|
内容記述 |
Superconducting accelerator magnets are fundamental components of high-energy particle accelerators. Very high field quality and stability of the magnets are required in order to operate the accelerator with minimizing beam instability and beam loss. Particularly, time-varying field quality during a period of the beam injection is an important subject to be understood. A possible reason of the time-varying magnetic field is the change of the superconductor magnetization due to local magnetic field change induced by redistribution of imbalance current in superconducting cables. In order to clarify and to evaluate the mechanism, experimental studies and their evaluations with computer simulations were performed．<br /> Magnetic field measurements of the superconducting quadrupole magnets (MQXAs) developed by KEK for the CERN/LHC insertion regions were carried out. The measurements at the beam injection current were performed by using a set of rotating harmonic coils after pre-excitations of the magnet. The periodic patterns of the magnetic field along the magnet axis, of which pattern length is equal to the cable twist pitch, was observed. The amplitude of the periodic pattern was decreased with time. The average field over the even number of the wave-length of the periodic pattern corresponding to the effective field quality, was also changed with time. It was thought that the decrease of amplitude is caused by equalization of imbalance currents in cables, and the change of average is caused by re-magnetization of superconducting filament． From the measurements of 18 magnets, it was clarified that the behaviors of the amplitude and the average indicate strong dependences on the pre-excitation．<br /> A change of superconductor magnetization due to external field change was experimentally evaluated. By using a vibration sample magnetometer (VSM), the magnetization of superconductor used in the LHC magnets was measured. The behavior of re-magnetization of filaments induced by the external magnetic field with arbitrary direction was clarified．<br /> In order to evaluate the mechanism of time-varying magnetic field, a simulation program was developed. At first, the numerical model of re-magnetization was developed. In this model, persistent current, which flowed in a superconductor filament, was assigned as “nested cosine theta current distribution” in coaxial shell as division of filament. Magnetization behavior calculated by cosine theta model indicated good agreement with results measured by VSM. As the next step, a numerical model of imbalance currents was developed. The imbalance currents were ascribed as zigzag currents given randomly. The amplitude of periodic pattern induced by the zigzag currents was evaluated by means of Monte Carlo simulation. To reproduce the measured periodic pattern amplitudes, current imbalance with a level of hundred amperes was required in calculation.<br /> A 3-dimensinal full coils model based on the LHC magnet was developed as extension of above programs. Imbalance currents in cables were assigned as zigzag current models on each cable position. Magnetic field in the magnet bore induced by the zigzag currents was computed, and a sinusoidal field pattern along the longitudinal axis was given. The local magnetic field at the strand in the coils was also calculated. Magnetization change of the superconductor filament in the coils, due to the local magnetic field change induced by zigzag current change, were computed by using the cosine theta model. Finally, the magnetic field in the bore induced by each magnetization of superconductor filaments in the coils was calculated. The simulation reproduced the magnetic field change in the magnet bore, by assuming the change of the zigzag currents which was needed to induce the periodic pattern consistent with measured results. It has been understood that magnetization change due to local magnetic field change induced by imbalance current redistribution causes the time-varying magnetic field during a period of beam injection in accelerator operation．<br /> In conclusion, the field measurement of the magnets showed the strong dependencies on periodic pattern amplitude and the average field to the pre-excitation history. The computation model for superconductor magnetization by cosine theta distribution was developed, and the calculation agreed with the VSM measurements. The simulation for the imbalance currents in the cables has shown the periodic pattern similar to the hat observed in the measurement. It has been understood that the simulation combining the magnetization and the imbalance currents has shown that the magnetization change due to the local field change induced by the decay of the imbalance currents are in good agreement with that observed in the measurement. |
要旨・審査要旨 / Abstract, Screening Result (297.8 kB)
