Nonlinear harmonic generation in a free-electron laser seeded with high harmonic radiationT. Tanikawa, G. Lambert, T. Hara, M. Labat, Y. Tanaka, M. Yabashi, O. Chubar and M. E. CouprieEPL, 94 3 (2011) 34001DOI: https://doi.org/10.1209/0295-5075/94/34001