Some Dynamical Properties of Very Strong Double Layers in a Triple Plasma Device

Since double layers observed in space and in simulations are rarely if every static, considerable attention has been given to studies of motions of double layers in the laboratory. Extensive reviews have recently been published of the dynamical properties of very strong double layers (eV/kTe 1000) in a Q machine (Sato et al., 1983; Iizuka et al., 1983) and strong double layers (eV/kTe 10) in a triple plasma device (Hershkowitz, 1985). In both cases the double layers were essentially planar. We report here on some of the dynamical properties of very strong double layers (eV/kT_ -200) seen in a differentially pumped triple plasma device (Torv6n, 1982). These double layers are V-shaped. In particular, we discuss the following findings: (1) Disruptions in the double layer potential and in the plasma current occur when an inductance is placed in series with the bias supply between the sources in the external circuit. These disruptions, which can be highly periodic, are the result of a negative resistance region that occurs in the I-V characteristic of the device. This negative resistance is due to a potential minimum which occurs in the low potential region of the double layer, and this minimum can be explained as the self-consistent potential required to maintain charge neutrality in this region. (2) When reactances in the circuit are minimized, the double layer exhibits a jitter motion in position approximately equal to the double layer thickness. The speed of the motion is approximately constant and is on the order of 2 times the ion-sound speed. The shape of the double layer does not change significantly during this motion. (3) When the bias between the sources is rapidly turned on, the initial phase in the double layer formation is the occurrence of a constant electric field (uniform slope of the potential) for the first few microseconds. The potential then steepens in the region where the double layer will eventually be formed and flattens in regions above and below this. The double layer is completely formed after about 100 microseconds and then engages in the jitter motion discussed above.