Radio-frequency atomic magnetometry with a rubidium Bose-Einstein condensate

This thesis details progress in radio-frequency atomic magnetometry with ultracold rubidium atoms. Motivations and context are first covered, before an introduction of the main concepts required to understand the underlying physics is given. At first, a cold atom magnetometer is designed, built and characterised. Consistent 20 µK atoms are produced. Radio-frequency (RF) atomic magnetometry (AM) is performed by placing the atoms in a bias magnetic field and generating coherent precession with an external AC field. A noise floor at 330 pT/√Hz defines the sensor’s sensitivity, with a range of applications. RF-AM is then performed with a Bose-Einstein condensate (BEC). The 20 µK atoms are loaded into a magnetic trap, where RF evaporation increases their phase space density (PSD = nλ^3dB, n is the density and λdB is the thermal de Broglie wavelength of the atoms). Next, atoms are transferred into a hybrid dipole trap, collecting in a dimple created at the intersect of two high power laser beams. Production and stabilisation of these beams is described, which are focused down to a 75 µm beam waist at the trap position with a total power of 7 W. Optimisation of the evaporation process in both traps leads to consistent BEC production. A pure condensate with 4x10^4 atoms at 25 nK is reported. Radio-frequency magnetometry is performed at various probe volumes. With systematic optimisation a best AC sensitivity of 24 pT/√Hz with 3.4 × 10^8 atoms in the magnetic trap before evaporation is achieved. This is extended to the BEC with 4 × 10^4 atoms, where an AC sensitivity of 84 nT/√Hz and DC sensitivity of 14 nT/√Hz is reported, bringing previously achieved atomic magnetometry into the micrometer regime. A trade-off must be considered due to reduction in sensitivity at lower probe volumes. Volumes between 1.4×10−7 m^3 and 1.6×10−14 m^3 can be accessed, highlighting the sensors adaptability and tunability for different applications. The results are contextualised in the background of previously achieved magnetometers of various types. Finally, proof-of-concept electromagnetic induction imaging (EMI) measurements are made to confirm the sensor’s viability for high resolution imaging

Non-fragile H∞ control with randomly occurring gain variations, distributed delays and channel fadings

This study is concerned with the non-fragile H∞ control problem for a class of discrete-time systems subject to randomly occurring gain variations (ROGVs), channel fadings and infinite-distributed delays. A new stochastic phenomenon (ROGVs), which is governed by a sequence of random variables with a certain probabilistic distribution, is put forward to better reflect the reality of the randomly occurring fluctuation of controller gains implemented in networked environments. A modified stochastic Rice fading model is then exploited to account for both channel fadings and random time-delays in a unified representation. The channel coefficients are a set of mutually independent random variables which abide by any (not necessarily Gaussian) probability density function on [0, 1]. Attention is focused on the analysis and design of a non-fragile H∞ outputfeedback controller such that the closed-loop control system is stochastically stable with a prescribed H∞ performance. Through intensive stochastic analysis, sufficient conditions are established for the desired stochastic stability and H∞ disturbance attenuation, and the addressed non-fragile control problem is then recast as a convex optimisation problem solvable via the semidefinite programme method. An example is finally provided to demonstrate the effectiveness of the proposed design method

Stabilization via delay feedback for highly nonlinear stochastic time-varying delay systems with Markovian switching and Poisson jump

Little work seems to be known about stabilization results of highly nonlinear stochastic time-varying delay systems (STVDSs) with Markovian switching and Poisson jump. This paper is concerned with the stabilization problem for a class of STVDSs with Markovian switching and Poisson jump. The coefficients of such systems do not satisfy the conventional linear growth conditions, but are subject to high nonlinearity. The aim of this paper is to design a delay feedback controller to make an unstable highly nonlinear STVDSs with Markovian switching and Poisson jump H∞-stable and asymptotically stable. Besides, an illustrative example is provided to support the theoretical results

Far-UVC (222 nm) efficiently inactivated an airborne pathogen in a room-sized chamber

