Chen, J.F. Bertucci, M. Bosotti, A. Eshraqi, M. Lindroos, M. Michelato, P. Monaco, L. Pagani, C. Paparella, R. Pirani, S. Sertore, D. Åkesson, T.P.Å. JACoW Geneva, Switzerland 978-3-95450-191-5 10.18429/JACoW-SRF2017-TUPB047 English 489-493 TUPB047 ion cavity site electron simulation Contribution to a conference proceedings 2018 2018-01 https://doi.org/10.18429/JACoW-SRF2017-TUPB047 http://jacow.org/srf2017/papers/tupb047.pdf During the first vertical test of ESS Medium Beta large-grain prototype cavity in INFN-LASA, a phenomenon of coexisting two passband-modes was observed – 4π/6 mode was excited spontaneously during the power rise of 3π/6 mode. This phenomenon is most likely due to the field-emission electrons that transfer their energy gained from the 3π/6 mode to the 4π/6 mode. In this paper, we present the experimental results, the excitation mechanism and the related simulation results.