Low-frequency electrostatic waves in a magnetized, current-free, heavy negative ion plasma

We report experimental observations of a low-frequency (<< ion gyrofrequency) electrostatic wave mode in a magnetized cylindrical (Q machine) plasma containing positive ions, very few electrons and a relatively large fraction (n _-/n_e > 10³) of heavy negative ions (m _-/m ₊ ~ 10), and no magnetic field-aligned current. The waves propagate nearly perpendicular to B with a multiharmonic spectrum. The maximum wave amplitude coincided spatially with the region of largest density gradient suggesting that the waves were excited by a drift instability in a nearly electron-free positive ion-negative ion plasma