon detector modules which operate in vacuum close to the LHC beam in a high radiation environment. The construction and quality assurance tests of these modules are described in this paper. The modules incorporate 200 \mum thick, n-on-p silicon sensors bump-bonded to 130 \nm technology ASICs. These are attached with high precision to a silicon microchannel substrate that uses evaporative CO$_2$ cooling. The ASICs are controlled and read out with flexible printed circuits that are glued to the substrate and wire-bonded to the chips. The mechanical support of the module is given by a carbon fibre plate, two carbon fibre rods and an aluminium plate. The sensor attachment was achieved with an average precision of 21 $\mathrm{\mu m}$, more than 99.5\% of all pixels are fully functional, and a thermal figure of merit of 3 \mathrm{Kcm^{2}W^{-1}}$ was achieved. The production of the modules was successfully completed in 2021, with the final assembly and installation completed in time for data taking in 2022.

physics.ins-det↗

arXiv2023

Bi-planar magnetic stabilisation coils for an inertial sensor based on atom interferometry

Inertial sensors that measure the acceleration of ultracold atoms promise unrivalled accuracy compared to classical equivalents. However, atomic systems are sensitive to various perturbations, including magnetic fields, which can introduce measurement inaccuracies. To address this challenge, we have designed, manufactured, and validated a magnetic field stabilisation system for a quantum sensor based on atom interferometry. We solve for the magnetic field generated by surface currents in-between a pair of rectangular coils and approximate the surface current using discrete wires. The wires are wound by-hand onto machined panels which are retrofitted onto the existing mounting structure of the sensor without interfering with any experimental components. Along the central $60$ mm of the $y$-axis, which aligns with the trajectory of the atoms during interferometry, the coils are measured to generate an independent uniform axial magnetic field with a strength of $B_z=\left(22.81\pm0.01\right)$ $\mu$T/A [$\mathrm{mean}\pm2\sigma\mathrm{std. error}$] and an independent linear axial field gradient of strength $\mathrm{d}B_z/\mathrm{d}y=\left(10.6\pm0.1\right)$ $\mu$T/Am. The uniform $B_z$ field is measured to deviate by a maximum value of $1.3$% in the same region, which is a factor of three times more uniform than the previously-used on-sensor rectangular $B_z$ compensation set.

physics.atom-ph↗

arXiv2022

Benchtop magnetic shielding for benchmarking atomic magnetometers

Here, a benchtop hybrid magnetic shield containing four mumetal cylinders and nine internal flexible printed circuit boards is designed, constructed, tested, and operated. The shield is designed specifically as a test-bed for building and operating ultra-sensitive quantum magnetometers. The geometry and spacing of the mumetal cylinders are optimized to maximize shielding efficiency while maintaining Johnson noise $<15$ fT/$\sqrt{}$Hz. Experimental measurements at the shield's center show passive shielding efficiency of $\left(1.0\pm0.1\right){\times}10^6$ for a $0.2$ Hz oscillating field applied along the shield's axis. The nine flexible printed circuit boards generate three uniform fields, which all deviate from perfect uniformity by ${\leq}0.5$% along $50$% of the inner shield axis, and five linear field gradients and one second-order gradient, which all deviate by ${\leq}4$% from perfect linearity and curvature, respectively, over measured target regions. Together, the target field amplitudes are adjusted to minimize the remnant static field along $40$% of the inner shield axis, as mapped using an atomic magnetometer. In this region, the active null reduces the norm of the magnitudes of the three uniform fields and six gradients by factors of $19.5$ and $19.8$, respectively, thereby reducing the total static field from $1.68$ nT to $0.23$ nT.

physics.ins-det↗

arXiv2021

Magnetic Field Design in a Cylindrical High-Permeability Shield: The Combination of Simple Building Blocks and a Genetic Algorithm

