Abstract: A mechanical scanning system for a batch ion implanter includes a disk for mounting wafers at sites near its periphery and a disk drive assembly for rotating the disk. The disk and the disk drive assembly are supported in an end station vacuum chamber by a scan arm which extends through a sealed opening in an access door. A scan drive assembly attached to the access door scans the scan arm and the disk in an arc-shaped path. Simultaneous rotation and scanning of the disk cause the ion beam to be distributed over the surfaces of the wafers mounted on the disk. A pivot drive assembly rotates the scan arm and the disk about a pivot axis between an implant position and a load/unload position. A bellows seals the scan arm to the vacuum chamber while tranmitting arc-shaped movement into the vacuum chamber. A bellows guide apparatus guides an intermediate portion of the bellows so as to limit lateral distortion caused by pressure differences applied to the bellows in combination with asymmetrical deflection thereof.