Future Linear Colliders and Accelerators require rf systems and components that are capable of handling hundreds of megawatts power levels at x-band frequencies and higher. Standard rf components that have been in use for a long time such as waveguide bends, directional couplers and hybrids, can not be used because of peak field considerations. Indeed, one has to reinvent most of these components taking into account the constraints imposed by ultra-high-power operation. Here, we present a new

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... sign for circular waveguides bends propagating the low-loss TE 01 mode. The bend has smooth walls and low field levels. We present a simple synthesis process for designing such device. The general philosophy of this technique can be applied to other components as well. We describe the detailed design of the bend and compare our design with finite element simulations and experimental data. The bend has very low ohmic losses, and the TE 01 mode is transmitted with virtually perfect mode purity. Recently, ultra-high-power rf systems at X-band and above have received a lot of attention in different laboratories around the world because of the desire to design and construct a future linear collier. For a review of these activities the reader is referred to [1] [2] . These systems are required to generate and manipulate hundreds of megawatts. Standard rf components that have been in use for a long time such as waveguide bends, directional couplers and hybrids, can not be used directly because of peak field considerations. Indeed, one has to reinvent most of these components while taking into account the constraints imposed by ultra-high-power operation. Usually most of these comports are made with oxygenfree high-conductivity copper and the operation takes place under ultra-high vacuum conditions. Experimental work at x-band showed that peak electric fields should not exceed 500 kV/cm [3] . Peak magnetic field should be limited so that the pulsed surface heating does not exceed 30 C o [4] For these systems the TE 01 mode in circular waveguide is very attractive because it has low loss properties, and there are no electrical field lines that terminate on the walls of the waveguide. The low-loss TE 01 mode in circular waveguide has been utilized for several decades. It was used in several applications including communication systems, antenna feeds and rf systems for high energy accelerators. Waveguide bends are important components for waveguide rf circuits and transport lines. The first analysis for bends in circular guides carrying the TE 01 mode is found in [5] . Since then several design concepts have been introduced to for these bends [6, 7, 8] . However, none of these designs are really suitable for ultra-high-power operations. Here, we present a new idea for perturbing the cross section of the waveguide smoothly to allow ultra high power operation. There are several numerical techniques for the analysis of waveguide bends, see for example [9] . However, since there are several constraints on the design including mode purity, these are not convenient for the design process. We present a simple synthesis process for designing such bends that guarantees mode purity. We later verify our results with finite element simulations and experimental data.