A control system for fine tuning or spreading a charged particle pencil beam includes a low-inductance, low-power compensation or fine-tuning magnet assembly. The feedback loop that includes the compensation magnet assembly has a faster response rate than the feedback loop that includes the scan nozzle. The compensation or fine-tuning magnet assembly is preferably disposed upstream of the scan nozzle magnet(s) with respect to the beam path to make rapid but minor adjustments to the beam position between iterations of the scan nozzle.