A miniaturized, optically driven, therapeutic radiation source is disclosed in which the voltage gradient between a high electron accelerating voltage and the ground potential can be controlled. The electron source and the target element are disposed within a capsule which defines a substantially evacuated region extending along an electron beam axis. The inner surface of the capsule is coated with a weakly conductive or semiconductive coating, so that a substantially uniform voltage gradient is maintained within the evacuated capsule. In this way, the chances of electric flashover or breakdown are reduced. Also, secondary emissions of electrons striking the inner wall of the capsule are reduced. X-ray production efficiency is optimized by maximizing the percentage of electrons propagated directly to the target.