Performance of a superconducting, high field subcentimeter undulator

A superconducting 8.80 mm wavelength undulator is under construction for the 500 nm free electron laser at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory. We present results on the design, construction and performance of this novel undulator structure. A field on axis of 0.51 T has been measured for a 4.40 mm gap, with a current 15% below the quench current. Our simple design focuses on minimizing the accumulation of errors by minimizing the numbers of parts and by using a ferromagnetic yoke. The magnetic field error is less than 0.30% rms as manufactured (without shimming). The third harmonic content is less than 0.1% of the fundamental.