Determine Ion Composition using Alfven Wave Measurements

Natural plasmas, such as the solar wind and Earth's magnetosphere, contains multiple ion species. The ion composition provides important information about the origin of the plasma, and it may also reveal the underlying energization process of the plasma. However, direct measurement of ion composition is sometimes very difficult, especially when the plasma is cold. Alternatively, measurements of Alfven waves can be used to determine the composition because wave properties such as the dispersion relation is strongly altered by ion composition. In this work, we present results from Alfven wave experiments in a proton-helium plasma on the Large Plasma Device. Two methods are used to estimate composition. The first one involves nonlinear interactions of three waves near the helium cyclotron frequency. Two large-amplitude counter-propagating Alfven waves are launched to excite a third wave through three wave resonance. The measured properties of these waves are then used to estimate relative density of helium ions. In the second method, we scan the frequency of a small-amplitude Alfven waves launched from the antenna near helium cyclotron frequency to obtain a linear dispersion relation. The two methods show good agreement when waves are well resolved in the experiment. The outcome of this study will have implications in developing new technology to measure cold ion populations in space plasmas, which is very challenging using traditional methods.