Electron beam processors have advanced a long way since the early scanned beam systems associated with radiation research and radiography. 'Curtain' beam equipment is today used in a wide range of industrial processes such as printing, magnetic media manufacture and the crosslinking of plastic film and wire insulation. This first part of a two-part article describes the technology that has been developed to deliver beam currents of as much as 1000 mA at accelerating voltages of up to 300 kV. The second part, in a future issue, will discuss the diverse applications of such processors.