Plasma species mix diagnostic using ion–ion hybrid layer reflectometry

Abstract A superheterodyne reflectometer could provide a direct and inexpensive measurement of ion species mixes with different charge-to-mass ratios. Using the cold plasma dispersion relation, the ion–ion hybrid cutoff frequency is uniquely determined by the density ratio and cyclotron frequencies of the two different species. The phase of a 20 MHz wave that travels from the launching point to the cutoff layer to the receiving antenna provides a direct measure of the hydrogen : deuterium species mix. In the first experiment, a fast Alfvén wave is launched perpendicular to a hydrogen–deuterium plasma from the low field side of the DIII-D tokamak. Quantitative measurements observe a hydrogen concentration range of 3–67% and a maximum penetration depth of 0.60 m. Corroborative values are obtained from two independent diagnostics. In the second experiment, the fast Alfvén wave is launched from the high field side (HFS) during a hydrogen puffing experiment. The results suggest that a wave launched from the HFS is able to tunnel through the resonance layer and reflect back to the receiving antenna.