Design, Construction, and First Testing of a 41.5 T All-Resistive Magnet at the NHMFL in Tallahassee

Abstract

The National High Magnetic Field Laboratory (NHMFL) has developed the design of its next generation 32-mm bore resistive magnet. This magnet upgrade includes enlarging the size of the outmost coil from a 610 mm to a 1000 mm outer diameter and from a 430 mm to a 731 mm maximum height. First, a general design optimization was performed to decide on the number, the geometries and the materials of the nested resistive coils. As a result of that analysis, a six coil design was chosen (upgraded from four coils) with the innermost two coils electrically connected in parallel and the remaining coils connected in series. Next, a series of systematic detailed analyses of the winding pack were iterated coil by coil as well as section by section along each coil axis employing current density grading all in an effort to achieve a new world record field of 41.5 Tesla. The project was executed within an aggressive 2.5 years total time line from the start of the earnest design to completion of the fabrication drawing package taking the first year followed by a 1.5 year period for procurement and construction. Finally, first testing of this water-cooled magnet has just been concluded in August 2017, including the collection of hydraulic pump curves and a first successful charge to the full current of 48 kA. An extrapolated hall-probe measurement suggests this 32-mm bore all-resistive magnet generated a $\text{41.4}{{\,\text{T}\,(}}+ / -\text{0.2}{{\,\text{T})}}$ peak field consuming 31.9 MW of electrical power leaving a 5% voltage margin towards the available 700 V limit. Minimal further winding adjustments accompanied by accurate field calibrations are planned next to tune and confirm the magnet to 41.5 T before it is delivered to the NHMFL user program in the near future.