I zyxwvutsrqponml II zyxwvutsrqponmlkji IEEE TRANSACTIONS ON zyxwvutsrqpo PLASMA SCIENCE, zyxwvutsrqponm VOL. 20, NO. 6, DECEMBER 1992 601 Double-Layer-Relevant Laboratory Results D. Diebold, Member, IEEE, C. E. Forest, N. Hershkowitz, Fellow, zyxwv lEEE, M.-K. Hsieh, T. Intrator, D. Kaufman, G.-H. Kim, S.-G. Lee, and J. Menard A bstract-over the past few years, many double-layer-relevant laboratory experiments have been carried out at the University of Wisconsin. Laboratory stair-step double layers, which resem- ble three or more weak double layers joined in series, have been produced without ionization for the first time. Double- layer floating potential fluctuations have been investigated and progress has been made in developing a novel technique for measuring electron energy distribution functions in low-density double layers (i.e., AD >> probe dimensions). A new inductive plasma source has been developed. With this source, magne- tized double layers can be routinely produced. These magnetized double layers are often weak stair-step double layers that are oblique to the magnetic field. Laboratory data of emitting probe characteristicstaken in tenuous plasmas have helped to quantify space-chaqge-enhanced plasma gradient induced error in double- layer electric field measurements made by satellite double probes. zyxwvutsr Also, magnetic sheaths have been experimentally studied and compared with theory. I. INTRODUCTION LTHOUGH it has long been thought that double layers A(DL ’s) may be responsible for the acceleration of charged particles associated with the aurora borealis, it is still not clear whether it is strong double layers (SDL’s), i.e., DL’s with eAq5 >> lOT,, where A4 is the potential drop across the double layer and T, is the plasma electron temperature, or weak double layers (WDL’s), i.e., DL’s with eAq5 5 lOT,, that are primarily responsible for auroral particle acceleration. Temerin et al. [ l ] found that probes on board the S3-3 polar- orbiting satellite observed many WDL’s, between altitudes of 6000 and 8000 km, aligned with and moving along the magnetic field. They proposed that a series of these DL’s might account for a large portion of the parallel potential drop that accelerates auroral particles along field lines. More recently, but again from S3-3 data, Mozer [2], [3] has found evidence of field-aligned SDL’s and has suggested that SDL’s may be primarily responsible for auroral particle acceleration. Analysis of Viking data by Bostrom et al. [4]-[6] revealed a predominance of WDL’s in regions of relatively weak overall potential drop, i.e., less than 1 keV as inferred from particle data. Mozer [3] has noted in his analysis of S3-3 data that when S3-3 was in regions of strong overall potential drop (roughly as much as 10 keV or more as inferred from particle Manuscript received February 18, 1992; revised May 29, 1992. D. Diebold, C.E. Forest, N. Hershkowitz, T. Intrator, G.-H. Kim, S.-G. Lee, and I. Menard are with the Department of Nuclear Engineering and Engineering Physics, University of Wisconsin-Madison, Madison, WI 53706. M.-K. Hsieh is with the Department of Electrical and Computer Engineer- ing, University of Wisconsin-Madison, Madison, WI 53706. D. Kaufman is with the Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706. IEEE Log Number 9205361. data) its data were dominated by SDL’s and that when S3-3 was in regions of weak overall potential drop its data were dominated by WDL’s. Although Viking has flown in both these types of regions, the analysis of Viking data to date has been concentrated on those regions of weak overall potential drop [6]. It may be that in regions of weak overall potential drop WDL’s are primarily responsible for charged particle acceleration while in regions of strong potential drop it is SDL‘s that are primarily responsible [3], [6]. Our group has been experimentally addressing DL issues for the last decade at the University of Wisconsin, and before that at the University of Iowa. This laboratory work has included both SDL’s [7]-[ll] and WDL’s [12]-[22]. More specifically,this work includes the first laboratory observations of ion-acoustic-type DL’s [18], slow ion acoustic DL’s [20], two-step DL’s [16], very weak DL‘s (i.e. Aq5 zyx N T,) [NI, DL’s with no external electric field applied [13], currentless DL’s [20], stair-step DL’s [21], magnetized stair-step DL’s [22], SDL’s in a triple plasma device [7], WDL’s in a triple plasma device [12], and U-shaped DL’s in a magnetic field in a triple plasma device [8]. It should be noted that all our DL’s have been produced in triple plasma devices [9] and that with the exception of the DL’s described in [lo], [18], and [20], these DL’s have been stationary. For a review of many of these, as well as other DL experiments, the reader is referred to Hershkowitz [23]. 11. STAIR-STEP DOUBLE LAYERS Chan and Hershkowitz [16] were the first to show that it is possible to obtain laboratory two-step DL’s (which resemble two single WDL’s joined in series) without ionization occur- ring in the DL’s. zyxwv An example of a two-step DL, measured by Bailey, is shown in Fig. 1. Chan and Hershkowitz [16] argued that two-step DL’s can be produced when the ratio XDIL, where AD is the Debye length and L is the length of the chamber in which the DL’s are produced, is made to be less than approximately 1 x In their experiment, they achieved this both by increasing the plasma density and by increasing L. However, subsequent efforts in which both the plasma density and the device length were further increased failed to produce stair-step DL’s with three or more single DL’s joined in series. Bailey and Hershkowitz [21] were the first to achieve, without ionization, laboratory stair-step DL’s with three or more single WDL’s joined in series. These stair-step DL’s were achieved by careful control of the boundary conditions and appeared to be a new class of laboratory DL’s that were intermediate between anomalous resistivity [24] and 0093-3813/92$03.00 0 1992 IEEE