Photo rf guns are a source of electron beams for X-ray FELs such as LCLS and European XFEL. In existing photo-guns power is coupled into the cavity by rectangular waveguides through the cell walls, , or use coaxial coupling, . We are considering feeding a gun using a circular waveguide with the
TM01 mode. To do that we need a mode launcher, a matched device that couples the rectangular TE01 mode waveguide to a TM01 mode in a circular waveguide. Use of
the mode launcher reduces complexity of the gun cavity and increases flexibility of positioning the input waveguide relative to the gun body. Mode launchers have been successfully used at SLAC and elsewhere for X-band high gradient tests. Because the existing mode launchers were not built for high brightness rf guns, they have a significant quadrupole field component. High brightness rf guns have tight requirements on output beam properties. The quadrupole component of the mode launcher adversely affects the beam quality. We have designed a mode launcher free of this disadvantage. We present an example S-band mode launcher with minimized quardupole field component.