Measured Performance of Different Solders in Bi2223/Ag Current Leads

Abstract

In superconducting magnet systems the current leads are usually divided into two parts. Normal metal like brass or copper is often used in the first part from the room temperature to the temperature of the radiation shield. The second part down to the magnet is made of high temperature superconductors (HTS). HTS leads can reduce the conductive heat load because they have poor thermal conductivity. Since HTS wires are lossless with direct current and have fair tolerance to the magnetic field only Ohmic losses are generated in the contact resistances at the current terminals. Thus, efficient current leads require that appropriate solders are used to reduce the contact resistances. In this paper, the thermal and contact resistances as well as thermal conduction losses of Bi-2223/Ag current leads are experimentally investigated using indium-and tin-based solders at operation temperatures between 20 and 77 K. Finally, the measured data is utilized to design an efficient HTS current lead to the current range of 0-1000 A.

Key designs of a short-bore and cryogen-free high temperature superconducting magnet system for 14 T whole-body MRI

“…10 cm overlapping length is about 20 nano-Ohm at 77 K [21]. As a result, we have about 30 µΩ resistance for the entire magnet (for operating temperature less than 77 K, the resistance is even smaller [49]). By the operating current of 185 A, the Ohmic heating power turns out to be about 1.0 W. Note that longer overlapping area can further reduce the resistance, so the total Ohmic heating can be lower than 1.0 W.…”

Winding, cooling and initial testing of a 10 H superconducting MgB2coil for an induction heater

“…The current leads conducting the current from the terminals at ambient temperature to the coils at below 20 K were made up of two parts: one copper current lead between the terminals and the top plate (at the temperature of the first stage), and one HTS current lead between the first stage and the coil (at the temperature of the second stage) [7]. Both current leads were designed to minimize the heat load (from conduction and resistive losses) to the respective cooling stages at 250 A.…”

Influence of Self-Heating on Measured $n$-Value

“…A 30 cm long sample was placed into a nylon groove and veiled with 20 layers of superinsulation to make the situation as thermally adiabatic as possible. Both ends of the sample were fixed to copper current terminals with 97Sn3Ag solder which is good option at low temperatures [8]. Voltage was measured using two voltage taps 98 mm apart using HP nanovolt meter.…”