An 8-km long MgB
 2
 wire for a prototype klystron magnet was made and evaluated. The wire was made by a typical in situ method; it has 10 filaments and 0.67 mm in outer diameter. The homogeneity of I
 c
 of this wire was evaluated by several methods. Deviation of I
 c
 values in short sample wires was very small. In addition, the current sharing temperature of the MgB
 2
 magnet (made of two reels of wire 2.9 km long each) agreed well with the estimated value of the I
 c
-B-T properties in short sample wires. Based on the obtained results, it can be said that the I
 c
 properties of the entire wire length are quite uniform.

/pdf/performance-of-mgb2-superconductor-developed-for-high-2ascilkcij.pdf

Performance of MgB2 Superconductor Developed for High-Efficiency Klystron Applications

A wind-and-react MgB2 solenoid magnet for klystrons has been developed. While the current normal-conducting (Cu) magnet consumes 20 kW per magnet, this MgB2 magnet consumes less than 3 kW in refrigerator power. The conduction-cooled half coil of the magnet is 337 mm in inner diameter; the winding pack, 19.4 mm wide × 136.6 mm high, uses 2.7 km of 10 filament circular conductor, which is insulated with glass 0.83 mm in diameter, and is reacted after being wound onto a stainless steel bobbin. The coil has Cu plates of 0.2 mm in thickness between each coil layer and on the inner and outer sides. The magnet has two coils and produces 0.8 T in the center and its stored energy is 11.8 kJ. Together with the above-mentioned coil structure, these coils can consume stored energy in itself at quench without a special quench protection system. A performance test of the magnet was successful.

Development of Prototype MGB 2 Superconducting Solenoid Magnet for High-Efficiency Klystron Applications

Current feeders system (CFS) is one of the most critical part of a large-scale superconducting (SC) magnets based machines like a tokamak or accelerator. One end of the SC feeder is connected to magnets and another end is connected to the power supply via optimised current leads (CL) for energizing the magnets. So far, low temperature superconductor (LTS) is the popular choice for such applications. For high temperature superconductor (HTS) applications, cuprate oxide based coated conductors or magnesium diboride (MgB2) wires are the options. MgB2 despite of its superconducting transition temperature of 39 K,is now gaining wide attention for SC feeders system due to its lower raw material cost and ease of availability than 2G HTS tapes. To assess its suitability, we report 1-D thermo-hydraulic study of 10 kA rated MgB2 current feeders system cooled with 20 K helium for SST-1 tokamak as a case study. For varying mass flow rate, temperature margin along the feeder are calculated considering helium at 20 K, 4 bar (a) with cooling inlet from magnet side. Such a feeder yields a temperature margin of 6 K–16 K across its entire length depending up on mass flow rate of helium. There is also a possibility of cooling the binary current lead (HTS + MgB2) from cold helium coming out from the feeder. From our study, MgB2 current feeder system provides benefits of higher temperature margin, lower mass flow requirement and cryogenic savings by use of 20 K helium as coolant compared to LTS based feeders. In future, it could provide a safe, reliable and cryogenic operational cost saving solution for SC current feeders for Tokamak applications.

Preliminary Design and Analysis of 20 K Helium Cooled MgB2 Based Superconducting Current Feeder System for Tokamak Application

High-temperature superconductors (HTS) and MgB2 may potentially improve the usability of superconducting magnets dramatically owing to their large energy margin. When HTS and MgB2 wires are used for magnets operated in the persistent current mode, such as in magnetic resonance imaging (MRI) scanners, the electric field generated in the wires must be lower than 10−10 V m−1. In this paper, critical current density, J c, defined at an electric field criterion of 10−10 V m−1 is evaluated from a magnetisation decay measurement for state-of-the-art monofilamentary MgB2 wires. By using the obtained J c, a critical line of our multifilamentary MgB2 wire is shown on the temperature-magnetic field plane. Here, the critical line is defined as a line on which an electric field of 10−10 V m−1 is generated at a coil current density of 150 A mm−2. The area inside the critical line is demonstrated to be large enough to fulfil the requirement of 1.5 T MRI scanners operated at 10K–15K. In addition, the iso energy-margin lines are shown on the temperature-magnetic field plane and compared with those of NbTi wires. The MgB2 wire has an order of magnitude greater energy margin than the NbTi wires in most of the area inside the critical line. This suggests that the MgB2 wire is highly unlikely to be quenched due to mechanical disturbances.

https://iopscience.iop.org/article/10.1088/1361-6668/abced3/pdf

Critical current density defined at low electric field criterion and energy margin of superconducting MgB 2 wires in wide temperature and magnetic field range

An MgB2 superconducting solenoid magnet has been developed for electron beam focusing in X-band (12 GHz) klystrons for particle accelerator RF systems, to provide a central field of 0.8 T at 57 A and at ≥20 K. It has successfully realized significant AC-plug power saving in one order of magnitude compared with that for a conventional Cu solenoid magnet. The large-scale application may be expected for the Compact Linear Collider (CLIC) project proposed as a future accelerator candidate at CERN. It requires ~5,000 klystrons, and the MgB2 magnet application will realize significant AC-plug power saving. This paper describes progress in a prototype MgB2 superconducting solenoid magnet development and discusses the future prospect.

/pdf/applying-superconducting-magnet-technology-for-high-1b4ij6mkhc.pdf

Tomoyuki Koga

Papers

Performance of MgB2 Superconductor Developed for High-Efficiency Klystron Applications

Development of Prototype MGB 2 Superconducting Solenoid Magnet for High-Efficiency Klystron Applications

Applying Superconducting Magnet Technology for High-Efficiency Klystrons in Particle Accelerator RF Systems