Axial ion source with magnetic field adjustment

A method of operating an ion source, comprising:

• generating a magnetic field in an ionization volume of an ionization chamber of the ion source using a magnetic field generator;
• passing electrons through a paramagnetic section of the ionization volume wherein the magnetic field confines the electrons to a center axis of the ionization volume;
• and
• passing the electrons through a ferromagnetic section of the ionization volume generated by a ferromagnetic element disposed within or adjacent to the ionization chamber, the electrons diverging away from the center axis within the ferromagnetic section of the ionization volume.

An ion source, comprising:

• an electron source configured to produce electrons;
• an ionization chamber having an entrance aperture through an electron lens, an ion exit aperture through an end wall, and a center axis through an ionization volume within the ionization chamber, the ionization chamber configured to produce ions;
• and
• a ferromagnetic element at a distance from the end wall or the electron lens such that magnetic field lines are concentrated at the ion exit aperture or the entrance aperture, respectively.