▁on ▁its ▁location ▁along ▁the ▁cyl inder . ▁The ▁net ▁effect ▁of ▁the ▁dr ift ▁over ▁a ▁period ▁of ▁several ▁or bits ▁along ▁the ▁long ▁axis ▁of ▁the ▁re actor ▁nearly ▁adds ▁up ▁to ▁zero . ▁ ▁Rot ational ▁transform ▁The ▁effect ▁of ▁the ▁hel ical ▁field ▁is ▁to ▁b end ▁the ▁path ▁of ▁a ▁particle ▁so ▁it ▁describes ▁a ▁loop ▁around ▁the ▁cross ▁section ▁of ▁the ▁contain ment ▁cyl inder . ▁At ▁any ▁given ▁point ▁in ▁its ▁orbit ▁around ▁the ▁long ▁axis ▁of ▁the ▁tor oid , ▁the ▁particle ▁will ▁be ▁moving ▁at ▁an ▁angle , ▁ θ . ▁ ▁In ▁the ▁simple ▁case , ▁when ▁the ▁particle ▁has ▁completed ▁one ▁orbit ▁of ▁the ▁re actor ' s ▁major ▁axis ▁and ▁returned ▁to ▁its ▁original ▁location , ▁the ▁fields ▁will ▁have ▁made ▁it ▁complete ▁one ▁orbit ▁of ▁the ▁minor ▁axis ▁as ▁well . ▁In ▁this ▁case ▁the ▁rot ational ▁transform ▁is ▁ 1 . ▁ ▁In ▁the ▁more ▁typical ▁case , ▁the ▁fields ▁do ▁not ▁" line ▁up " ▁this ▁way , ▁and ▁the ▁particle ▁will ▁not ▁return ▁to ▁exactly ▁the ▁same ▁location . ▁In ▁this ▁case ▁the ▁rot ational ▁transform ▁is ▁calculated ▁thus : ▁ ▁where ▁R ▁is ▁the ▁major ▁radius , ▁ ▁the ▁minor ▁radius , ▁ ▁the ▁pol oid al ▁field ▁strength , ▁and ▁ ▁the ▁tor oid al ▁field . ▁As ▁the ▁fields ▁typically ▁vary ▁with ▁their ▁location ▁within ▁the ▁cyl inder , ▁ ▁var ies ▁with ▁location ▁on ▁the ▁minor ▁radius , ▁and ▁is ▁expressed ▁i (