511 research outputs found
Development of a SQUID magnetometry system for cryogenic neutron electric dipole moment experiment
A measurement of the neutron electric dipole moment (nEDM) could hold the key to understanding why the visible universe is the way it is: why matter should predominate over antimatter. As a charge-parity violating (CPV) quantity, an nEDM could provide an insight into new mechanisms that address this baryon asymmetry. The motivation for an improved sensitivity to an nEDM is to find it to be non-zero at a level consistent with certain beyond the Standard Model theories that predict new sources of CPV, or to establish a new limit that constrains them.
CryoEDM is an experiment that sought to better the current limit of cm by an order of magnitude. It is designed to measure the nEDM via the Ramsey Method of Separated Oscillatory Fields, in which it is critical that the magnetic field remains stable throughout. A way of accurately tracking the magnetic fields, moreover at a temperature K, is crucial for CryoEDM, and for future cryogenic projects.
This thesis presents work focussing on the development of a 12-SQUID magnetometry system for CryoEDM, that enables the magnetic field to be monitored to a precision of pT. A major component of its infrastructure is the superconducting capillary shields, which screen the input lines of the SQUIDs from the pick up of spurious magnetic fields that will perturb a SQUID's measurement. These are shown to have a transverse shielding factor of , which is a few orders of magnitude greater than the calculated requirement.
Efforts to characterise the shielding of the SQUID chips themselves are also discussed. The use of Cryoperm for shields reveals a tension between improved SQUID noise and worse neutron statistics. Investigations show that without it, SQUIDs have an elevated noise when cooled in a substantial magnetic field; with it, magnetostatic simulations suggest that it is detrimental to the polarisation of neutrons in transport. The findings suggest that with proper consideration, it is possible to reach a compromise between the two behaviours.
Computational work to develop a simulation of SQUID data is detailed, which is based on the Laplace equation for the magnetic scalar potential. These data are ultimately used in the development of a linear regression technique to determine the volume-averaged magnetic field in the neutron cells. This proves highly effective in determining the fields within the pT requirement under certain conditions
Analysis of three body decays in quasi-real photoproduction
This thesis presents preliminary measurements of the moments of angular distribution for the K∗K + mesonic final state using a quasi-real, linearly polarised photon.
Two of the main points of focus for this analysis were the development of the formalism for photoproduced vector-pseudoscalar decay and the tools by which the analysis was carried out. The formalism described how the moments of angular distribution could be extracted from the angular decay variables, and how they related to resonance decays in terms of partial waves. In extracting the moments, it was found that adapted Markov Chain Monte Carlo (MCMC) methods proved to be more effective in terms of extracting results from the data compared to the sole use of gradient descent based fitting algorithms.
These measurements are complementary to other decay channels and production mechanisms, with the global aim of probing the existence of exotic mesons. Data from this analysis was taken by the CLAS collaboration at the Thomas Jefferson National Accelerator Facility (JLab) by scattering from a liquid hydrogen target using a 10.6 GeV electron. This work was done within the MesonEx program at CLAS12 that aims to map the spectrum of mesons, and in doing so, gain a greater understanding of Quantum ChromoDynamics (QCD) and the strong force
The Effective Mean-Free-Path of the Solar Wind
The high temperature and rarefied ionised gas (plasma) that constitutes the corona of the sun escapes the gravitational bound and flows out into interplanetary space. This plasma is called the solar wind. It is characterised by a long collision mean-free-path (i.e., weakly collisional); it is not in thermodynamic equilibrium. While the plasma is ultimately governed by a kinetic equation, it does appear that the solar wind is described by fluid equations, where it is assumed to be at equilibrium. This is in stark contradiction to the long collision mean-free-path. The suggestion is that collisionless relaxation processes are playing a strong role in dictating the dynamics of the solar wind. These processes