Patent ID: 7911120

Claim:
A source for providing an electron beam of settable power, the source comprising: a plasma chamber including an enclosure having an internal surface area of a first value and an extraction grid having a surface area of a second value, the grid having a potential that is different from a potential of the enclosure and being settable; and multipolar or multipolar magnetic structures for exciting and confining a plasma, wherein a ratio of the second value to the first value is selected to be between an upper limit D and D/2, where D is equal to: 1 β ⁢ 2 ⁢ π ⁢ ⁢ m e m i ⁢ exp ⁡ ( - 1 / 2 ) where β is an electron ratio of the plasma, m e is a mass of an electron, and m i is a mass of positively-charged ions, and wherein an electron current density of an extracted beam is substantially steady when a grid-enclosure potential difference varies.