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Towards a Carbon Nanotube Ionization Source for Planetary Atmosphere Exploration


OZA Apurva




Auteur(s) supplémentaire(s)F.Leblanc, J.J Berthelier, J.Becker, P. Gilbert, L. Vettier, R.Coulomb, N.T. Hong*, S.Lee**.
Institution(s) supplémentaire(s)* Center for High Technology Development, VAST, Vietnam. **Ajou University, South Korea.


   The characterization of planetary exospheres today, relies on the development of a highly efficient ionization source, due to the scant neutral molecules ($n < 10 ^ 8 cm^{-3}$) present in diffuse planetary coronae. These tenuous atmospheres provide insight on to physical processes known to occur such as: space weathering, magneto-atmosphere interactions, as well as atmospheric escape mechanisms, all of which are being heavily investigated via current 3D Monte Carlo, Exosphere General Model (EGM) simulations (Turc et al. 2014, Leblanc et al. 2016 in prep) at LATMOS.  Validation of these studies will rely on \textit{in-situ} observations in the coming decades. 

Neutral detection strongly depends on electron-impact ionization which via conventional cathode-sources, such as thermal filaments (heated up to $2000K$), may only produce the target ionization essential for energy-measurements with large power consumption. Carbon nanotubes (CNTs) however, are ideal low-power, cold cathodes, when subject to moderate electric fields ($E \sim 1 M V/m$). We present our current device, a CNT electron gun (CNTEG), powered by a $15 mm^2$ CNT chip. The CNTEG currently extracts hundreds of $\mu$Amperes of field electrons with applied external voltages of $\sim -150$ Volts, approaching minimum power consumption $< 0.1$ Watts. The 3D modeling of field effect electrons ionizing a standard influx of neutrals is shown, using the multiphysics suite \texttt{COMSOL}. 

To better anticipate the species an ideal \textit{in-situ} spacecraft equipped with such an ionization source would observe, for a given position and time, we simulate  Europa's exosphere. We observe atmospheric inhomogeneities shaped by (1) Jovian gravitation, (2) solar insolation of water ice, and (3) sputtering of the icy regolith from Iogenic plasma torus ions, S$^{n+}$, O$^{n+}$ (Cassidy et al. 2013), ejecting predominately molecular oxygen as predicted by Johnson et al. 1982, and potentially observed by HST (Saur et al. 2011). 

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