  In this paper, we discuss the design and whole-cavity simulation of a
Multi-Beam Inductive Output Tube (MBIOT) that uses a 3rd harmonic component to
the drive voltage on the grid. High-efficiency inductive output tubes (IOTs)
are typically characterized by efficiencies up to 70 - 75%. However, the
achievement of efficiencies greater than 80% would substantially reduce the
operating costs of next-generation accelerators. To achieve this goal, we
consider the addition of a 3rd harmonic component to the drive signal on the
grid. We anticipate that the MBIOT will be used to provide the rf power to
drive RF linacs. We discuss and model an 8-beam MBIOT with a 700 MHz resonant
cavity using beams with a voltage of 35 kV and an average current of 7.25 A
yielding a perveance of about 1.1 microP. We simulate this MBIOT using the
NEMESIS simulation code which has been extended using a three-dimensional
Poisson solver based upon the Petsc package from Argonne National Laboratory.
The effect of the 3rd harmonic on the efficiency is greatest when the phase of
the 3rd harmonic is shifted by pi radians with respect to the fundamental drive
signal and with 3rd harmonic powers greater than about 50% of the fundamental
drive power. For the present example, we show that efficiencies approaching 82%
are possible. Designs for the MBIOT input coupler, grids and output cavity have
been developed based on these simulations and will be discussed.
