Electron-Field Instability: Excitation of electron plasma waves by an ambipolar electric field*

Electric fields are common in plasmas, especially ambipolar fields in quasineutral regions, and affect transport by driving currents and, in some cases, instabilities. The condition for instability in collisionless plasmas is understood via the Penrose criterion, which quantifies the relative drift between different populations of particles that must be present for wave amplification via inverse Landau damping. For example, electric fields drive drifts between electrons and ions that can excite the ion-acoustic instability. Here, we use particle-in-cell simulations and linear stability analysis to show that electric fields can drive a fundamentally different type of kinetic instability, named the electron-field instability. This instability excites electron plasma waves with wavelengths >∼ 30λDe, has a growth rate that is proportional to the electric field strength, and does not depend on the relative drift between electrons and ions, and so is not described by the Penrose criterion. Fluctuations formed by the instability oscillate near the electron plasma frequency, further distinguishing it from the ion-acoustic instability, which oscillates near the ion plasma frequency. The ubiquity of electric fields in plasmas suggests that this instability is possible in a host of systems, including low-temperature and space plasmas. In fact, damping from neutral collisions in such systems is often not enough to completely damp the instability, adding to the robustness of the instability across plasma conditions.