Development and testing of a three-period, subsize 2G AIMI MgB₂ planar undulator

Abstract

Compared to Nb3Sn- and NbTi-wound superconducting undulators (SCUs), MgB2-wound SCUs are of interest for future electron synchrotron beam light sources owing to their higher temperature operating margin and associated stability. In this study, a three-period undulator consisting of twelve racetrack coils wound with 2nd generation (2 G) multifilamentary advanced-internal-magnesium-infiltration MgB2 strands were fabricated and tested in liquid and gaseous helium (He) over a temperature range of 4.2 K–20 K. The coil winding cross sections (in each coil) were 5 mm wide and 4.8 mm thick. At 4.2 K, a critical current (I c) of 325.7 Amps produced a maximum undulator bore field of 1.19 T. It should be noted that the short, 3-period nature of the coil led to an asymmetry in the field profile (the maximum positive field was 1.19 T, the maximum negative was −0.25 T), suggesting a peak field of 0.72 T in the absence of end effects. Finite element modeling (FEM) results of simulations for a one meter long undulator of otherwise identical design gave 0.85 T (larger because of higher currents enabled by the lower field). But in any case, the I c value coil reached is 94% of that of the short sample (dictated by the 1.19 T positive field for the coil as tested). FEM was performed to study the magnetic field profile, which was validated experimentally. The magnetic field was measured using a Hall probe which was translated along the beam axis during measurement to explore the spatial field variation along the beam travel direction. The spatially alternating field was asymmetric, and the maximum field was more prominent in the positive direction than in the negative direction, the difference being due to broken symmetry, that is, short coil end effects. In this work, we show useful fields are possible for MgB2 undulators; the use of such conductors can allow a larger thermal margin and enable conduction-cooled operation.