Patent Number: 
Section: claims

1. A particle irradiation system for repeating an operation of moving an incident particle beam along a first axis and for making the incident particle beam dwell so as to irradiate the particle beam onto a target, comprising:a first deflector having a maximum deflection amount which enables the first deflector to move the particle beam along the first axis to a maximum width of the target;a second deflector having a maximum deflection amount which enables the second deflector to move the particle beam along the first axis, the maximum deflection amount by the second deflector being less than the maximum deflection amount by the first deflector; anda scanning control apparatus which controls the first deflector and the second deflector,wherein the scanning control apparatus is configured to (i) perform a control in which the particle beam is moved by increasing at least a deflection amount by the second deflector when the particle beam is moved, and (ii) subsequently perform a deflection substitution control in which a deflection by the second deflector is substituted by a deflection by the first deflector by decreasing the deflection amount by the second deflector and changing a deflection amount by the first deflector so as to make a position of the particle beam in the target dwell. 2. The particle irradiation system according to claim 1, wherein the scanning control apparatus is configured to control a rate of change of deflection amount by the second deflector, which increases the deflection amount when the particle beam moves, to be faster than a rate of change of deflection amount by the first deflector which changes the deflection amount when the particle beam dwells. 3. The particle irradiation system according to claim 1, wherein the first deflector deflects the particle beam by an electromagnet. 4. The particle irradiation system according to claim 3, wherein the second deflector deflects the particle beam by an electromagnet. 5. The particle irradiation system according to claim 2, wherein the second deflector deflects the particle beam by an electric field. 6. The particle irradiation system according to claim 1, further comprising:a beam position monitor which obtains an information of a position of the particle beam which is scanned, wherein a feedback control is added to the control, in which the deflection amount by the second deflector is decreased, by the information from the beam position monitor when the particle beam dwells. 7. The particle irradiation system according to claim 4, wherein the first deflector comprises an excitation coil of an electromagnet and a first power source which supplies a current to the excitation coil and the second deflector comprises the excitation coil of the electromagnet and a second power source which supplies a current to the excitation coil. 8. The particle irradiation system according to claim 4, whereinan electromagnet comprises a first excitation coil and a second excitation coil on the same iron core;the first deflector comprises the first excitation coil and a first power source which supplies a current to the first excitation coil; andthe second deflector comprises the second excitation coil and a second power source which supplies a current to the second excitation coil. 9. The particle irradiation system according to claim 3, wherein the second deflector is disposed at a position which is a particle beam incident side of a deflection electromagnet which deflects a main direction of particle beam traveling so as to guide the particle beam to the target. 10. The particle irradiation system according to claim 3, wherein (i) the system has the configuration of a rotating gantry and (ii) the second deflector is disposed at a particle beam incident side of a deflection electromagnet which is disposed at a most downstream side of a particle beam traveling the rotating gantry. 11. A particle beam irradiation method in which a target is irradiated by repeating an operation of moving the particle beam along a first axis and making the particle beam dwell so as to scan by a first deflector having a maximum deflection amount which enables the first deflector to move an incident particle beam along the first axis to a maximum width of the target and a second deflector having a maximum deflection amount, which enables the second deflector to move the particle beam along the first axis, and is less than the maximum deflection amount by the first deflector, the method comprising:increasing at least a deflection amount by the second deflector so as to move the particle beam in the target when the particle beam moves; andonce the particle beam is moved within the target, decreasing the deflection amount by the second deflector; andchanging a deflection amount by the first deflector in such a manner that a deflection by the second deflector is gradually substituted by a deflection by the first deflector so as to make a position of the particle beam in the target dwell. 12. The particle beam irradiation method according to claim 11, wherein a rate of change of deflection amount by the second deflector, which increases the deflection amount when a particle beam moves, is faster than a rate of change of deflection by the first deflector, which changes the deflection amount when the particle beam dwells.