Ultrafast Nonlinear Double Excitations of He in Intense EUV FEL FieldsM. Fushitani, Y. Hikosaka, A. Matsuda, C.-N. Liu, T. Morishita, E. Shigemasa and A. HishikawaEPJ Web of Conferences, 41 (2013) 02009DOI: https://doi.org/10.1051/epjconf/20134102009