Design and Construction Progress of a Cryogen-Free, Shielded 23.5-T REBCO Magnet for Benchtop 1-GHz NMR Spectroscopy.

We present the detailed design and construction progress of a cryogen-free, shielded 23.5-T, 30-mm warm-bore REBCO magnet for use in benchtop 1-GHz microcoil nuclear magnetic resonance (NMR) spectroscopy. The magnet design, initially based on constant current density, exhibits a high homogeneity of <0.1 ppm over a 10-mm diameter, 10-mm length cylindrical volume or 10-mm diameter of spherical volume (DSV). This benchtop magnet features an all-REBCO composition and operates at above 4.2 K without the use of cryogens. A surface-shunting no-insulation (NI) winding technique is employed to prevent quenching during potential fault modes, enabling self-protection. The magnet comprises 50 double pancake coils, each co-wound with 3-mm width REBCO and Hastelloy tapes. This co-winding design reduces overall current density and thus stress, and increases mechanical strength within the winding. Additionally, a vibration-reduced, conduction-cooled cryostat, equipped with an iron yoke for fringe field shielding, has been designed and is currently under construction. The 5-gauss fringe field radius is reduced to 1.25 m due to the iron shield. We analytically investigate electromagnetic, thermal, and mechanical properties of the REBCO coils with the iron shield.