Magnetically-sensitive experiments and newly-developed quantum technologies with integrated high-permeability magnetic shields require increasing control of their magnetic field environment and reductions in size, weight, power and cost. However, magnetic fields generated by active components are distorted by high-permeability magnetic shielding, particularly when they are close to the shield's surface. Here, we present an efficient design methodology for creating desired static magnetic field profiles by using discrete coils electromagnetically-coupled to a cylindrical passive magnetic shield. We utilize a modified Green's function solution that accounts for the interior boundary conditions on a closed finite-length high-permeability cylindrical magnetic shield, and determine simplified expressions when a cylindrical coil approaches the interior surface of the shield. We use an analytic formulation of simple discrete building blocks to provide a complete discrete coil basis to generate any physically-attainable magnetic field inside the shield. We then use a genetic algorithm to find optimized discrete coil structures composed of this basis. We use our methodology to generate an improved linear axial gradient field, $\mathrm{d}B_z/\mathrm{d}z$, and transverse bias field, $B_x$. These optimized structures increase, by a factor of seven and three compared to the standard configurations, the volume in which the desired and achieved fields agree within $1\%$ accuracy, respectively. This coil design method can be used to optimize active--passive magnetic field shaping systems that are compact and simple to manufacture, enabling accurate magnetic field control in spatially-confined experiments at low cost.

physics.app-ph↗

arXiv2018

Opportunities in Flavour Physics at the HL-LHC and HE-LHC

Motivated by the success of the flavour physics programme carried out over the last decade at the Large Hadron Collider (LHC), we characterize in detail the physics potential of its High-Luminosity and High-Energy upgrades in this domain of physics. We document the extraordinary breadth of the HL/HE-LHC programme enabled by a putative Upgrade II of the dedicated flavour physics experiment LHCb and the evolution of the established flavour physics role of the ATLAS and CMS general purpose experiments. We connect the dedicated flavour physics programme to studies of the top quark, Higgs boson, and direct high-$p_T$ searches for new particles and force carriers. We discuss the complementarity of their discovery potential for physics beyond the Standard Model, affirming the necessity to fully exploit the LHC's flavour physics potential throughout its upgrade eras.

hep-ph↗

arXiv2018

Physics case for an LHCb Upgrade II - Opportunities in flavour physics, and beyond, in the HL-LHC era

The LHCb Upgrade II will fully exploit the flavour-physics opportunities of the HL-LHC, and study additional physics topics that take advantage of the forward acceptance of the LHCb spectrometer. The LHCb Upgrade I will begin operation in 2020. Consolidation will occur, and modest enhancements of the Upgrade I detector will be installed, in Long Shutdown 3 of the LHC (2025) and these are discussed here. The main Upgrade II detector will be installed in long shutdown 4 of the LHC (2030) and will build on the strengths of the current LHCb experiment and the Upgrade I. It will operate at a luminosity up to $ 2 \times 10^{34} \rm cm^{-2}s^{-1}$, ten times that of the Upgrade I detector. New detector components will improve the intrinsic performance of the experiment in certain key areas. An Expression Of Interest proposing Upgrade II was submitted in February 2017. The physics case for the Upgrade II is presented here in more depth. $CP$-violating phases will be measured with precisions unattainable at any other envisaged facility. The experiment will probe $b\to s \ell^+\ell^-$ and $b\to d \ell^+\ell^-$ transitions in both muon and electron decays in modes not accessible at Upgrade I. Minimal flavour violation will be tested with a precision measurement of the ratio of $B(B^0\to\mu^+\mu^-)/B(B_s^0\to \mu^+\mu^-)$. Probing charm $CP$ violation at the $10^{-5}$ level may result in its long sought discovery. Major advances in hadron spectroscopy will be possible, which will be powerful probes of low energy QCD. Upgrade II potentially will have the highest sensitivity of all the LHC experiments on the Higgs to charm-quark couplings. Generically, the new physics mass scale probed, for fixed couplings, will almost double compared with the pre-HL-LHC era; this extended reach for flavour physics is similar to that which would be achieved by the HE-LHC proposal for the energy frontier.