are wave-particle interactions that cause the plasma to relax towards equilibrium, i.e., they are effective collision processes. This thesis takes a novel route to measure the effective mean-free-path of the solar wind, by modelling compressive fluctuations of arbitrary effective mean-free-path, and making a robust comparison to solar wind observations. The effective mean-free-path is measured to be approximately 10 times shorter than the collisional mean-free-path. It is shown to be consistent with and justify decades of past solar wind research that use fluid equations. The theory for the numerical model is derived from first principles and is shown to coincide with previous results, and draw together many concepts about compressive plasma waves. The solar wind dataset used in this thesis was not previously used for scientific analysis, so verification of the data quality is demonstrated. In addition, data analysis tools are constructed to measure some of the potential effective collision mechanisms. The analysis is tested on simulation data, to verify the accuracy, by measuring key quantities in identifying the relevant role of various effective collision mechanisms. The analysis of the numerical simulation data is shown to be satisfactory and can be employed on spacecraft data. This measurement resolves a long-standing debate on the utility and accuracy of fluid equations in studying the solar wind. The direct measurement of the effective mean-free-path is important for the field of plasma physics because it dictates the transport and thermodynamics of weakly collisional plasmas
Real-time simulations of transmon systems with time-dependent Hamiltonian models
In this thesis we study aspects of Hamiltonian models which can affect the
time evolution of transmon systems. We model the time evolution of various
systems as a unitary real-time process by numerically solving the
time-dependent Schr\"odinger equation (TDSE). We denote the corresponding
computer models as non-ideal gate-based quantum computer (NIGQC) models since
transmons are usually used as transmon qubits in superconducting prototype
gate-based quantum computers (PGQCs).We first review the ideal gate-based
quantum computer (IGQC) model and provide a distinction between the IGQC, PGQCs
and the NIGQC models we consider in this thesis. Then, we derive the circuit
Hamiltonians which generate the dynamics of fixed-frequency and flux-tunable
transmons. Furthermore, we also provide clear and concise derivations of
effective Hamiltonians for both types of transmons. We use the circuit and
effective Hamiltonians we derived to define two many-particle Hamiltonians,
namely a circuit and an associated effective Hamiltonian. The interactions
between the different subsystems are modelled as dipole-dipole interactions.
Next, we develop two product-formula algorithms which solve the TDSE for the
Hamiltonians we defined. Afterwards, we use these algorithms to investigate how
various frequently applied assumptions affect the time evolution of transmon
systems modelled with the many-particle effective Hamiltonian when a control
pulse is applied. Here we also compare the time evolutions generated by the
effective and circuit Hamiltonian. We find that the assumptions we investigate
can substantially affect the time evolution of the probability amplitudes we
model. Next, we investigate how susceptible gate-error quantifiers are to
assumptions which make up the NIGQC model. We find that the assumptions we
consider clearly affect gate-error quantifiers like the diamond distance and
the average infidelity.Comment: Dissertation, 203 pages, RWTH Aachen University, 2023. This
dissertation includes and extends the results of arXiv:2201.02402 and
arXiv:2211.1101
Computational and experimental studies of selected magnesium and ferrous sulfate hydrates: implications for the characterisation of extreme and extraterrestrial environments
Magnesium sulfate hydrates are considered important rock-forming minerals on the outer three Galilean moons of Jupiter (i.e., Europa, Ganymede,
Callisto) and, alongside ferrous sulfate hydrates, are promising candidate minerals for the widespread sulfate deposits that occur in the equatorial region of
Mars. In such extraterrestrial environments, these minerals experience extreme high-pressure conditions in the interiour of the Galilean moons and low temperature conditions on the surface of these moons and Mars. The aim of this thesis is to understand the structural stability, compressibility, and thermal
expansion of these compounds in such extreme environments and aid their identification in ongoing and future space missions.