Funding: We acknowledge the financial assistance of the United Kingdom’s Department for Health and Social Care (2020/092).Many infectious diseases, including COVID-19, are transmitted by airborne pathogens. There is a need for effective environmental control measures which, ideally, are not reliant on human behaviour. One potential solution is Krypton Chloride (KrCl) excimer lamps (often referred to as Far-UVC), which can efficiently inactivate pathogens, such as coronaviruses and influenza, in air. Research demonstrates that when KrCl lamps are filtered to remove longer-wavelength ultraviolet emissions they do not induce acute reactions in the skin or eyes, nor delayed effects such as skin cancer. While there is laboratory evidence for Far-UVC efficacy, there is limited evidence in full-sized rooms. For the first time, we show that Far-UVC deployed in a room-sized chamber effectively inactivates aerosolised Staphylococcus aureus. At a room ventilation rate of 3 air-changes-per-hour (ACH), with 5 filtered-sources the steady-state pathogen load was reduced by 98.4% providing an additional 184 equivalent air changes (eACH). This reduction was achieved using Far-UVC irradiances consistent with current American Conference of Governmental Industrial Hygienists threshold limit values for skin for a continuous 8-h exposure. Our data indicate that Far-UVC is likely to be more effective against common airborne viruses, including SARS-CoV-2, than bacteria and should thus be an effective and “hands-off” technology to reduce airborne disease transmission. The findings provide room-scale data to support the design and development of effective Far-UVC systems.Publisher PDFPeer reviewe

Stabilisation of Time Delay Systems with Nonlinear Disturbances Using Sliding Mode Control

This paper focuses on a class of control systems with delayed states and nonlinear disturbances using sliding mode techniques. Both matched and mismatched uncertainties are considered which are assumed to be bounded by known nonlinear functions. The bounds are used in the control design and analysis to reduce conservatism. A sliding function is designed and a set of sufficient conditions is derived to guarantee the asymptotic stability of the corresponding sliding motion by using the Lyapunov-Razumikhin approach which allows large time varying delay with fast changing rate. A delay dependent sliding mode control is synthesised to drive the system to the sliding surface in finite time and maintain a sliding motion thereafter. Effectiveness of the proposed method is demonstrated via a case study on a continuous stirred tank reactor system

Carrier-envelope phase control for the advancement of attosecond pulse generation

