Bare-Tether Sheath and Current: Comparison of Asymptotic Theory and Kinetic Simulations in Stationary Plasma

Abstract

Abstract-Analytical expressions for current to a cylindrical Langmuir probe at rest in unmagnetized plasma are compared with results from both steady-state Vlasov and particle-in-cell simulations. Probe bias potentials that are much greater than plasma temperature (assumed equal for ions and electrons), as of interest for bare conductive tethers, are considered. At a very high bias, both the electric potential and the attracted-species density exhibit complex radial profiles; in particular, the density exhibits a minimum well within the plasma sheath and a maximum closer to the probe. Excellent agreement is found between analytical and numerical results for values of the probe radius R close to the maximum radius -R max for orbital-motion-limited (OML) collection at a particular bias in the following number of profile features: the values and positions of density minimum and maximum, position of sheath boundary, and value of a radius characterizing the no-space-charge behavior of a potential near the high-bias probe. Good agreement between the theory and simulations is also found for parametric laws jointly covering the following three characteristic R ranges: sheath radius versus probe radius and bias for R <^i -Rmax! density minimum versus probe bias for R = _R max ; and (weakly bias-dependent) current drop below the OML value versus the probe radius for R > _R max .

Increased electric sail thrust through removal of trapped shielding electrons by orbit chaotisation due to spacecraft body

“…(35) numerically, one finds even larger values than our parameter R above. The reason for the difference between R and r s is that the parameter r s of Sanmartín et al (2008) corresponds to the distance where the potential V (r s ) becomes comparable to T e ≈ 12 eV, whereas our R corresponds to the distance where V (R) ∼ V 1 ≈ 1 kV. Thus the result of Sanmartín et al (2008) is consistent with our results.…”

On the feasibility of a negative polarity electric sail

“…(3) includes the extra shielding effect of these trapped electrons. As shown by Sanmartín et al (2008), if trapped electrons were absent, the potential structure would be wider. Presumably, the thrust would also then be larger than predicted by Eq.…”

Low work-function thermionic emission and orbital-motion-limited ion collection at bare-tether cathodic contact

“…17), the curve lies entirely above the diagonal in case of OML regime (R R max , I i ¼ I iOML ), otherwise the non-OML regime (R > R max , I i < I iOML ) applies. 18,19 In the presence of emission, emitted electrons result in negative space charge, which decreases the electric field that accelerates them outwards, or even reverses it, decelerating electrons near the emitting probe. In the case of a monotonic potential as curve c in Fig.…”

“…[16][17][18][19] The impact of relativistic effects using similar methods has been analyzed for a possible Jupiter mission with electrodynamic bare tethers. 20,21 The effects of emission were first investigated by Langmuir for a planar sheath problem, not fully self-consistent but a good approximation for strong double layers.…”