LEG 41 Electron Gun - for charge neutralisation

The LEG41 is an electron source designed for charge neutralisation. It operates in two energy modes. Mode 1 is user variable between 0 and 80eV and is suitable for charge neutralisation in XPS. Mode 2 is a fixed 500eV mode particularly suitable for charge neutralisation in SIMS. It features four poles enabling the beam to be deflected to the required position on the sample. The current arriving at the sample is both stable and user controlled.
The gun consists of an electron source, optical column, and deflection poles mounted inside a stainless steel tube. An optional mu-metal outer shield can be fitted which provides shielding from magnetic fields. This is designed to be used in stainless steel chambers.

Electron source
The electron source is a Thoria coated Rhenium tape filament with the emission (beam current) controlled by a variable potential grid in front of the filament. It can be operated in feedback control mode giving a constant beam current irrespective of other conditions or the user can directly vary the grid potential to give the required beam current (emission). The filament and grid are in one assembly fixing their relative distances. This assembly can be demounted by the user and replaced if required after extended running or hostile environment.

Modes
Mode 1 operation : The filament (cathode) is at the user selected energy. A1 and A3 are at + 500eV, the deflection poles are centred at 500eV. This extraction potential creates a stable field accelerating the electrons through the optical column. In particular it allows high sample currents to be generated at low beam energies.
Mode 2 operation : The filament (cathode) is at -500eV with A1 and A3 at earth potential. The deflection poles are centred at 0 V.