Design And Implementation Of Dynamic Magnetic Measuring System For A Kicker Magnet Applied In A Proton Therapy Facility

Kicker is a kind of special dipole magnet working in fast pulse mode,which is mainly used for fast small-angle deflection on beams in the beamline.In HUST-PTF(Huazhong University of Science and Technology-Proton Therapy Facility),a kicker magnet locating upstream of the degrader is used for fast beam switch during the spot scanning and energy modulation processes.In order to reduce the irradiation uncertainty to patients,switch time must be less than 100 μs.When the exciting current is triggered on,the proton beam will be deflected to the Faraday cup upstream of the degrader.When the exciting current is triggered off,the proton beam will be delivered to treatment rooms through the beamline.Fast deflection function of the kicker magnet relies on the integral magnetic field and its dynamic response feature.Dynamic properties of the kicker magnet can not be obtained by the static measurement,which means that the fast deflection function can not be verified.Therefore,dynamic measurement experiments under pulsed current are needed to be performed.In order to measure the integral field and the dynamic performance of the kicker magnet,a magnetic measurement system based on the induction coil method is designed and implemented,with PCB coils array technique applied in the kicker magnet measurement for the first time in China.The core components of the system are the induction coil and the integrator.In this dissertation,comparative studies of the two components are carried out: the induced voltage signals are obtained by hand-wound coils or PCB coils and integrated by analog or digital integrators.Different methods are adopted by hand-wound coil scheme and PCB coils scheme to measure the magnetic field.In the hand-wound coil scheme,integral field at each position is measured and then compared to calculate the uniformity;while in the PCB coils scheme,the uniformity is acquired by measuring the differential signal between array coils.Comparing the magnetic measurement results of different schemes,it is found that:(1)the PCB array coils scheme has higher geometric accuracy due to the circuit etching process,thus the measurement error(0.072%)is much smaller than the hand-wound coils scheme(10.61%)without calibration;(2)the measurement uniformity of PCB coils scheme within a single pulse is barely affected by the repeatability of the power supply as a result of its differential measurement method,so the standard deviation of its uniformity(0.00574%)is also much less than the hand-wound single group coil scheme(0.176%).When the digital integration scheme is used,the sampling noise of the oscilloscope will cause distortion of the integrated waveform and it is difficult to eliminate it through data processing;while in the analog integrator scheme,the influence of the sampling noise on the measurement results can be ignored.Based on the experimental data analysis,PCB coils which have higher geometric accuracy and RC integrators which are less sensitive to zero drifts are adopted as the final solution for the measurement system.The experiment is performed under 510 A pulsed current and the integral length is set to 600 mm.The experimental results are as below: rise time is 63 μs and fall time is 60 μs;integral field at the center of the magnet is 0.02576 T·m;integral field uniformity in the good field region(60 mm×24 mm)is 0.5976%.The kicker magnet and its measurement system meet the design specifications.