The current baseline design for the 500‐GeV SLAC/KEK future collider requires approximately 5000 75‐MW, 1.6 &mgr;s, PPM pencil‐beam klystrons. A prototype is currently on test. Although the estimated cost of the klystrons is a small part of the total collider cost, this number of klystrons is at least an order of magnitude higher than the klystron population in any scientific or military system ever fielded. A back‐up sheet‐beam klystron design has been under study at SLAC for the last six years. It offers several advantages: If two sheet beams were employed in parallel, the current density at the two cathodes would be low, and the power density at the output cavity a fraction of that in the pencil‐beam klystron. Furthermore, because of significantly fewer vacuum parts, the 150‐MW SBK should have a substantially lower cost than the baseline 75‐MW pencil‐beam klystron. Finally, it is considered that because of the lower power density, a longer rf pulse (3.2 &mgr;s) could be employed. All this means is that, with more pulse compression, the total number of klystrons in the collider could be reduced by a factor of 4, to approximately 1250. The total cost of the klystrons would be cut by an even larger factor. Since a practical SBK has never been designed before, two major problems had to be solved before a meaningful computer simulation of the entire tube could be performed. First, a sheet‐beam gun had to be designed, along with a periodically‐focused beam transport system outside the vacuum. Secondly, since extended interaction cavities are used throughout, new techniques had to be developed to provide useful designs with adequate stability and mode separation. This work is essentially complete. The work to parallel 24 CPUs, and modify the MAGIC 3D code so simulations of the complete SBK can be performed in a reasonable time, has progressed sufficiently for an interim report on the project to be presented. © 2003 American Institute of Physics