TY - CPAPER AU - MacArthur, J.P. AU - Huang, Z. AU - Lutman, A.A. AU - Marinelli, A. AU - Nuhn, H.-D. TI - Energy Spread Constraints on Field Suppression in a Reverse Tapered Undulator J2 - Proc. of FEL2015 AB - A 3.2 m variable polarization Delta undulator[1] has been installed at the end of the LCLS undulator line. The Delta undulator acts an an afterburner in this configuration, using bunching from upstream planar undulators to produce radiation with arbitrary polarization. To optimize the degree of polarization from this device, a reverse taper[2] has been proposed to suppress background radiation produced in upstream undulators while still microbunching the beam. Here we extend previous work on free electron lasers with a slowly varying undulator parameter[3] to show there is a strong energy spread dependence to the maximum allowable detune from resonance. At LCLS, this energy spread limitation keeps the reverse taper slope in the slowly varying regime and limits the achievable degree of circular polarization. PB - JACoW CY - Geneva, Switzerland SP - 597 EP - 601 KW - undulator KW - laser KW - FEL KW - bunching KW - simulation DA - 2015/12 PY - 2015 SN - 978-3-95450-134-2 DO - 10.18429/JACoW-FEL2015-WEP004 UR - http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/wep004.pdf ER -