Impact of Miniaturized Fixed-Bias Multineedle Langmuir Probes on CubeSats

Abstract

Kinetic simulation results are presented to study the response of multi-Needle Langmuir Probes of the type used on many satellites. Simulations of isolated probes are used to parametrize the current collected as a function of voltage for a set of densities and temperatures of relevance to Earth ionosphere. These simulations also serve to assess the validity of analytic results obtained from the orbit motion limited (OML) theory used in recent studies. Computed probe characteristics are then fitted with empirical scaling laws and used to account for electron current collected by needle probes on a typical triple CubeSat. These fits are then used to determine the impact of the probes and guards on the spacecraft floating potential for a nominal configuration of bias voltages, over the plasma parameters of interest. In order for the probes to work as intended, they must operate at a positive potential with respect to the ambient plasma. However, results show that for the cases considered, the spacecraft floating potential is so low that the probe with the lowest voltage becomes negative. Possible solutions are examined and proposed to ensure that all probes remain at a positive voltage with respect to surrounding plasma.

Finite-length effects on cylindrical Langmuir probes

“…β is similar for the two approaches of including a guard, to within the accuracy of the model. Our results support previous findings [14,16] that cylindrical Langmuir probes must be much more than ten Debye lengths in order to make end effects negligible, even with an ideal guard. Too short a guard may have nontrivial effects on the characteristics, and β in particular, and is not covered by our model.…”

“…It turns out that this expression also holds true for spherical probes but with β = 1 [1]. For this reason, it has several times been assumed that this expression will also hold for thin, finite-length cylindrical probes, approaching 0.5 as the length increases and 1 as it decreases and the probe approaches the shape of a small grain [13,15,16].…”

“…The current collected by the probe can then be computed similarly as in Eq. (16), except that the integration should start at a lower limit λ g . Figure 7 shows the current profile for a probe with λ p = 30 and η = 10 both without a guard (λ g = 0) and with a finite-length guard with λ g = 5.…”

“…Increasing the probe length will of course help, although there is an upper limit to what is practical. Besides, the literature reports different results on how long probes must be to overcome these effects, ranging from about 10 to more than 50 Debye lengths [8,[13][14][15][16]. Some of these references also attempt to find expressions for the characteristics of the probes.…”

“…However, present studies on finite-length effects are limited to very specific ranges of probe lengths, plasma parameters and/or experimental setup, which may explain the discrepancy between these studies; one laboratory experiment is performed using a conducting guard [13], the other with an insulating one [14]. Numerical experiments and analytical results often disregard the guard altogether, focusing on an idealized cylinder, possibly with some discussion on the expected changes due to the guard [8,15,16]. Clearly, there is a need for a more fundamental understanding of finite-length effects in cylindrical Langmuir probes.…”