Analysis of chamber simulations of long collecting probes in high-speed dense plasmas

Abstract

Chamber tests of simulated electrodynamic tethers (EDTs) of different geometries operating in a dense high-speed plasma are described. The geometries tested and described here are cylindrical and flat-ribbon tape. By moving the probe samples relative to the plasma source it was possible to vary the density and therefore the effective width over a range of approximately 1 to 2 Debye lengths ( ) for the cylinder sample and 6 to 19 for the tape samples. Several important conclusions can be drawn from the tests. 1) The current-voltage characteristics of the cylinder behave as predicted by orbital-motion-limited (OML) current collection theory in the saturation region. 2) The tape tether had comparable current levels to a theoretical equal area OML cylinder up to an effective width of at least 11 and possibly wider. 3) Orienting the tape samples parallel or perpendicular to the plasma flow yielded different current responses (perpendicular is larger) above a bias potential that is near the estimated energy of the incoming beam ions. The observed difference was generally more pronounced at larger effective widths (higher densities). 4) It was also necessary to be above this bias potential to have a 0 5 current-voltage character appropriate for an ideal cylinder in the OML regime. It is concluded that wide ribbon-like tape tethers can be effective current collectors but that velocity effects will be a factor to consider, especially as relative width of the tape tether (with respect to ) grows.

Measurement of Cross-Section Geometry Effects on Electron Collection to Long Probes in Mesosonic Flowing Plasmas

“…Nonetheless, a low duty cycle pulsing of the applied voltages was necessary to allow the samples to cool off thus preventing melting; details of this procedure are reported in Ref. 5.…”

“…The effective solid tape widths spanned from 4.9 to 42 Debye lengths, which extends our previous range of tested widths. 5 As for the three slotted samples, they were designed with the same overall widths as their solid counterparts; this strategy allowed us to compare solid and porous samples spanning equivalent widths. In addition, the design is such that each of the four lines on every slotted sample has the same perimeter as our reference cylinder, allowing one to consider the effects and measure the extent of sheath interactions.…”

“…In a previous investigation, 5 it was determined that tape tethers with widths up to 10 Debye lengths would perform close to an equal-area reference cylinder, and that the perpendicular tape orientation would consistently outperform the parallel orientation in terms of collected current.…”

“…In the present communication, we describe the results of a new set of chamber tests that addresses questions of end effects and plasma source current limitations discussed in Ref. 5, in addition to testing a larger breadth of "solid tape" widths and a new "slotted tape" geometry with various porosities. The issue of end effects was addressed by adding guards to the tether samples, which are described below.…”

A Comparison of Laboratory Experimental and Theoretical Results for Electrodynamic Tether Electron Collection Performance for Some Bare Tether Geometries

“…The primary objective of this experiment is to further understand the differences and similarities behind how distributed or sparse geometries perform compared to that of thin cylindrical tether geometries within a high-speed flowing plasma. It is important to note that he mechanisms behind electron current collection within these various tether geometries for a flowing plasma have yet to be understood [6,8]. Some particular objectives that an electrodynamic tether design needs to account for are lifetime, minimum mass, minimum drag, and maximum current collection.…”