hep-ex↗

arXiv2013

Continental Diversity of Chenopodium album Seedling Recruitment

Chenopodium album seedling emergence studies were conducted at nine European and two North American locations comparing local populations with a common population from Denmark. It is hypothesized that C. album seedling recruitment timing and magnitude have adapted to environmental and cropping system practices of a locality. Limitations in the habitat (filter 1) were reflected in local C. album population recruitment season length. Generally, the duration of seedling recruitment of both populations (local; DEN-COM) increased with decreasing latitude, north-to-south. In general, compared to the local population, DEN-COM recruitment at locations north of Denmark was longer and south of Denmark was shorter, and ended sooner. Generally, the local cropping system disturbances (CSD) period increased with decreasing latitude. The total duration of the CSD period was over twice as long in the south as that in the north. Recruitment at each locality possessed seasonal structure (time, number) consisting of 2-4 discrete seasonal cohorts. This may be an adaptive means by which C. album searches for, and exploits, recruitment opportunity just prior to, and after, predictable disturbances. The control of C. album seedling emergence is contained in the heteroblastic traits of its locally adapted seeds, and is stimulated by a complex interaction of light, heat, water, nitrate and oxygen signals inherent in the local environment. Our observations of complex recruitment patterns occurring at critical cropping times is strong evidence that C. album possesses a flexible and sensitive germination regulation system adaptable to opportunity in many different Eurasian and North American agricultural habitats.

q-bio.PE↗

arXiv2013

Improving computational efficiency of Monte-Carlo simulations with variance reduction

CCFE perform Monte-Carlo transport simulations on large and complex tokamak models such as ITER. Such simulations are challenging since streaming and deep penetration effects are equally important. In order to make such simulations tractable, both variance reduction (VR) techniques and parallel computing are used. It has been found that the application of VR techniques in such models significantly reduces the efficiency of parallel computation due to 'long histories'. VR in MCNP can be accomplished using energy-dependent weight windows. The weight window represents an 'average behaviour' of particles, and large deviations in the arriving weight of a particle give rise to extreme amounts of splitting being performed and a long history. When running on parallel clusters, a long history can have a detrimental effect on the parallel efficiency - if one process is computing the long history, the other CPUs complete their batch of histories and wait idle. Furthermore some long histories have been found to be effectively intractable. To combat this effect, CCFE has developed an adaptation of MCNP which dynamically adjusts the WW where a large weight deviation is encountered. The method effectively 'de-optimises' the WW, reducing the VR performance but this is offset by a significant increase in parallel efficiency. Testing with a simple geometry has shown the method does not bias the result. This 'long history method' has enabled CCFE to significantly improve the performance of MCNP calculations for ITER on parallel clusters, and will be beneficial for any geometry combining streaming and deep penetration effects.

physics.plasm-ph↗

arXiv2005

The Strengths of Near-Infrared Absorption Features Relevant to Interstellar and Planetary Ices

The abundances of ices in planetary environments have historically been obtained through measurements of near-infrared absorption features (lambda = 1.0-2.5 microns), and near-IR transmission measurements of materials present in the interstellar medium are becoming more common. For transmission measurements, the band strength (or absorption intensity) of an absorption feature must be known in order to determine the column density of an ice component. In the experiments presented here, we have measured the band strengths of the near-IR absorption features for several molecules relevant to the study of interstellar icy grain mantles and icy planetary bodies: CO (carbon monoxide), CO2 (carbon dioxide), C3O2 (carbon suboxide), CH4 (methane), H2O (water), CH3OH (methanol), and NH3 (ammonia). During a vacuum deposition, the sizes of the near-IR features were correlated with that of a studied mid-IR feature whose strength is well known from previous ice studies. These data may be used to determine ice abundances from observed near-IR spectra of interstellar and planetary materials or to predict the sizes of near-IR features in spectral searches for these molecules in astrophysical environments.

astro-ph↗