VACUUM CONSIDERATIONS IN THE DESIGN OF LOW-ENERGY ACCELERATORS

The use of low energy ion beams for material deposition and assisted deposition has led to the need for careful consideration of the vacuum requirements of both the beam transport and target chamber sections of accelerator systems. The maintenance of uhv conditions round the target during bombardment with intense ion beams requires appreciation of beam gas load and ion and electron impact desorptions, particularly when significant beam retardation close to the target is required. This also places exacting demands on the control of the charge state of the beam prior to retardation and, hence, on the design of the beam line from a vacuum standpoint. The system under investigation is a 20 keV-10ev mass analyzed 'clean' accelerator with a Whickham Engineering 'Freeman-type' ion source and a uhv target chamber with an electrostatic retarding lens at the target.