State Key Laboratory of Intense Pulsed Radiation Simulation and Effect ( Northwest Institute of Nuclear Technology ) , Xi ’ an 710024 , China

Vibrating wire technique is a promising measurement method for small-aperture Permanent Magnet Quadrupoles (PMQs) in linear accelerators and scanning nuclear microprobes. In this paper, we describe the improved vibrating wire setup for measuring an individual PMQ with the minimum aperture of several millimeters. This setup is aiming at measuring the magnetic center. The advantage of this setup is that any mechanical measurement on the wire, which may be the main error source, is avoided. Experiments of the 20 mm-aperture Halbachtype PMQs for Xi’an Proton Application Facility (XiPAF) DTL has been carried out. The research results of the magnetic center measurements show a precision of about 10 μm and robustness against the background magnetic field. Results of the magnetic center and field multipoles measurements agree with the ones obtained from the rotating coil. INTRODUCTION The 325 MHz Alvarez DTL for the Xi’an Proton Application Facility (XiPAF) will be used to accelerate the Hbeam from 3 to 7 MeV. As shown in Figure 1, the 20 mm-aperture Halbach-type Permanent Magnet Quadrupoles (PMQs) are used in the DTL. The tolerance for the PMQs are list in Table 1. The measurement of the magnetic center is very important. The misalignment of the magnetic center will cause the beam to deviate. Rotating coil is the conventional method to achieve the magnetic center position as well as the multipole contents of the quadrupoles. However, it is difficult to measure the magnets with small apertures of several millimeters. To achieve high efficiency, the apertures of the magnets has been smaller and smaller in new linear accelerators. A new measuring method which is free from the restriction of the aperture should be developed. Vibrating wire method is a high accuracy method adapt for measuring the magnetic center of the small-aperture magnets. This method is quite sensitive to make the wire coincides with the magnetic center. However, it is difficult to relate the wire to external fiducials. The improved fiducialization method is described in ref [1]. The advantage of this method is that the measurement of the wire position, which may be the main error source, is no longer needed. This method has been adopted to measure the XiPAF PMQs. Figure 1: The PMQ to be used in XiPAF. Table 1: PMQ Tolerances Errors Name Values PMQ position δx, δy ± 0.17 mm PMQ pitch and yaw φx, φy ± 3 deg PMQ roll φz ± 0.6 deg PMQ integrated gradient ΔGl/Gl ± 1.5 % PMQ multipole contents Bn/B2 @R=8 mm n=3...15 1% MEASUREMENT SETUP As shown in Figure 2, a vibrating wire setup was constructed as described in [1]. A 1.2 m-long wire is stretched vertically by a plumb. The plumb is submerged into the water in order to rebalancing quickly after some disturbances. The 100 μm-diameter wire carries the sinusoidal current. The function generator is used to excite the wire to vibrate in the fundamental mode. The lower end of the wire is connected to the function generator, and the upper end is connected to the ground in the consideration of isolation. The fixed point of the plumb is separated from the lower end of the wire. The extra slack between the fixed point and the lower end is long enough in order to avoid disturbing the hanging plumb. The PMQ to be measured is on the top of the rotary table. The rotary table is installed on the mover in order to move in x and y directions. A vertical linear guide is beside the mover. It is used to check the perpendicularity of the PMQ. Two optocouplers, mounted in the perpendicular directions, are used as the wire vibration detectors to transduce the wire displacement into an electrical voltage. The signals from the sensor are sent to an oscilloscope. Two dial indicators are used to measure the magnet position change. _________________________________________ † email address zhengsx@tsinghua.edu.cn TUPMB009 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 1124 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s 07 Accelerator Technology T09 Room-temperature Magnets Figure 2: The vibrating wire setup. EXPERIMENTS AND RESULTS Robustness in the Background Magnetic Field In Ref. [1], it has been illustrated that the results of the method mentioned will not be influenced by a stable background magnetic field. As shown in Figure 3, a 0.5 T dipole magnet is placed beside the set up. The magnetic center of one PMQ is measured with and without the dipole magnet beside. As shown in Figure 4, the difference between the results are smaller than 10 μm. Figure 3: Experiments on the influence of background magnetic field. -0.085 -0.080 -0.075 -0.070 90 °-2 70 ° Y 90 °-2 70 ° X 0° -18 0° Y 0° -18 0° X Without the magnet With the magnet