Quadrupole Radiation of Ion-Acoustic Waves

An experiment to measure the radiation of ion-acoustic waves from a quadrupole antenna is described. The quadrupole consists of 4 hemispherical grids located at the corners of a square structure. The results are contrasted with those obtained from an antenna with the 4 hemispheres excited in phase and compared with a fluid model prediction. Electrostatically coupled excitation of ion-acoustic waves from 3 hemispheres when only the fourth is excited is also observed in this experiment.