Ion gun for production of ion beams with particular radial current density profile

摘要

An ion gun device capable of producing circularly shaped ion beams in the keV energy range with constant radial current density across a large part of the beam diameter. The device consists of an ion source, a cylindrical extraction cup electrode and a cylindrical acceleration tube electrode, all disposed on the same axis. Positive ions are axially extracted from the ion source by the extraction electrode which is kept at a small negative potential (less than one tenth of the acceleration potential) with respect to the ion source. The extracted ions are accelerated and focused by an immersion lens which is formed by the potential distribution within and between the extraction and acceleration electrodes, the acceleration electrode being at such a negative potential with respect to the ion source that the focused ions possess the desired energy after having passed through the immersion lens. Under observance of well-defined restrictive conditions for the lay-out of several geometrical dimensions of the construction and under observance of well-defined restrictive operation conditions of the ion gun it is possible to obtain circularly shaped ion beams with an approximately trapezoidal radial current density distribution at axial target position far removed from the acceleration field.