Field Enhanced Secondary Electron Emission on Glassy Carbon Grains

We have investigated the charging of spherical glassy carbon grains of 3 to 6 micrometers in diameter under the influence of high energy electron bombardment. The single grain is trapped in the electrodynamic quadrupole trap and hold there for a long time (up to days). The grain is isolated and therefore its charge can be altered by impinging and leaving electrons only. We are able to continuously monitor the grain charge and we can determine the net current on the grain. We have measured the secondary electron-electron emission yield and we have observed the increase of the yield that was attributed to the electric field at the surface of the grain. The relation between the measured yield and primary energy follows the universal curve suggested by Draine and Salpeter [1979] and the total yield increases with square root of the surface electric field according to our results. Introduction Low mass of dust grains allow them to reach comparatively high charge-to-mass ratio and therefore the electro-magnetic forces has a strong influence on the motion of the grain [Horányi, 1996]. Understanding the charging processes is important for description of the dynamics of dust particles. The charge of the grain can be altered by various processes. Among others, the secondary electron-electron emission plays an important role in setting the potential of the grain [Meyer-Vernet, 1982]. The secondary electron emission (SEE) from small grains (diameter of the order of microns) can be considerably different than that from planar bulk materials and the grain size is an important parameter determining its potential in the plasma. When a penetration depth of primary electrons is comparable to the size of the grain, the yield of the backscattered electrons increases [Richterova et al., 2007]. Due to high curvature of the surface, the surface electric field is relatively high. Therefore, the electron field emission can be observed at surface potentials in order of 100V even on smooth surfaces [Pavlu et al., 2008]. With lower fields, the yield of secondary emission can be altered. The yield of dielectric surfaces can be significantly increased by the electric field penetrating into the material and causing the additional acceleration of electrons [Jacobs et al., 1952]. Since the increase is caused by the field inside the material this effect should not increase the yield of conductors. However, the secondary yield from glassy carbon (conducting) surface is enhanced too, although weakly. This was noted in the previous paper by Beránek et al. [2009]. In the presented paper, the research is expanded further and the increase is quantified. We have treated the single glassy carbon grain using the beam of electrons of adjustable energy. The continuous monitoring of the charge of the grain has been used for determination of the surface potential and the net current flowing on the grain. Using this technique, we have observed variations of secondary electron-electron yield with the electric field at the surface. Experiment set-up The single spherical dust grain was trapped in the quadrupole electrodynamic trap under UHV conditions (about 10−7 Pa) [Cermak, 1994]. Being held in the centre of the trap, the grain is isolated and its charge is determined by the balance between the primary electron current 188 WDS'09 Proceedings of Contributed Papers, Part II, 188–194, 2009. ISBN 978-80-7378-102-6 © MATFYZPRESS