Multiple double layers in laboratory experiments relevant for space plasma phenomena

Double layers (DLs) are nonlinear potential structures in plasmas, consisting of two adjacent layers of positive and negative space charges, in which the potential jumps, thereby creating an electric field. They were intensively investigated in the last decades because of their frequent occurrence in almost all possible devices: tokamaks, spheromaks, thermionic converters, Q-machines, double plasma and triple plasma machines. Also in natural plasmas DLs have been observed or postulated: auroras, solar flares, ball lightnings, binary X-ray pulsars, extragalactic jets [1]. Under certain experimental conditions, more complex structures in form of two or more subsequent or adjacent DLs were observed. Such a structure, called multiple double layer (MDL), can appear in two different geometries, concentric and non-concentric, respectively. Concentric MDLs appear as bright and concentric plasma shells, the successive DLs being precisely located at the abrupt changes of luminosity between two adjacent plasma shells. Probe measurements showed clearly that the axial profile of the plasma potential has a stair steps shape, with potential jumps close to the ionization potential of the used gas. These structures are partly similar to other types of multiple DLs, but not so extended [2]. Non-concentric MDLs appear as a network of plasma spheres, near each other, equally distributed on the surface of the positive electrode used for their generation. Each of the plasma spheres consist of a positive nucleus (an ion-rich plasma) confined by a DL. Many auroral photos seem to suggest that MDL structures are involved in its phenomenology. In fact, many authors [3,4] have suggested that, along magnetic field lines, a series of weak double layers exists, appearing and disappearing at random. Here, we report on experimental results concerning the generation and dynamics of MDLs in laboratory plasmas. In these experiments additional positively biased electrodes have been inserted into a weak background plasma. Depending on the conditions and on the size and form of the additional anodes either type of MDL can appear. In the case of concentric MDLs sometimes up to six concentric shells have been found, similar to other authors [5]. These results reveal the important role of elementary processes, in particular electron-neutral impact excitations and ionizations in the physical mechanism of the MDLs creation. The similarities with the auroral phenomenology are emphasized. The potential role of the MDLs in cosmic particle acceleration is discussed.