First performance test of a 25 T cryogen-free superconducting magnet

Abstract

A 25 T cryogen-free superconducting magnet (25T-CSM) was developed and installed at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University. The 25T-CSM consists of a high-temperature superconducting (HTS) coil and a low-temperature superconducting (LTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with a reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. All the coils were impregnated using an epoxy resin for conduction cooling. Initially, a GdBa2Cu3Oy (Gd123) coil was designed as the HTS insert coil, and then a Bi2Sr2Ca2Cu3Oy (Bi2223) coil was also developed. The HTS insert and the LTS (CuNb/Nb3Sn and NbTi) outsert coils are cooled by two 4K GM and two GM/JT cryocoolers, respectively. The LTS coils successfully generated a central magnetic field of 14 T at an operating current of 854 A without any training quench. The Gd123 coil generated 10.15 T at an operating current of 132.6 A in the absence of a background field. Subsequently, the operating current of the Gd123 insert was increased in a step-by-step manner under a background field of 14 T. The Gd123 coil could be operated up to 124.0 A stably, which corresponds to 23.55 T, but quenched at around 124.6 A (23.61 T). The Bi2223 insert coil using a Ni-alloy reinforced Bi2223 tape successfully generated 11.48 T at an operation current of 204.7A in a stand-alone test and 24.57 T in a background field of 14 T. The differences between the calculated and the measured values of the central magnetic fields are about 0.4 T for the Gd123 insert and 0.1 T for the Bi2223 insert around 24 T.

Stable, predictable and training-free operation of superconducting Bi-2212 Rutherford cable racetrack coils at the wire current density of 1000 A/mm2

“…The baseline magnet technology for future high energy proton colliders is the 12-16 T wind-and-react, A vital concern for any HTS magnet is the need to detect quench so that active protection can be Important concerns with REBCO conductors, despite their high stability, are that they contain localized defects either pre-existing 15 or created during coil winding or during magnet operation 22 , that their critical current is anisotropic in magnetic field, and that this anisotropy depends on flux pinning and therefore REBCO processing conditions; these uncertainties mean that quench is often localized, unpredictable, unexpected, and catastrophic [21][22][23]27 . This is illustrated by a single-layer coil wound from a 2 m long REBCO single tape (2 mm wide, 100 µm in thickness with 40 µm Cu) with a known defected 2 cm section (I c = 48 A versus ~55 A for other sections at 77 K).…”

“…This coil went into a thermal run-away at 60 K at ~200 A, with the last recorded terminal voltage of 1.2 mV (DAQ rate at 10 samples/s) and the voltage across its defective section of 0.4 mV, resulting in unexpected burnout in another coil section. In spite of these dangers, the default quench detection method for REBCO magnets is still a voltage measurement in the mV range with a control limit of several or tens of mV [21][22][23][24][25] . It is thus no surprise that many REBCO magnets have been damaged during quench.…”

“…However, the paradox for HTS magnets is that, when quench does occur, it is a much bigger threat than in LTS magnets because normal zones are smaller, quench velocities much smaller, with the consequence of much higher energy density in much smaller normal zones. Several REBCO and Bi-2223 magnet systems have been degraded during quench [21][22][23][24][25][26][27] , likely when small normal zones generated too little voltage for warning, making active quench protection unable to prevent local overheating as magnetic energy turned into very localized heating. A key challenge of HTS magnets is thus the timely detection of small hot spots that grow only at several cm/s, rather than the two orders of magnitude larger rate in Nb-Ti and Nb 3 Sn magnets.…”

NMR spectroscopy up to 35.2 T using a series-connected hybrid magnet

“…While HTS materials were discovered more than 30 years ago, progress in using these materials at fields above 23.5 T has been slow. An NMR magnet with an HTS insert operated for a while at 1020 MHz at Riken [4] and a non-NMR magnet with an HTS insert became operational at 24.5 T in Sendai, Japan early in 2017 [22]. Consequently, a resistive/SC hybrid was clearly the only option when this project was initiated.…”

Summary of the FESAC Transformative Enabling Capabilities Panel Report

“…Continued R&D following the discovery of high-temperature superconductivity in the late 1980's has resulted in superconductors that can now be considered for high field magnetic fusion applications. The high field and temperature properties of HTS allow the possibility of eliminating cryogens 20 and enabling the use of demountable resistive joints 21 . In addition, the high critical temperature could also allow operating in a nuclear heating environment significantly higher than allowed in low-temperature superconductor (LTS) magnets.…”