DC septum magnet for beam extraction

A septum magnet with DC excitation has been fabricated for slow beam extraction at TARN II. Its gap height, core length, and septum thickness are 25 mm, 1 m, and 9 mm, respectively. At the maximum excitation current of 2500 A in four-turn septum coils, corresponding to the magnetic field of 5 kG, the current density amounts to 78 A/mm² . Measured temperature rise in the air is less than 27°C at the septum coil under the condition that the flow rate of cooling water is 2.4 l/min for each coil. This is well below the tolerance for DC operation, although a slightly higher temperature rise is anticipated for real application in a vacuum chamber. The measured field homogeneity in the aperture is better than 0.8%. The leakage field strength at the entire region outside of the septum is less than 0.24% even at its largest point, which is considered to have no significant effect on the circulating beam