An apparatus for forming a beam of electromagnetic radiation. The apparatus includes a plasma radiation source, a foil trap provided with a plurality of thin foils that extend substantially parallel to the direction of radiation from the plasma source, and a grid disposed between the plasma radiation source and the foil trap. A space is located between the grid and the foil trap. An electrical potential application circuit is constructed and arranged to apply an electrical potential to the grid so that the grid repels electrons emitted by the plasma radiation source and creates a positive space charge between the grid and the foil trap to deflect ions emitted by the plasma radiation source to the foil trap. A distance between the grid and the foil trap is at least equal to one-half of a radius of the foil trap.