Summary The proposal is primarily a theoretical project aimed at resolving several of the most important outstanding problems associated with a promising type of cryogenic detector, the superconducting Transition Edge Sensor (TES), which offers unique capabilities far exceeding that of traditional semiconductor technology. Over the past decade TES-based detectors have found application in diverse areas from dark matter searches, X-ray astrophysics, time-resolved X-ray absorption spectroscopy, quantum information processing, biological sensors, industrial material analysis and homeland security. Practical instruments require a complex optimization of speed, linearity, energy resolution and array size. However, lack of understanding of the superconducting transition in TESs limits our ability to optimise performance and predict the behaviour of a new detector designs. The present models of TESs have played an important role during a period of extensive development of technology. However, based on empirical observations the models lack knowledge of the fundamental details of superconductivity, which determine the transition, and ultimately the performance of TESs. They cannot explain the observable energy resolution, and such fundamental properties as recently-discovered weak superconductivity of TESs. As a result, the current development path of TES detector for a certain applications is still very time consuming and costly, being in many aspects based on trial and error. Significant advances are expected if better understanding of the fundamental physics of TESs is achieved, because this would underpin accurate and streamlined design processes, leading to shorter periods of experiments with targeted design options. The project aims to develop new a theoretical model of the resistive transition in TESs based on fundamental superconductivity theory. The objectives are: 1. Understanding the mechanisms of the resistive transition in TESs as spatially inhomogeneous superconducting systems, simulating electrical and thermal fluctuations, which determine the energy resolution of TES micro- and nano- calorimeters and noise performance of bolometers 2. Developing a model of non-local energy transport in multilayered TES structures, including energy escape and fluctuations over the extremely short time scale of energy deposition and down-conversion. 3. stimulating the development of the next generation of high-performance TESs by evaluating the potential of graphene and few-layer boron nitride for engineering the coupling to a thermal bath and shaping the resistive transition An expected outcome of this project is a new approach to complex optimization of speed, linearity, energy resolution and array size for individual applications. A few examples illustrate the potential impact. An improvement of the energy resolution of TES-based soft X-ray detectors below 2 eV will allow the Athena X-ray mission proposal to ESA to study turbulence in the hot gas of clusters of galaxies, and will also allow the mapping of chemical shifts in X-ray fluorescence signals in Transmission Electron Microscopy (TEM), thus opening exciting possibilities for Industrial Materials Analysis. An increase in the number of pixels per array would lead to efficient imaging on a future X-ray telescope, and also provides the ability to sustain higher flux levels in emerging synchrotron applications, such as time-resolved X-ray spectroscopy. With several potential markets for high-resolution X-ray spectroscopy equipment, most notably synchrotron facilities and manufacturers of TEM equipment, the emergence of new companies is a likely consequence. For gamma-ray and neutron spectroscopy, larger arrays of TES detectors with higher energy resolution imply more efficient and faster screening, facilitating assessment tasks in such fields as non-destructive assay of spent nuclear fuel, and the operational detection of nuclear materials.

Clouds formed by aircraft (contrails) are the most easily visible human forcing of the climate system. Trapping energy in the Earth system, they contribute more than half of the total climate impact of aviation. This makes reducing contrails an important goal to achieve the UK's climate commitments. Theoretical considerations indicate two pathways for reducing contrails. First, improving engine design to emit fewer particulates may reduce contrail lifetimes and so their climate impact. Second, rerouting aircraft to avoid contrail forming regions. Assessing these pathways requires accurate models of contrail formation, evaluated at the level of an individual aircraft. This evaluation requires observations of contrails across their lifetime, coupled to details of the generating aircraft. Even where they are matched to specific aircraft, existing observations typically view a contrail once, (limiting their use for measuring contrail lifecycles) or cannot provide the detail on the contrail microphysical properties (such as ice crystal number or shape) necessary to assess the efficacy of different pathways to contrail reduction. Improving confidence in our contrail models urgently requires novel observations of contrail properties and lifecycles from individual aircraft. The impact of aircraft on clouds is not limited to contrails forming in clear air. Over half of contrails form embedded in existing clouds and the particulates emitted by aircraft can affect cloud formation several days after they were released. These effects produce a cooling, potentially large enough to offset all other warming effects of aviation, but are not represented in aircraft-level models used for planning contrail avoidance strategies. There are few observational constraints of these effects, targeted observations of the impact of individual aircraft on cloud microphysics are required to assess them and to improve future model simulations. To address these uncertainties and around contrail formation, persistance and climate impact as well as aerosol-cloud interactions, COBALT has three core components: 1. A measurement campaign in the southern UK, combining an array of ground-based cameras with a steerable cloud radar, to make high resolution observations of contrail formation from individual aircraft. Guided by aircraft transponder information, these observations will be focused on contrails and clouds modified by aircraft, characterising contrail formation and perturbed cloud properties within the first few hours of their lifecycle. 2. Counterpart satellite observations, using novel techniques to characterise contrail and cloud development from an hour to several days behind the aircraft. Building on techniques for studying natural cirrus, this will produce a complete characterisation of the contrail lifecycle, along with the first estimate of the aviation aerosol impact on existing cirrus clouds at a global scale. 3. The complete lifecycle characterisation will be combined with flight data from aircraft operators to produce a unique dataset designed specifically for the evaluation of aircraft-level models of contrail formation. An initial focus will be placed on evaluating aircraft-scale models, as these are currently being used to plan aircraft diversions. A comparison of climate model parametrisations of contrail formation will assess the ability of the parametrisations to reproduce the wide-area (>1000km2) contrail observations taken by the camera array. Led by an inter-disciplinary team of scientists and engineers, with partners in key international research centres and industry groups, COBALT will provide the tools necessary to evaluate our current models and ability to avoid contrails, guiding future modelling and operational trials of sustainable fuels and contrail avoidance.