Ionized Molecular Hydrogen Build-Up and Confinement Inside the Space Charge of a Continually Resupplied Electron Cloud

Abstract

A cylindrically symmetric electromagnetic ion trap has been constructed that creates a potential well suitable for confining ions with the space charge of a continually resupplied electron cloud. The trap uses permanent magnets to form a linear array of magnetic field ring cusps in a cylindrical geometry and includes electrostatic plugging of the cusping magnetic fields. The ion source is electron bombardment ionization of a controlled leak of thermal molecular hydrogen directly inside the trap, and a potential well created by those selfsame electrons is the mechanism of ion confinement. The ion trap is unique in that the confinement volume has relatively minimal external static electric and magnetic field intrusion into the regions intended to confine ions and no overall axial magnetic field. Another standout feature is the inclusion of electrostatic elements in two locations inaccessible to electrons inside the trap that provide axial ion confinement in conjunction with electrons in the trap. Build-up of ions, and therefore confinement, was demonstrated via an ion signal that increased in intensity over a period of several dozen expected axial ion transit times while injecting electrons. A study has been conducted for a range of incident electron currents from the electron flood gun for several fixed partial pressures of molecular hydrogen and interpreted using a full particle-in-cell simulation of the trap. Data were acquired via a microchannel plate and phosphor screen assembly biased in a manner that could only be excited by positively charged particles exiting the trap. An order of magnitude estimate for mean lifetime is made using a combination of simulated charged particle population estimates and measured data.