Search

All high field superconductors producing magnetic fields above 12 T are brittle. Nevertheless, they will probably be the materials of choice in commercial tokamaks because the fusion power density in a tokamak scales as the fourth power of magnetic field. Here we propose using robust, ductile superconductors during the reactor commissioning phase in order to avoid brittle magnet failure while operational safety margins are being established. Here we use the PROCESS systems code to inform development strategy and to provide detailed capital-cost-minimised tokamak power plant designs. We propose building a 'demonstrator' tokamak with an electric power output of 100 MWe, a plasma fusion gain Qplasma = 17, a net gain Qnet = 1.3, a cost of electricity (COE) of \$ 1148 (2021 US) per MWh (at 75 % availability) and high temperature superconducting operational TF magnets producing 5.4 T on-axis and 12.5 T peak-field. It uses Nb-Ti training magnets and will cost about \$ 9.75 Bn (2021 US). An equivalent 500 MWe plant has a COE of \$ 608 per MW suggesting that large tokamaks may eventually dominate the commercial market. We consider a range of designs optimised for capital cost (as the reactors considered are pilot plants) consisting of both 100 MWe and 500 MWe plants with each of two approaches for the magnets: training and upgrading. With training magnets, the plant is cost-optimised for REBCO TF magnets. For a 100 MWe plant, the Nb-Ti training magnets typically produce 70 % peak field on the toroidal field coils compared to REBCO magnets, 65 % peak field on the central solenoid and cost approx. 10 % of the total machine cost. Training magnets could in principle be reused for each of say 10 subsequent (commercial) machines and hence at 1 % bring only marginal additional cost., Comment: 71 pages. 14 Tables. 11 Figures

We investigate the in-field critical current density Jc(B) of SNS Josephson Junctions (JJs) and polycrystalline superconductors with grain boundaries modelled as Josephson type planar defects, both analytically and through computational time-dependent Ginzburg-Landau (TDGL) simulations in 2D and 3D. For very narrow SNS JJs, we derive analytic expressions for Jc(B) that to our knowledge are the first high field solutions for Jc(B) for JJs across the entire field range up to the effective upper critical field. They generalise the well-known (low-field) exponential junction thickness dependence for Jc from de Gennes, often used in the Josephson relation. We extend the new analytic expressions to describe wider junctions and confirm their validity using TDGL optimised for high field applications. They provide an explanation for the B^{-0.6} field dependence found for Jc(B) in high temperature superconductors, and the Kramer dependence and inverse power-law grain size widely found in many low temperature superconductors., Comment: 39 pages, 11 figures and 1 table

YBCO fabrics composed of nanowires, produced by solution blow spinning (SBS) are so brittle that the Lorentz force produced by induced currents can be strong enough to damage them. On the other hand, it is known that silver addition improves the mechanical and flux pinning properties of ceramic superconductors. Thus, in this work, we show how we successfully obtained a polymeric precursor solution containing YBCO$+$Ag salts, which can be spun by the SBS route to produce ceramic samples. Yttrium, barium, copper, and silver metal acetates, and polyvinylpyrrolidone (PVP) (in a ratio of 5:1wt [PVP:acetates]) were dissolved in a solution with 61.5 wt\% of methanol, 12 wt\% of propionic acid, and 26.5 wt\% of ammonium hydroxide, together with 6 wt\% of PVP in solution. Three different amounts of silver (10 wt\%, 20 wt\%, and 30 wt\%) were used in YBa$_2$Cu$_3$O$_{7-x}$. The TGA characterizations revealed a lowering of crystallization and partial melting temperatures by about \SI{30}{\celsius}. SEM images show that after burning out the polymer, a fabric composed of nanowires of diameters up to \SI{380}{\nano \metre} is produced. However, after the sintering process at \SI{925}{\celsius} for \SI{1}{\hour}, the nanowires shrink into a porous-like sample., Comment: 6 pages, 5 figures, submited to Ceramics International

We present a detailed spectroscopic investigation of a thermal $^{87}$Rb atomic vapour in magnetic fields up to 0.4T in the Voigt geometry. We fit experimental spectra with our theoretical model \textit{ElecSus} and find excellent quantitative agreement, with RMS errors of $\sim 0.3$%. We extract the magnetic field strength and the angle between the polarisation of the light and the magnetic field from the atomic signal and find excellent agreement to within $\sim 1$% with a commercial Hall probe. Finally, we present an investigation of the relative sensitivity of this technique to variations in the field strength and angle with a view to enabling atom-based high-field vector magnetometry., Comment: 9 pages, 8 figures

Maxwell's four differential equations describing electromagnetism are amongst the most famous equations in science. Feynman said that they provide four of the seven fundamental laws of classical physics. In this paper, we derive Maxwell's equations using a well-established approach for deriving time-dependent differential equations from static laws. The derivation uses the standard Heaviside notation. It assumes conservation of charge and that Coulomb's law of electrostatics and Ampere's law of magnetostatics are both correct as a function of time when they are limited to describing a local system. It is analogous to deriving the differential equation of motion for sound, assuming conservation of mass and that Hooke's static law of elasticity holds for a system in local equilibrium. We demonstrate that Faraday's law can be derived without any relativistic assumptions about Lorentz invariance and discuss creation of charge.

Flux pinning scaling laws were developed to explain the observed increase in the critical current density Jc caused by increased density of grain boundaries in polycrystalline low temperature superconductors (LTS) such as Nb3Sn. For four decades they have provided the framework for the successful development of the LTS materials that carry high Jc in high magnetic fields. However, the discovery of the weak-link problem suggested that transmission of supercurrent flow through the grain boundaries limited Jc in high temperature superconductors (HTS) such as YBa2Cu3O7-{\delta} (YBCO). Here we provide evidence that a single mechanism - flux flow along grain boundaries - confirmed by time-dependent Ginzburg-Landau (TDGL) computational visualisation in LTS materials, explains the functional form of Jc in polycrystalline LTS Nb3Sn and HTS YBCO and (Bi,Pb)2Sr2Can-1CunOx (BiSCCO) materials in low and high magnetic fields. Data presented show that standard flux pinning theory cannot explain comprehensive Jc data for YBCO because Jc is a function of strain but the superconducting properties are not. We conclude that grain boundaries are narrow and metallic in Nb3Sn and YBCO but wide and semiconducting in BiSCCO. Strain alters Jc by changing the superconducting properties of the grains in Nb3Sn but by changing the grain boundaries in YBCO, Comment: The paper has been withdrawn following revised publication in SuST