Determination of ionization efficiencies of thermal ionization cavity sources by numerical simulation of charged particle trajectories including space charge

Abstract

An in-house developed code, Sofie, and the methods used for numerical determination of the ionization efficiency of thermal ionization cavity (TIC) sources up to intermediate temperatures (<2500K for cavity dimensions of a few millimetres) are presented, thus, creating a tool to identify cavity geometries offering better ionization efficiencies and to aid in matching the ion beam produced by a TIC source to the ion optics of a beam line or a mass analyser as used in conventional thermal ionization mass spectrometry (TIMS).Essential is the code’s ability to approximate the solution of Poisson’s equation, in which the space charge predominantly originates from electrons thermally emitted from the inner cavity walls. The assumption of a quasi-neutral plasma forming in the volume of the cavity is no longer valid at lower temperatures and/or cavity dimensions, and charged particles can be regarded as moving freely throughout the cavity only interacting via the electrostatic potential shaped by the particles’ space charge. Under such conditions the geometry of the cavity has got a large effect on the ionization efficiency of sample atoms loaded into the source. The code also allows the simulation of surface adsorption and re-emission of various particle species simultaneously, making the calculation of ionization efficiencies possible by determining the currents of sample ions and neutral atoms out of the cavity. The validity of the code is successfully demonstrated by simulating problem sets, for which analytical solutions exist, and also by reproducing experimental data from TIC sources found in the literature. The necessity of including space charge in such simulations is demonstrated. A non-exhaustive search for a best cavity geometry has been performed and a geometry has been identified that could enhance the ionization efficiency by up to a factor of 100 over the ionization efficiency of the Saha-Langmuir equation, which corresponds to the efficiency achievable with conventional flat filament TIMS sources.