The excitation of whistler waves in a radial inhomogeneous plasma is investigated experimentally, using a slow-wave structure consisting of a helix of variable length surrounding the plasma column. The excited waves were observed to have a wave-vector parallel to the external magnetic field. The possibility of exciting the waves in different radial regions is demonstrated.
Balmashnov, A. A., Lynov, J-P., Michelsen, P., & Juul Rasmussen, J. (1981). Exitation of Whistler Waves by a Helical Wave Structure. Journal of Physics D: Applied Physics, 14(10), 1803-1809. https://doi.org/10.1088/0022-3727/14/10/014