Self-Generated Wide Solitons and Ion Beam Instabilities in a Double Plasma Device.

Investigations of ion beam instabilities in the source and target chambers of a double plasma device are presented. Most notably, a self-generated "wide" soliton is observed when the target potential is set in the range of approximately 0.5 T_ e/e to 2 T_ e/e (where T_ e is the electron temperature and e is an electronic charge) below the source potential. This soliton is unusual in that it is not launched directly but rather is self -generated by the plasma and ion beam. In addition, the soliton has an unusually large width W ~eq 14 lambda_ D(delta n/n) ^{-1/2}, where lambda _ D is the Debye length and delta n/n is normalized density fluctuation (for an ion acoustic soliton in a stationary plasma without an ion beam, a typical width is W ~eq 2.5 lambda_ D(delta n/n)^ {-1/2}).. A self-generated low frequency ({ ~10}^{-2}f_{pi }, where f_{pi} is the ion plasma frequency) ion-ion beam instability is observed to be associated with the soliton. In contrast, when the target potential is <=q T_ e/e above the source potential, no solitons are observed and the self-generated ion instability that is observed has a frequency that is on the order of f _{pi} and is approximately the inverse of the transit time of an ion across the sheaths of the grid separating the source and target chamber plasmas. This instability, which is similar in frequency to the instabilities observed under similar conditions by others, is identified here as an ion beam-ion beam instability that occurs in the sheaths of the grid. When the target potential is well below that of the source, the plasma is quiescent (i.e., there are no solitons and no self-generated ion instabilities observed). This makes possible the detection of small amplitude ("wave packets" launched (i.e., externally generated) by Langmuir probes. The source of the wave packets has been determined to be the Johnson noise of a 50 Omega resistor in a simple amplifying circuit. The random wave packets are oscillations, at approximately the ion plasma frequency, grow and then diminish in amplitude within a few periods. Launched and detected wave packets show a time delay. The wave packets propagate on the slow beam mode which is identified from the measured time delays between wave packets.