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TY - UNPB AU - Bettoni, S. AU - Dax, A. AU - Huppert, M. AU - Pedrozzi, M. AU - Trisorio, A. AU - Vicario, C. ED - Schaa, Volker RW TI - Effect of Long Response Time Photocathode Materials on Microbunching Instability in Free Electron Laser Facilities J2 - Proc. of IPAC2020, Caen, France, 10-15 May 2020 CY - Caen, France T2 - International Particle Accelerator Conference T3 - 11 LA - english AB - Microbunching instability may be initiated by any intensity modulation on the bunch at low energy or by shot-noise along the machine. This instability may also be very detrimental in Free Electron Laser (FEL) facilities, because the resulting distortion of the beam phase space can strongly deteriorate the FEL intensity. The Laser Heater, which induces an increase of the beam energy in a controlled way, is used to mitigate this instability. In this paper we propose an alternative or complementary way to reduce the induced energy spread by using a long response time material as photocathode. By choosing a photocathode material with a response time larger than the value corresponding to the maximum amplification of the microbunching instability, the photocathode acts as a low pass filter of the laser profile, reducing in this way one of the possible sources of microbunching instability. The present experience shows that with this approach SwissFEL, the FEL facility in user operation at the Paul Scherrer Institute, efficiently lases without or with a very limited use of the laser heater. PB - JACoW Publishing CP - Geneva, Switzerland ER -