A technique for providing an ion beam of variable focussing (concentration) is described using a flexible grid for extracting and accelerating ions from an ion plasma. The grid is electrically conducting and will bow depending on a voltage difference between it and the ion plasma. This bowing of the grid from its initial planar configuration provides focussing of the ion beam. The amount of focussing depends upon the amount the grid is bowed, which in turn depends upon the voltage difference between it and the ion plasma. The same ion source/flexible grid combination can be used for different operations as for example, providing a collimated, low energy ion beam over a large area and then for providing a focussed ion beam of high energy onto a small area.