Most magnesium sulfate hydrates lack accurate reference elastic tensors, which hinders their seismological identification in lander missions on the icy moons of the outer solar system, as envisioned for the near future. In this thesis, the accuracy of recent advancements in density functional theory to predict the compressibility and elastic constants of icy satellite candidate minerals (i.e., epsomite (MgSO₄·7H₂O), gypsum (CaSO₄·2H₂O), carbon dioxide (CO₂), and benzene (C₆H₆)) was assessed by benchmarking them against experimental
reference data from the literature. Key findings are that density functional theory calculations do not yield elastic constants accurate enough to be used as a
reference for the seismic exploration of icy moons. However, the bulk compressibility of such materials is very accurately reproduced by density functional theory, which was therefore used to predict the compressibility of the icy satellite candidate minerals starkeyite (MgSO₄·4H₂O) and cranswickite (MgSO₄·4H₂O). Knowledge of the compressibility of such minerals is critical to model mantle processes (e.g., salt diaprisim, plate tectonics, subduction) and the density structure of the outer three Galilean moons. The thermal expansion and structural stability of three sulfate minerals (i.e., rozenite (FeSO₄·4H₂O), starkeyite, and cranswickite) was characterised for
the first time using neutron diffraction. Cranswickite transforms to starkeyite at 330 K, well above the maximum surface temperature of 308 K hitherto reported on Mars. Starkeyite likely undergoes a structural phase transition at around 245
K. The structure of this proposed low-temperature polymorph could not be determined but would be of great interest since the temperature drops below 245 K on equatorial Mars at night-time. Starkeyite was also studied by means of synchrotron X-ray diffraction but suffered radiation damage. No phase transition
was observed in rozenite from 290 – 21 K, which contrasts with Raman data reported in the literature, where sharpening of vibrational modes upon cooling
was misinterpreted as mode splitting and evidence for two phase transitions at temperatures relevant to the Martian surface. First-principles phonon frequency
calculations provide evidence supporting the absence of vibrational mode splitting. A workflow to obtain reliable reference Raman spectra for space
exploration was proposed and an optical centre stick for the simultaneous acquisition of neutron diffraction and Raman spectroscopy data at the HRPD
instrument was commissioned. Lastly, the structure of a polymorph of hexahydrite (MgSO₄·6H₂O), most recently proposed in the literature, was shown to be
unambiguously wrong
Electron Thermal Runaway in Atmospheric Electrified Gases: a microscopic approach
Thesis elaborated from 2018 to 2023 at the Instituto de AstrofÃsica de AndalucÃa under the supervision of Alejandro Luque (Granada, Spain) and Nikolai Lehtinen (Bergen, Norway). This thesis presents a new database of atmospheric electron-molecule collision cross sections which was published separately under the DOI :
With this new database and a new super-electron management algorithm which significantly enhances high-energy electron statistics at previously unresolved ratios, the thesis explores general facets of the electron thermal runaway process relevant to atmospheric discharges under various conditions of the temperature and gas composition as can be encountered in the wake and formation of discharge channels
Dusty star-forming galaxies and their environments
Since their discovery over three decades ago, there has been astounding progress in our understanding of DSFGs. Roughly half all optical and UV emission from star formation is absorbed and re-radiated as infrared light by dust. This dust enshrouded star formation is contained within DSFGs, with some harbouring SFRs of thousands of solar masses per year, yielding infrared luminosities greater than \,L. DSFGs are thought to be the progenitors of elliptical galaxies that are predominantly found within massive, galaxy clusters in the local Universe. This thesis aims to build on this progress by investigating the nature of DSFGs and their environments. We first investigate the molecular gas properties of galaxies in the Antlia cluster, a potentially useful low redshift analogue to developing protoclusters at higher redshifts. We find that the disturbed cluster environment is not strong enough to strip the molecular gas reservoirs of its member galaxies, yielding the observed high SFRs in this population. We also study the properties of candidate members of the massive SpARCS-0330 galaxy cluster at , where we find significant ongoing star formation. Quenching in SpARCS-0330 is likely driven by secular processes that scale with stellar mass rather than environment, suggesting that either the cluster environment cannot truncate star formation or the galaxy members have not yet inhabited the cluster for long enough to be significantly affected. We also investigate the multiplicities and properties of four candidate z > 4 DSFGs. We find that three resolve into multiple components, suggesting that this population is more diverse than predicted by simulations. We additionally investigate SDSS1607, a fascinating quasar-SMG system at z = 3.65, using observations from the VLA, SMA and HST. Finally, we discuss the implications of these results and the exciting avenues for future research produced by this work.Open Acces
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