Emittance balancing technique for the resonant slow extraction from a synchrotron

Abstract

A new method is proposed to produce identical beam emittances in the transverse phase planes from a severely asymmetric beam extracted from a slow-cycling synchrotron. This is important to obtain a good beam distribution at the irradiation spot when the spot scanning method is used for the treatment in hadron therapy. The method makes use of the beam rotation effect either by a solenoid or by a rotational quadrupole section, referred to as rotator, to produce identical emittances in the two transverse phase planes. It is also helpful for the stabilization of the beam spot at the irradiation point. Simulation studies with an artificially generated beam have been carried out to prove the principle. The method is not only applicable to fixed treatment nozzles but also to rotational gantry nozzles. Both a solenoid and a section of quadrupole rotator can be used, with the former being ideal for proton gantries and the latter for fixed carbon nozzles. There is a practical limit in using room-temperature solenoid due to the high beam rigidity in the case of a carbon beam, for which a superconducting solenoid has to be used. The comparison with other methods and the practical applications of the method are also presented.