Accelerator design concept for future neutrino facilities by Apollonio, M, Berg, J, Blondel, A, Bogacz, A, Brooks, S, Campagne, J, Caspar, D, Cavata, C, Chimenti, P, Cobb, J, Dracos, M, Edgecock, R, Efthymiopoulos, I, Fabich, A, Fernow, R, Filthaut, F, Gallardo, J, Garoby, R, Geer, S, Gerigk, F, Hanson, G, Johnson, R, Johnstone, C, Kaplan, D, Keil, E