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A 208-mm inner diameter, 62-mm-tall, wind-and-react Nb3 Sn prototype magnet was tested to demonstrate its suitability for use in a compact superconducting cyclotron. The magnet and its 270 kg iron return yoke were cooled together by conduction using a 2-stage Gifford-McMahon cycle refrigerator. This paper presents thermal, electrical and electromagnetic data collected during the tests. Although the radiation shield for the assembly cooled to 47 K within one day, a total of nine days were needed to cool the magnet assembly to below 4 K, at an ultimate heat load of 0.2 W. The dominant heat load at each stage of the cold head was due to thermal conduction along the current leads. The coil was charged without quench to 211 A, generating a pole tip magnetic induction of 3 T. The test demonstrates the technical feasibility to design, manufacture and operate compact superconducting cyclotron magnets at currents in excess of 200 A. The results are being used to improve the design for future high-field, conduction-cooled superconducting cyclotrons.