Fast electron heating due to the interplay of electron-and ion-acoustic waves in a current-driven turbulence*

We report a one-dimensional particle-in-cell simulation of fast electron heating in collisionless current-driven turbulence due to the coupling between electron- and ion-acoustic waves. We used a condition for the ramp-up phase in the tokamak startup, but similar physics should apply to e.g. plume of hollow cathode at low pressure. PIC simulation results show that the drift velocity difference between electrons and ions excites the ion-acoustic waves, which interact with slow electrons and modify the local distribution function, thereby leading to the marginal instability that generates fast electron-acoustic waves. Electron-acoustic waves slow down the high-energy electrons, and then, ion-acoustic wave further traps these decelerated electrons in the wave trough and thus creates a giant electron hole. In this hole, strong phase mixing is generated, producing strong heating to the electrons. The numerical simulations are consistent with the measurements and provide insight into the key processes responsible for electron heating and the generation of nonlinear waves in a collisionless current-driven instability.