Observation of the backward electrostatic ion-cyclotron wave

Abstract
The backward branch of the electrostatic ion‐cyclotron wave has been observed for the first time. The wave, which was driven by a phased antenna structure inserted in a neon plasma, exists in the parameter ranges 2Ti/mi≪(ω/k)2≪2Te/me, nΩi<ω<(n+1)Ωi, TeTi, and ωpii. Double‐tip probe interferometry data agree with the theoretical dispersion relation. The antenna couples into the wave more readily on the side of the antenna where it has its smallest wavenumber.