The effects associated with implanting high power ion beams into photoresist masked silicon wafers have been investigated. Outgassing from the photoresist mask can result in significant dose control errors when Faradays with conventional electrostatic suppression schemes are used. Sheet resistivity measurements show that these errors can be reduced by a factor of 3 when a magnetic suppression technique is employed. With magnetic suppression of secondary charged particles from target impact with an arsenic primary ion beam dose control accuracy is better than 2%, even under severe outgassing conditions. A semiempirical model confirms the sheet resistivity results.