Operation of free-electron lasers using spherical whistler and rippled density plasma Lenic Anshal, Alireza Paknezhad, Niti Kant and Oriza Kamboj Eur. Phys. J. Plus, 139 9 (2024) 833 Published online: 20 September 2024 DOI: 10.1140/epjp/s13360-024-05642-w