When the optical pulses emitted by a laser become so short in time that they encompass only a few cycles of the carrier wave, the phase between carrier and envelope becomes a crucial parameter. The ability to control this carrier-envelope phase (CEP) is elemental to experiments probing the fastest processes in the microcosm, occurring on the time-scale of attoseconds. More than a decade into the attosecond era, the limitations of the established CEP stabilisation technique have begun to curtail experimental progress. First, increasingly complex experiments require many hours of uninterrupted operation at the same waveform. Second, the pulses used in experiments are approaching the single-cycle boundary, calling for ever-decreasing CEP noise. With the conventional stabilisation technique, already these two requirements cannot be fulfilled simultaneously. Ultimately, the low efficiency of the underlying nonlinear processes can only be compensated by driver lasers at a higher repetition rate than available at present. In order to advance attosecond pulse generation, novel approaches to CEP control thus face a threefold challenge that outlines this thesis: To simultaneously provide low CEP noise and long-term operation to present-day few-cycle lasers and amplifiers, and to investigate CEP control capability in high average power sources that are currently under development. This thesis describes the adaptation of cavity-external CEP stabilisation for use with few-cycle pulses. The intrinsic limitations of the conventional feed-back technique are lifted. A laser oscillator is demonstrated to maintain record-low CEP noise for tens of hours of operation free from phase discontinuities. In addition, a modification of the technique is presented that further enhances the applicability to amplified systems. Two routes are investigated to achieve CEP control in system architectures that represent potential megahertz repetition rate driver sources. In combination with temporal pulse compression, a thin-disk laser is shown to yield few-cycle pulses. Experiments are presented that provide the groundwork towards the first CEP-stabilised thin-disk oscillator. The second approach targets the seed oscillator of a fibre chirped-pulse amplifier. The CEP noise properties of different amplification regimes are examined. Intensity enhancement of the output pulses in a passive resonator is shown to benefit greatly even from a coarse lock of the CEP slip rate. For few-cycle pulse energy to reach the millijoule level and above, amplification and temporal compression will remain indispensable in the foreseeable future. Maintaining CEP stability across such stages is crucial, irrespective of the technology employed. Cavity-external CEP control is demonstrated to enable more than 24 hours of constant-CEP operation in chirped-pulse amplifiers. Furthermore, a novel actuator is introduced that, in conjunction with a fast means of measuring the CEP, is able to provide phase correction of the amplified waveform up to several kilohertz bandwidth. The result is a train of millijoule-level pulses with residual CEP noise comparable to that of state-of-the-art nanojoule oscillators. Eventually, an experiment is presented to examine the influence of different types of hollow-core fibre-based temporal compression on the CEP. The findings shed new light on the origin of adverse effects introduced by this technique, and point out ways towards effective compensation.Wenn die von einem Laser emittierten Lichtpulse so kurz werden, dass ihre Dauer nur noch wenige Schwingungszyklen des elektrischen Feldes umfasst, kommt der Phase zwischen Trägerwelle und Einhüllender (CEP) eine entscheidende Rolle zu. Ihre Regelung ist essentiell für jene Experimente, die die schnellsten Prozesse in der Natur auf der Zeitskala von Attosekunden ausloten. Mehr als zehn Jahre nach Beginn der Attosekunden-Ära ist die etablierte Methode der CEP-Regelung zum Hindernis für experimentelle Fortschritte geworden. Einerseits erfordern immer komplexere Experimente, dass das elektrische Feld der Pulse über viele Stunden konstant bleibt. Andererseits zeichnet sich eine Entwicklung der Pulsdauer zu immer kürzerer Dauer in Richtung eines einzigen Zyklus ab, was eine steigende Präzision der Regelung erfordert. Die gleichzeitige Erfüllung schon dieser beiden Anforderungen ist mit der konventionellen Methode nicht zu erreichen. Schlussendlich kann die niedrige Effizienz der zugrunde liegenden nichtlinearen Prozesse nur die Verwendung von Lasersystemen mit deutlich erhöhter Wiederholrate ausgeglichen werden. Um die Erzeugung von Attosekunden-Pulsen voranzutreiben, müssen neue Ansätze zur CEP-Regelung einer dreifachen Herausforderung gerecht werden, die dieser Dissertation ihren Rahmen gibt: Einerseits hohe Präzision und andererseits hohe Langzeittauglichkeit zur Verfügung zu stellen, und überdies neue Wege zur CEP-Regelung von derzeit in Entwicklung befindlichen Laserquellen mit hoher Durchschnittsleistung aufzuzeigen. Diese Dissertation beschreibt die Anpassung einer alternativen Methode der CEP-Regelung auf Pulse mit einer Dauer von wenigen Zyklen. Die intrinsischen Beschränkungen der konventionellen Technik werden damit behoben. Der solchermaßen stabilisierte Oszillator bietet geringstes CEP-Rauschen über mehrere zehn Stunden Laufzeit ohne Phasensprünge. Zusätzlich wird eine Abwandlung der Methode beschreiben, die deren Anwendbarkeit für Verstärkersysteme erweitert. Die CEP-Regelung in Systemarchitekturen für hohe Durchschnittsleistungen wird an zwei Lasersystemen untersucht, die exemplarisch für potentielle Attosekunden-Quellen mit Megahertz-Wiederholrate stehen. Es wird gezeigt, dass ein Scheibenlaser in Kombination mit zeitlicher Pulskompression genutzt werden kann, um Pulse in der Größenordnung von 10 fs zu erzeugen. Erste Experimente zu deren CEP-Stabilisierung ebnen den Weg für den ersten CEP-stabilen Scheibenlaser. Der zweite Ansatz betrifft die CEP-Regelung eines Oszillator-Verstärker-Systems. Das CEP-Rauschverhalten verschiedener Faserverstärker wird untersucht. Es wird gezeigt, dass die Überhöhung des Pulszugs in einem passiven Resonator auch von einer groben Stabilisierung der CEP-Änderungsrate deutlich profitiert. Um Pulse von wenigen Zyklen Dauer auf eine Energie von Millijoule und darüber hinaus zu bringen, wird Verstärkung und zeitliche Kompression auf absehbare Zeit unverzichtbar bleiben. Unabhängig von der hierzu gewählten Technologie ist es von entscheidender Bedeutung, den Einfluss dieser Prozesse auf die CEP gering zu halten. Die Verwendung eines mit der alternativen CEP-Regelung ausgestatteten Oszillators zur zeitlich gestreckten Verstärkung wird beschrieben, was in hochenergetischen Pulsen mit über 24 Stunden konstanter Wellenform resultiert. Alsdann wird ein neuartiger CEP-Aktuator beschrieben, der in Kombination mit einer schnellen Messmethode die CEP-Korrektur eines jeden Pulses bei einer Bandbreite von mehreren Kilohertz leistet. Das Resultat ist ein Pulszug auf Millijoule-Niveau, dessen CEP-Rauschen mit dem eines Nanojoule-Oszillators vergleichbar ist. Abschließend wird ein Experiment vorgestellt, mit dem der Einfluss von Hohlfaser-Kompression auf die CEP untersucht wird. Die Ergebnisse werfen neues Licht auf den Ursprung zusätzlichen Rauschens in solchen Aufbauten, und zeigen Wege zu dessen Vermeidung auf

