Three dimensional physics in reversed field pinch and stellarator

Fusion research is motivated by the dilemma between the dramatic increase of energy demands and the limitation of total energy storage on earth. Thermal nuclear fusion has been developed addressing to this issue, along with many other ways of exploring new energy source and it is considered to be one of the most promising new energy sources for sustainable development of human civilization. Toroidal magnetic confiurations are the main magnetic fusion research field. There are mainly three different types of toroidal magnetic configurations: • The first type is tokamaks. Tokamaks are considered to be the most advanced toroidal configurations with most of the magnetic field being in the toroidal direction. The toroidal field is generated by external coils and the poloidal field is generated by both the plasma current induced externally and the bootstrap current generated by plasmas. The safety factor q in tokamaks is typically above 2. • The second type is stellarators. Stellarators feature complex coil design because the helical magnetic field is generated by external coils only. This makes it free of plasma current so that stellarators are free of current driven instabilities, which makes one of the biggest advantages for stellarators. However, stellarators suffer some drawbacks like low β value and complex design and manufacturing of field coils. The q profile is usually below 1. • The third type is reversed field pinches (RFPs). It has similar design with tokamaks. The magnetic field, however, is dominated by the poloidal component in the plasma edge. In fact, part of the toroidal field is generated via the so-called dynamo mechanism. Consequently, it could couple higher plasma current than tokamaks. The q profile is below 1 and becomes negative in the plasma edge. One of the main challenges for toroidal magnetic configurations is to maintain high plasma confinement properties. Particles and energy are constrained on the flux surfaces with losses via radial transport process across the nested flux surfaces. Transport study has been intensively carried on tokamaks on one dimension with the assumption of toroidal symmetry. Indeed, the design of tokamaks is to achieve toroidal symmetry. However, in real experiments, error fields or MHD activities could distort the magnetic surfaces, leading to the increase of transport properties. Among those factors, magnetic islands, being resistive MHD tearing modes, exist commonly in all fusion devices. The formation of magnetic islands relates to the reconnection process of magnetic field lines and this makes the system intrinsically three dimensions. (…)Programa Oficial de Doctorado en Plasmas y Fusión NuclearPresidente: Francesco Gnesotto.- Secretario: José Ramón Martín Solís.- Vocal: Jean Manie Noterdaem

An Alternating Direction Algorithm for Matrix Completion with Nonnegative Factors

This paper introduces an algorithm for the nonnegative matrix factorization-and-completion problem, which aims to find nonnegative low-rank matrices X and Y so that the product XY approximates a nonnegative data matrix M whose elements are partially known (to a certain accuracy). This problem aggregates two existing problems: (i) nonnegative matrix factorization where all entries of M are given, and (ii) low-rank matrix completion where nonnegativity is not required. By taking the advantages of both nonnegativity and low-rankness, one can generally obtain superior results than those of just using one of the two properties. We propose to solve the non-convex constrained least-squares problem using an algorithm based on the classic alternating direction augmented Lagrangian method. Preliminary convergence properties of the algorithm and numerical simulation results are presented. Compared to a recent algorithm for nonnegative matrix factorization, the proposed algorithm produces factorizations of similar quality using only about half of the matrix entries. On tasks of recovering incomplete grayscale and hyperspectral images, the proposed algorithm yields overall better qualities than those produced by two recent matrix-completion algorithms that do not exploit nonnegativity

Performance Limits of Fluid Antenna Systems

Fluid antenna represents a concept where a mechanically flexible antenna can switch its location freely within a given space. Recently, it has been reported that even with a tiny space, a single-antenna fluid antenna system (FAS) can outperform an L-antenna maximum ratio combining (MRC) system in terms of outage probability if the number of locations (or ports) the fluid antenna can be switched to, is large enough. This letter aims to study if extraordinary capacity can also be achieved by FAS with a small space. We do this by deriving the ergodic capacity, and a capacity lower bound. This letter also derives the level crossing rate (LCR) and average fade duration (AFD) for the FAS.Comment: 4 pages, 5 figure

Fluid Antenna Systems

Over the past decades, multiple antenna technologies have appeared in many different forms, most notably as multiple-input multiple-output (MIMO), to transform wireless communications for extraordinary diversity and multiplexing gains. The variety of technologies has been based on placing a number of antennas at fixed locations which dictates the fundamental limit on the achievable performance. By contrast, this paper envisages the scenario where the physical position of an antenna can be switched freely to one of the N positions over a fixed-length line space to pick up the strongest signal in the manner of traditional selection combining. We refer to this system as a fluid antenna system (FAS) for tremendous flexibility in its possible shape and position. The aim of this paper is to study the achievable performance of a single-antenna FAS system with a fixed length and N in arbitrarily correlated Rayleigh fading channels. Our contributions include exact and approximate closed-form expressions for the outage probability of FAS. We also derive an upper bound for the outage probability, from which it is shown that a single-antenna FAS given any arbitrarily small space can outperform an L-antenna maximum ratio combining (MRC) system if N is large enough. Our analysis also reveals the minimum required size of the FAS, and how large N is considered enough for the FAS to surpass MRC.Comment: 26 pages, 5 figure

A Functionalised Aptamer Electrochemical Biosensor Platform

The ability to utilise new knowledge of biomarkers from genomic and proteomic data will have a great impact on molecular diagnosis. Biomarker detection could be achieved by utilising a capture molecule that associates specifically with the target biomarker. The work described in this thesis focuses on a platform comprising a lysozyme binding aptamer and an amperometric electrode (an electrochemical aptasensor). To couple the binding reaction to a change in current, the aptamer is modified with a redox group, ferrocene. Two types of signalling aptamer were constructed, one comprised the aptamer self-assembled on gold and hybridised to a short complementary oligonucleotide carrying a ferrocene group. The second incorporated the binding sequence into a molecular beacon, one end of which self-assembled onto the electrode, the other end carried the ferrocene group. Both of these showed a lysozyme dependent change in current on a gold electrode. Further characterisation of the first aptasensor suggested that the nucleic acid formed a multilayer structure on the electrode surface and that lysozyme binding induced conformational change moved ferrocene close to the surface, increasing the current. In contrast, the second aptamer usually showed a decrease in current in the presence of lysozyme suggesting that the binding resulted in the ferrocene moving away from the surface. In order to evaluate the possible use of these aptasensors for continuous in vivo measurement, needle shaped microelectrodes arrays were produced and the beacon aptamer immobilised on the surface. These electrodes had high impedance which resulted in low sensitivity, however lysozyme binding could still be detected using electrochemical impedance spectroscopy with ferrocyanide in solution. These microspike arrays could also be used for glucose sensing following modification with glucose oxidase