A Megawatt-Level Cusp Gun With Low Velocity Spread for <i>W</i>-Band Helical Gyro-TWT

A cusp gun has been designed for a high-power W-band gyrotron traveling-wave tube (gyro-TWT) with a helically corrugated waveguide operating at the second harmonic of the cyclotron frequency in a magnetic field of about 2 T. According to simulations performed using various software tools, the elaborated gun enables the generation of an axis-encircling electron beam with an energy of 90 keV, a pitch factor of 1.2, and a velocity spread of less than 6% at beam currents ranging from 10 to 15 A. A high beam current is achieved at a moderate cathode loading of 5 A/cm2 due to a very wide emitting ring with a radial width of 46% of its mean radius. In the case of ideal axial symmetry, the spread of transverse velocities can be as small as 2%, owing to the positive impact of space-charge forces.