High-harmonic slotted gyroklystron amplifier: linear theory and nonlinear simulation

A fluid theory and two self-consistent particle-tracing simulation codes are described for designing low voltage, s^th-harmonic slotted gyroklystron amplifiers, in which axis-encircling electron beams are in resonance with the s^th-order azimuthal modes of a series of magnetron-type cavities, allowing the gyrotron amplifier´s required magnetic field to be reduced by a factor of s. The linear fluid theory yields a convenient closed-form expression for gain and the nonlinear simulation code determines the large-signal device performance, while the much faster linear simulation code allows thorough, multi-dimensional parameter searches to be performed quickly. The simulation codes self-consistently account for shifts of the cavity´s resonant frequency and quality factor due to beam loading. The three theoretical approaches, which agree in the small-signal regime for weak beam loading, were used to design a 95 GHz, three-cavity, slotted twelve-vane, sixth-harmonic gyroklystron amplifier utilizing a 70 kV, 10 A, v_⊥/v_z=2, axis-encircling beam and a 6.1 kG magnet. The nonlinear self-consistent simulation code predicts that the sixth-harmonic gyrotron amplifier with an ideal beam will yield an electronic efficiency of 20% and a saturated gain of 37 dB, while the more realistic device with a 10% axial velocity spread will generate a peak output power of 84 kW with 12% efficiency, a saturated gain of 27 dB and a 0.2% constant-drive bandwidth