Patent Number: 
Section: claims

1. A method of neutralising space charge in an ion beam comprising ions of a first polarity, the method comprising the steps of generating ions of a second polarity, and introducing said second polarity ions to space charge neutralise the ion beam, wherein an external electric field is applied to a region of the ion beam, and said second polarity ions are introduced in said region. 2. A method as claimed in  claim 1 , wherein said second polarity ions are accelerated by said external electric field in a field gradient direction and said second polarity ions are introduced at a location in said region which is upstream in said field gradient direction. claim 1 3. A method as claimed in  claim 2 , wherein said second polarity ions have negative polarity and are introduced adjacent to an electron suppression electrode in the beam extraction system of a source of positive ions. claim 2 4. A method as claimed in  claim 1 , wherein said second polarity ions are cluster ions. claim 1 5. A method as claimed in  claim 4 , wherein said cluster ions have a mass/charge ratio of at least 400. claim 4 6. A method as claimed in  claim 1 , wherein the ion beam comprises positive ions and said second polarity ions are negative. claim 1 7. Apparatus for neutralising space charge in an ion beam, comprising means producing a beam of ions of a first polarity, means applying an external electric field to a region of said beam, means generating ions of a second polarity, and means introducing said second polarity ions in said region to space charge neutralise the ion beam. 8. Apparatus for neutralising space charge in an ion beam, comprising a source of ions of a first polarity, said source producing a beam of said first polarity ions, electrodes to apply an external electric field to a region of said beam, a chamber in which ions of a second polarity are generated, and an aperture from said chamber adjacent to said region of said beam through which said second polarity ions diffuse into said region to space charge neutralise said beam. 9. A method of neutralising space charge in an ion beam comprising ions of a first polarity, the method comprising the steps of generating ions of a second polarity, and introducing said second polarity ions to space charge neutralise the ion beam, wherein said second polarity ions are cluster ions having a mass/charge ratio of at least 400. 10. A method as claimed in  claim 9 , including applying an external electric field to a region of the ion beam, wherein said second polarity cluster ions are introduced in said region. claim 9 11. Apparatus for neutralising space charge in an ion beam, comprising a source of ions of a first polarity, said source producing a beam of said first polarity ions, a chamber in which cluster ions of a second polarity are generated, said cluster ions having a mass/charge ratio of at least 400, and an aperture from said chamber adjacent to said beam through which said second polarity ions diffuse to space charge neutralise said beam. 12. A method of neutralising space charge in an ion beam comprising ions of a first polarity, the method comprising the steps of generating ions of a second polarity, and introducing said second polarity ions to space charge neutralise the ion beam, wherein an external magnetic field is applied to a region of the ion beam, and said second polarity ions are introduced in said region.