Magnetic Measurement Developments for Undulators

Abstract

FLASH2 is an extension of the present VUV-FEL facility at DESY. It includes a separate tunnel with a 12×2.5m=30m long planar hybrid undulator. The undulators have 31.4mm period length and 1T field at a minimum gap of 9mm. The paper presents recent progress in the magnetic measurements of these undulators. Several specific details of the measurement tools will be discussed like peculiarities in the Hall probe calibration and noise, positioning accuracy and synchronization of voltage measurement with probes movement during scan, noise issues of various voltage integrators for stretched wire and search coil measurements. HALL PROBE NOISE AND POSITIONING ACCURACY The undulator with a period length of 31.4mm and 1T field amplitude has 0.2T/mm field gradients at the zero crossings. Having a noise level of the Hall probe of about 20 μT, the positioning accuracy should be better than 20μT / 0.2T/mm = 100nm in order not to add noticeable noise from the positioning jitter. This applies especially when magnetic field measurements are performed on-the-fly, continuously moving through the undulator with triggering of the Hall probe voltage readout by the position encoder. For shorter period in-vacuum undulators this problem could be more severe. When scannig with a speed of 100mm/s, the trigger signal timing should be as accurate as 100nm / 100mm/s = 1us and any additional trigger synchronization logic inside the voltmeter, that adds delay or timing jitter, could significantly degrade measurement quality.