Tandem accelerator

A tandem accelerator ion beam source for producing a beam of positive ions having energies greater than 1 MeV and a beam current greater than 10 mA, comprising a volume ion source for producing a beam of negative ions having an energy of the order of tens of kilowatts and a beam current of at least 10 mA, a windowless ion stripper cell arranged to convert the negative ions to positive ions, means for supplying an electron stripping gas to the stripper cell, a plurality of electric stress shields surrounds the ion stripper cell and arranged to produce an electric field of about 20 kV/cm in the region of the ion stripper cell, means for removing uncharged species from the beams of positive ions, means for controlling the direction of the beam of positive ions, a vacuum envelope enclosing the ion beam path within the tandem generator, the electric stress shields and the windowless stripping cell, and a vacuum system for producing a dynamic vacuum of at least 10.sup.-5 torr within the vacuum envelope.