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Test of horizontal field measurements using two-axis Hall probes at the APS Magnetic Measurement Facility

The free-electron laser (FEL) project at the Advanced Photon Source (APS) will use a 400-MeV particle beam from the APS linac with RMS beam transverse size of 100 micrometers and requires very high performance of the insertion devices in order to achieve high intensity radiation. Averaged over period, the trajectory must deviate from the ideal on-axis trajectory by not more than 10% of the RMS beam size. Meaning that the second field integral should be straight within {+-}1,300 G-cm{sup 2} over the length of the device for both horizontal and vertical directions for the 400-MeV particle beam. Under such conditions, special attention should be given to the measurement technique and tuning. Two types of probes were tested to examine the possibility to measure small horizontal field in presence of strong (up to 1 T) vertical magnetic field without the distortion associated with planar Hall probe effect. They are a two-axis Sentron analog Hall transducer and a two-axis Bell probe. The Sentron probe is a new type of Hall probe, a so-called vertical Hall device, which is sensitive to the magnetic field parallel to the chip plan. The Bell probe is a conventional-type probe sensitive to the magnetic field perpendicular to the chip plane. A flipping coil and an 81-mm-long moving coil were used to make the reference measurements. Insertion device Undulator A {number_sign}3 was used for these measurements with a gap of 11 mm and a peak field of about 0.85 T.