Abstract
Foil and foil‐less field emission diodes (current ∼10 kA, voltage ∼1.5 MV) used in free‐electron Raman lasers are studied experimentally with the view of reducing the perpendicular energy of the emitted beam. Microwave emission, attributed to a cyclotron instability in the beam, is used as the diagnostic. Removal of the outer portions of the beam leads to a significant reduction in the radiated intensity, indicating an overall improvement in beam quality.