Plasma Sheaths and Double Layers with Instabilities

Sheaths and associated potential structures in plasmas and resulting instabilities are almost ubiquitous in laboratory plasmas and also frequently found in space plasmas. Here we would like to present a few examples, describe their observations and focus on the basic physical explanations for the effects [1] . We will start with high frequency oscillations near the electron plasma frequency. Low frequency instabilities also occur at the ion plasma frequency. The injection of ions into an electron-rich sheath widens the sheath and forms a double layer. Likewise, the injection of electrons into an ion rich sheath widens and establishes a double layer which also occurs in free plasma injection into vacuum. The sheath widens and forms a double layer by ionization in an electron rich sheath. When particle fluxes in "fireballs" [2] get out of balance, the double layer performs relaxation instabilities. Fireballs inside spherical electrodes create a new instability due to the transit time of trapped electrons. On cylindrical and spherical electrodes, the electron rich sheath rotates in magnetized plasmas. Electrons rotate due to E x B0 which excites electron drift waves with azimuthal eigenmodes (see Figure 1 [3] ).