T. Nogi, T. Katsurayama, T. Kii, K. Masuda, K. Morita, T. Murata, \emph{et al.}, "Image Charge Effect on Emittance Reduction Phenomenon in Electron Gun", presented at the 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper TUC04, unpublished.