Instability as a source for traveling ion waves

Low-frequency instability is excited by passing a dc electron current in a collisionless Q-machine plasma. The current is driven by applying a positive bias to a grid situated between the hot plate and the cold end plate. The instability shows a standing-wave pattern between the hot plate and the grid. The frequency can be controlled by varying the hot plate-grid separation. This instability is used as a source for excitation of traveling waves behind the grid. The excited waves have the same dispersion relation as ion waves excited by applying external signals to the grid.