A pencil beam system includes a charged particle beam generator, a transport beamline apparatus, a scan nozzle, a fast deflector electromagnet, and a controller. After a therapeutic dose is delivered to a first target spot, the fast deflector electromagnet generates a first magnetic field that causes the net deflection of the charged particle beam to transition from the first target spot to an adjacent target spot. After the charged particle beam is directed to the adjacent target spot, the controller simultaneously adjusts the first magnetic field and the scan nozzle magnetic field to reduce and eliminate the contribution of the first magnetic field to the net deflection. The fast deflector electromagnet is deliberately designed with limited magnetic field and limited deflecting power to provide a higher slew rate, faster settling and less hysteresis contribution to beam position as compared to the scan nozzle electromagnets.
