Magnetic Field Control in Synchrotrons

Hadron beams delivered by normal conducting synchrotrons are highly attractive in various fundamental research experiments as well as in the field of tumour therapy. These applications require fast synchrotron operation modes with pulse-to-pulse energy variation and magnetic field slopes up to 10 T/s. The aims are to optimize the duty-cycle and to minimize treatment times for the patients as well as to provide extremely stable properties of the extracted beams, i.e. position and spill structure. Studies performed at the SIS18 synchrotron at GSI proved that the ring quadrupoles contribute to the deterioration of the slowly extracted beam as well as the dipoles. An attempt has been made to measure the magnetic fields in the synchrotron magnets with high precision and speed comparable to the current measurement with a DCCT used in the power supplies. Adding magnetic field monitoring into the current control feedback loop suppresses the unfavourable dynamic effects from hysteresis and eddy currents. The presentation describes this approach and the results obtained at the HIT [1] synchrotron will be discussed. THE SYNCHROTRON-CYCLE Two major effects in contradiction to high-performance operation like therapy requirements occur in normal conducting magnets: Figure 1: Cycle of main dipole current at HIT, with conditioning phase at the end. • Hysteresis, which can only be handled by adding a conditioning phase at the end of each synchrotroncycle. This means driving all magnets into saturation to create well defined initial conditions for the next injection, see Fig. 1. • Eddy currents appear due to fast ramping (in case of HIT: 1.5 T/s). After the acceleration it takes about 1s until the magnetic field reaches the nominal value and the beam attains an orbit sufficiently stable for the extraction process , e.g. by RF Knock-out. The relative magnetic field measurement in a high energy cycle, displayed in Fig. 2, shows the strong effect. The standard magnet-by-current control cannot deal with these both effects, time and energy consuming processes are necessary. Magnetic field control would eliminate these processes, improving the efficiency up to 30% as compared to the present HIT operation. Fig. 1 shows that the waiting phase at the beginning of the extraction flattop as well as the conditioning phase at the end leads to only 50% duty cycle concerning the spill-ontime. Figure 2: The plot shows the highly attenuated signal of the Hall probe mounted in a HIT dipole; the Hall voltage represents the difference of the reference value and the actual measurement, scaled by a factor of 10,000.