DECELERATION AND ION-BEAM OPTICS IN THE REGIME OF 10-200 EV

A deceleration lens system for low energy ion beam transport applications in the region of 10-200 eV has been designed, constructed and experimentally characterized. In this ion beam system, the target is required to be in an almost field-free region at the exit of the lens to allow for maximum target maneuverability. The lens design was optimized using the simulation data obtained with CHDEN, a computer program for calculating ion trajectories with space-charge included. The design chosen in this work was a five-electrode lens which can be operated in either a simplified two-electrode deceleration mode or, when the best system performance is required, in a five-electrode decelerator-focusing mode. The performance of the lens was experimentally evaluated by using a 3 keV pure argon ion beam with a beam size of about 1 mm2 and beam current of about 10-mu-A which was generated by a modified Colutron Ion Gun Model G-2 unit. The lens decelerated the incident beam to 10-200 eV and transported it to either a Faraday cage for beam current and beam profile measurements or to a quadrupole mass spectrometer equipped with a cylindrical ion energy analyzer for beam energy and mass characterization. The results indicated that the lens could deliver an ion current density of approximately 100-mu-A/cm2 at ion energies higher than 50 eV. The available current density dropped with decreasing energies but remained at more than 10-mu-A/cm2 at 10 eV.