An ion beam is sensed with a beam current sensor which has a sensing aperture that is smaller than a cross-sectional dimension of the ion beam at the beam current sensor. The sensed ion beam current is indicative of ion beam position relative to a desired ion beam path. The ion beam position may be adjusted if the sensed ion beam position differs from the desired ion beam path. One or more beam current sensors may be utilized in an ion implanter for calibration and/or alignment. The beam current sensor may be utilized to determine a relation between a characteristic of an ion beam, such as magnetic rigidity, and a parameter of a system element, such as magnetic field, required to direct the ion beam along a desired ion beam path.