Toufexis, F. Neilson, J. Tantawi, S.G. JACoW Geneva, Switzerland 978-3-95450-182-3 10.18429/JACoW-IPAC2017-TUPAB136 English 1647-1650 TUPAB136 undulator polarization coupling controls electron Contribution to a conference proceedings 2017 2017-05 http://dx.doi.org/10.18429/JACoW-IPAC2017-TUPAB136 http://jacow.org/ipac2017/papers/tupab136.pdf To reduce the linac energy required for an FEL radiating at a given wavelength, and hence its size, a smaller undulator period with sufficient field strength is needed. Previous work from our group successfully demonstrated a microwave undulator at 11.424 GHz using a corrugated cylindrical waveguide operating in the HE11 mode. Scaling down the undulator period using this technology poses the challenge of confining and coupling the electromagnetic fields while maintaining overmoded features for power handling capability and electron beam wakefield mitigation. We have designed a mm-wave undulator cavity at 91.392 GHz*. This undulator requires approximately 1.4 MW for sub-microsecond pulses to generate an equivalent K value of 0.1. Transferring such amounts of power in mm-wave frequencies requires overmoded corrugated waveguides, and coupling through irises creates excessive pulsed heating. We have designed a novel mode launcher that allows coupling power from a highly overmoded corrugated waveguide to the undulator through the beam pipe. Additionally, this mode launcher can be used along with grating polarizers to control the polarization of the produced light.