Bi-2212 is the only high temperature superconducting wire with the round geometry. It is multifilamentary, available in a wide range of fine filaments and twisted filament architectures and can be made into Rutherford and other cables. The properties of Bi-2212 conductors depend on powder quality, conductor fabrication and heat treatment. The heat treatment is still complex but much better understood, particularly the vital parameters of the maximum heat treatment temperature ( T max ), time-in-the-melt ( t melt ) and the cooling rate as Bi-2212 reforms on cooling. Here we report on the performance and microstructure variation with heat treatments for more than a dozen wires made with powders produced by Engi-Mat in recent years. Wire architectures include 37 × 18, 55 × 18 and 85x18 and wire diameters range from 0.8 to 1.0 mm. T max was varied between 884 and 897 °C. Wires with smaller filament diameter showed a peak JE at the low end of T ma x and also a JE that was more sensitive to T max . JE ( T max ) plots for all recent wires show a plateau between T max of 886 and 894 °C, where JE (4.2 K, 5 T) is 1100–1400 A/mm 2 . Some wires with filament size of 13–15 μm showed a 10 °C heat treatment window ( Δ T max ) with a plateau JE (4.2 K, 5 T) of about 1100 A/mm 2 .

Scandate cathodes represent a next-generation performance enhancement over current state-of-the-art tungsten emitters (i.e., the M-type dispenser cathode). The improvement in performance of scandate cathodes is attributed to a lower work function, which yields longer lifetimes, higher power, and/or higher brightness. In this study, we report on a series of scandate cathode emitters fabricated from a novel nano-scandia/tungsten composite powder. The work functions of the cathode surfaces have been measured directly by contact potential difference using a Kelvin probe. These are the first reported work function values for scandate cathodes to be measured directly (in situ) at high temperatures rather than calculated from emission current density via the Richardson-Dushman equation. Several scandate cathodes were determined to have work function values less than that of standard M-type cathodes, with a difference up to ~ 0.4 eV.

Scandate cathodes have the potential to replace conventional cathodes because of their improved emission characteristics. Nevertheless, it remains a challenge in this field to produce scandate cathodes with uniform surface emission in a reproducible manner. In this study, we report on scandate cathode emitters prepared from a novel nano-scandia/tungsten composite powder. Processing improvements have been developed to yield high-quality emitter surfaces. Emission testing demonstrates a greater than 200°C decrease (improvement) in knee temperature versus the standard M-type cathode. There is an on-going effort to quantify the work function of the cathode surface prior to and following activation.

Scandate cathodes have long attracted attention due to their higher emission current density, apparent low work function, and anticipated improved longevity compared to conventional cathodes. However, scandate cathodes have not been applied as extensively as their potential would imply. In this study, the compositional homogeneity of a scandia/tungsten composite powder was investigated by scanning electron microscopy, X-ray energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. From these techniques, a sample providing the optimal coverage of tungsten by scandia particles was then chosen. A cathode fabricated from this sample displayed a greater than 200°C decrease (improvement) in knee temperature versus the standard M-type cathode.