Chapter Three - Wien Filter Instrumentation

Abstract

The Wien filter or ExB velocity filter is probably the first charged particle analyzer and focusing device ever used. Invented by Wien, it appeared later also as ExB velocity filter, possibly reinvented independently. Despite its age, the Wien filter is not so frequently series manufactured as, quadrupole mass filters for instance. However the Wien filter popularity is higher. A test of this "popularity" tested on internet shows: "Wien velocity filter" mentioned in 830000 sites, "quadrupole mass analyzer" 234000 sites. The big obstacle in large scale use of the Wien filter was in the past its sensitivity for the differences between the electric and magnetic fringing field distribution shapes. These put in danger even the rectilinearity of the beam axis and are a prolific source of big aberrations. The invention of the multipole Wien filters is the sure cure of these inconvenients but this come to day light almost a century after its invention. Our book purpose is to assembly in coherent chapters the existing knowledge of Wien filters in both scientific and technical areas. The first chapter reviews the development of the Wien filter marking the milestones of this development. The second chapter gives the tools needed to describe the charged particles motion through Wien filters, taken independently or as part of complex systems, some aberrations and their cancellation being included. The third chapter deals with the technical aspects of the design and construction of the Wien filters, emphasizing the deep knowledge of multipole Wien filters. The fourth chapter is dedicated to Wien filter simulations. These allow to build realistic geometric configurations to become translated in electrostatic potential and magnetostatic potential distributions, allowing to draw real computed charged particle trajectories inside the filters and through their fringing fields. The fifth chapter concentrates on the use of Wien filters to mass analysis and ion handling. There are presented some single and double mass spectrometers, instruments and designs. The recoil mass separators, big space needing research instruments in the area of nuclear physics and astrophysics are reviewed. The sixth chapter reports the very successful application of the Wien filters in the area of electron microscopes, the use of these filters being responsible for a substantial increase in resolution. The content of these chapters is the starting basis for the future development of research and technology of Wien filters. The aberrations of various complex but specific electron optical system will be determined with the methods disclosed here. The theory will be developed to include the mechanical aberrations of Wien filters, still at the very beginning.