Ion source assembly with multiple elliptical filaments

An electron bombardment ion source assembly for use in a mass spectrometer, comprising:

• a cylindrical anode extending along an axis and surrounding an ionization volume;
• and
• a first filament and a second filament that are each configured to thermionically emit electrons, wherein the first and second filaments are positioned outside the ionization volume and proximate to the cylindrical anode;
• wherein the first and second filaments each comprise an elliptically-shaped central length and non-elliptical lengths on either end of the elliptically-shaped central length,
• wherein the non-elliptical lengths are configured to be mounted in a fixed position relative to the cylindrical anode to maintain a constant distance between the elliptically-shaped central length and the cylindrical anode, and
• wherein each elliptically-shaped central length extends along a plane that intersects a plane perpendicular to the axis of the cylindrical anode at a non-zero angle.