Search

The ac loss characteristics of superconducting wires were obtained and plotted against the field ramp rate of magnet operation. The observations were carried out via an isothermal calorimetric technique on single-strand wires made of niobium titanium, internal-tin and modified jelly-roll (MJR) internal-tin. The results confirmed numerical predictions except for the MJR sample which showed a local maximum during a slow field changing rate.

Further studies on the ramp-rate constraint manifested by the conductors of the US-Demonstration Poloidal Coil (US-DPC) were carried out on a 27-strand cable-in-conduit conductor. The experiment, which simulated the US-DPC test conditions, involved a ramped transport current tuned to a background field with a field ramp rate of 0.5T/s tto 2T/s. Although it did not identify the cause of the ramp-rate limitations, it led to a semi-empirical equation that is valid for both the US-DPC and experimental data.

Abstract: Evaporation of Ag in the presence of an electron cyclotron resonance (ECR) oxygen plasma was used to deposit Ag2−x O films with a range of stoichiometries onto r-plane sapphire substrates. A quartz crystal oscillator (QCO) was used to accurately measure the silver and oxygen arrival rates and establish the O/Ag flux ratio needed to produce films with nominal Ag2O stoichiometry. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis indicates that the Ag2−x O films are not single phase but contain signatures of coexisting Ag2O and AgO components. XRD shows that the lattice matching with the r-plane sapphire substrate causes the Ag2O phase to grow with <002> heteroepitaxial crystallites coexisting with crystallites having <111> normal and random in-plane orientation. The AgO phase also forms with crystallites having <002> heteroepitaxy as well as crystallites with <111> normal and random in-plane orientation. The mixed phase Ag2−x O films exhibit approximately 77% optical transmission over the visible range (500 nm to 700 nm) and have a single absorption edge near 3.3 eV. Four-point van der Pauw conductivity and Hall effect measurements indicate that the Ag2−x O films are p-type with a conductivity on the order of 3×10−3 Ω−1 cm−1. [Copyright &y& Elsevier]