NSLS-II Magnetic Measurement System Facility

The National Synchrotron Light Source II at Brookhaven National Laboratory (BNL) is a storage ring of 3 GeV electron beam energy that will provide ultra high brightness and flux and exceptional beam stability using advanced insertion devices (IDs). The magnetic characterization of these devices is important to achieve these performances. In order to perform precise and accurate measurements of the NSLS-II IDs, several recent improvements on actual magnetic measurement system have been developed. The system includes a flipping coil bench to measure the first and second magnetic field integrals and a Hall probe bench to perform magnetic field measurements of different types of IDs. The main features of the hardware system, control program and the recent improvements in the magnetic measurements system are presented. INTRODUCTION All insertion devices for NSLS-II have been bought from different vendors. It is planned to measure these devices at NSLS-II in order to confirm vendors. Our magnetic measurements are the final verification of the complex design and fabrication process, so that proper certification of device can be made. The magnetic measurement system facility at BNL consists in a Hall probe bench MMB-6500, built by Kugler, GmbH and an Integrated Field Measurement System (IFMS), provided by ADC [1]. HALL PROBE BENCH The Hall probe mapping bench has an ultra-precision granite guide beam. There are nine conrolled axes. The air-ride Z axis has a travel distance of 6.5 meters. The trajectory deviation for the Z axis carriage is ± 7 μm and the positioning accuracy is ± 1 μm. The X and Y axes are high precision stages. Mounted to the Y stage carriage is a rotary stage. Mounted to the rotary stage is a small X-Y stage. Mounted to that is a pair of goniometers. Thus the extended Hall probe can be positioned in any orientation and then translated (see Fig. 1). The Z’ axis is used as a follower stage for transporting the cables, so that cable loading does not affect the Z axis trajectory. Instrumentation is also carried on the follower stage (see Fig. 2). The Z axis has two encoders. A linear scale encoder is used for the motion control and a laser interferometric encoder is used for generating triggers. A Delta Tau controller is used for motion control. A Senis 3D probe is used as the Hall sensor. Each Hall sensor, arranged along X-axis, has been accurately calibrated to provide a calibration curve of its response to the strength of the magnetic field up 1.9 Tesla. The magnetic field sensitive volume for each sensor is 150x150x1 μm and the angular accuracy of axes with respect to the reference surface is about of ± 2°. Figure 1: A: Machine coordinate system. Axis movements viewed from undulator towards granite. B: Arrangement of the goniometer head axes V, W, A, B and C. C: Arrangement of the three main linear axes X, Y, Z and auxiliary axis Z’. In order to improve the accuracy and the repeatability of the Hall probe measurements, the electronics of the data acquisition have been upgraded. In particular a new time synchronization system has been developed in order to optimize the first integral repeatability and the precision of magnetic field measurements. Figure 3 shows the vertical field of a Damping Wiggler with period length of 100 mm at the minimum gap of 15 mm (blue line) and the field difference between two consecutive Hall scans (red line), which represents an absolute error of the magnetic field, measured with the initial triggering method. This error is a periodic and systematic curve with precise harmonic frequencies (see Fig. 4). The peak-to-peak amplitude is about of 30 G and occurs around the magnetic field gradient. It is a strong indication of a small jitter in the start trigger of the Hall probe measurements. This trigger jitter was reduced by changing from a motion controller generated trigger to a trigger generated by external Hardware, the parallel digital signal from the laser encoder is processed in external logic which feeds a trigger signal into an FPGA aboard a National Instruments CompactRIO real-time PAC. The THPAC15 Proceedings of PAC2013, Pasadena, CA USA ISBN 978-3-95450-138-