The development of magnetic field measurement system for drift-tube linac quadrupole

In the China Spallation Neutron Source (CSNS) linac, a conventional 324 MHz drift-tube linac (DTL) accelerating an H− ion beam from 3 MeV to 80 MeV has been designed and manufactured. The electromagnetic quadrupoles (EMQs) are widely used in a DTL accelerator. The main challenge of DTLQ׳s structure is to house a strong gradient EMQ in the much reduced space of the drift-tube (DT). To verify the DTLQ׳s design specifications and fabrication quality, a precision harmonic coil measurement system has been developed, which is based on the high precision movement platform, the harmonic coil with ceramic frame and the special method to make the harmonic coil and the quadrupoles coaxial. After more than one year׳s continuous running, the magnetic field measurement system still performs accurately and stably. The field measurement of more than one hundred DTLQ has been finished. The components and function of the measurement system, the key point of the technology and the repeatability of the measurement results are described in this paper.