A microwave electron velocity spectrograph

An electron-velocity spectrograph is described which can be used to measure the velocity distribution of electrons in a high-density electron beam modulated at microwave frequencies. The spectrograph employs a crossed electric and magnetic deflection system, has a resolution of less than one per cent in velocity, and a deflection region of very short length in order to minimize space-charge effects in the instrument. Results of measurements made with the instrument on a Brillouin-focused, gap-modulated electron beam of microperveance one are given. Measurements made on the dc beam indicate that nonlaminar flow effects are important for focusing fields of the order of and greater than the Brillouin focusing condition. Measurements of the RF velocity distribution at a point in the beam near the cavity gap have been found to agree with the predictions of kinematic bunching theory. A consequence of small-signal, space-charge-wave theory is that RF velocities should go to zero at a point in the beam a quarter of a reduced plasma wavelength from the cavity gap. This is confirmed by measurements made with this spectrograph.