With the development of ever higher energy particle accelerators comes the need for compactness and high gradient, which in turn require very high frequency high power rf sources. Recent development work in W-band accelerating techniques has spurred the development of a high-power W-band source. Axisymmetric sources suffer from fundamental power output limitations(Psat∼&lgr;2)brought on by the conflicting requirements of small beam sizes and high beam current. The sheet beam klystron allows for an increase in beam current without substantial increase in the beam current density, allowing for reduced cathode current densities and focussing field strengths. Initial simulations of a 20:1 aspect ratio sheet beam/cavity interaction using the 3 dimensional particle-in-cell code Magic3D have demonstrated a 35&percent; beam-power to RF power extraction efficiency. Calculational work and numerical simulations leading to a prototype W-band sheet beam klystron will be presented, together with preliminary cold test structure studies of a proposed RF cavity geometry. ©1999 American Institute of Physics.