Per- and polyfluoroalkyl substance (PFAS) retention by colloidal activated carbon (CAC) using dynamic column experiments

Developing effective remediation methods for per- and polyfluoroalkyl substance (PFAS)-contaminated soils is a substantial step towards counteracting their widespread occurrence and protecting our ecosystems and drinking water sources. Stabilisation of PFAS in the subsurface using colloidal activated carbon (CAC) is an innovative, yet promising technique, requiring better understanding. In this study, dynamic soil column tests were used to assess the retardation of 10 classical perfluoroalkyl acids (PFAAs) (C-5-C-11 perfluoroalkyl carboxylic acids (PFCAs) and C-4, C-6, C-8 perfluoroalkane sulfonates (PFSAs)) as well as two alternative PFAS (6:2 and 8:2 fluorotelomer sulfonates) using CAC at 0.03% w/w, to investigate the fate and transport of PFAS under CAC treatment applications. Results showed high retardation rates for long-chain PFAS and eight times higher retardation for the CAC-treated soil compared to the non-treated reference soil for the Sigma PFAS. Replacement of shorter chain perfluorocarboxylic acids (PFCAs), such as perfluoropentanoic acid (PFPeA), by longer chained PFAS was observed, indicating competition effects. Partitioning coefficients (K-d values) were calculated for the CAC fraction at similar to 10(3)-10(5) L kg(-1) for individual PFAS, while there was a significant positive correlation (p < 0.05) between perfluorocarbon chain length and K-d. Mass balance calculations showed 37% retention of Sigma PFAS in treated soil columns after completion of the experiments and 99.7% higher retention rates than the reference soil. Redistribution and elution of CAC were noticed and quantified through organic carbon analysis, which showed a 23% loss of carbon during the experiments. These findings are a step towards better understanding the extent of CAC's potential for remediation of PFAS-contaminated soil and groundwater and the limitations of its applications

A novel descriptor redundancy method based on delay partition for exponential stability of time delay systems

This is an Author's Accepted Manuscript of an article published in Antonio González (2021) A novel descriptor redundancy method based on delay partition for exponential stability of time delay systems, International Journal of Systems Science, 52:8, 1707-1718, DOI: 10.1080/00207721.2020.1869344, available online at: http://www.tandfonline.com/10.1080/00207721.2020.1869344[EN] This paper investigates the exponential stability of uncertain time delay systems using a novel descriptor redundancy approach based on delay partitioning. First, the original system is casted into an equivalent descriptor singular state¿space representation by introducing redundant state variables so that the resulting delay is progressively reduced. From the equivalent model and applying Lyapunov Functional method, a sufficient condition based on Linear Matrix Inequalities (LMIs) for exponential stability with guaranteed decay rate performance is obtained. As a result, the inherent conservatism of Lyapunov¿Krasovskii functional techniques can arbitrarily be reduced by increasing the number of delay partition intervals including decay rate performance and model uncertainties in polytopic form. Various benchmark examples are provided to validate the effectiveness of the proposed method, showing better trade-off between conservatism and performance in comparison to previous approaches.This work was supported by project PGC2018-098719-B-I00 (MCIU/AEI/FEDER,UE).González Sorribes, A. (2021). A novel descriptor redundancy method based on delay partition for exponential stability of time delay systems. International Journal of Systems Science. 52(8):1707-1718. https://doi.org/10.1080/00207721.2020.18693441707171852