Numerical simulations were performed to predict the performance of microwave plasma reactors with various reactor geometries. The simulations include the calculation of the electric field distribution using the finite integration theory and the determination of the plasma density distribution based on a breakdown field algorithm. One reactor geometry with a cavity having the shape of a rotational ellipsoid turned out to be very promising. The electric field within this cavity exhibits two pronounced maxima at the two focal points of the ellipsoid. By coupling microwave energy into one maximum via an antenna, large electric field strengths can be generated in the counter maximum. This effect has been used to excite intense discharges that are very stable, spatially extended, homogeneous, and free from wall contact. These discharges were employed for the chemical vapor deposition of large area diamond wafers.