Temperature control of ion guiding through tapered capillaries

We investigate the guiding of Ar7+ ions (kinetic energy of 4.5 keV) through a single macroscopic tapered glass capillary of conical shape as a function of capillary tilt angle with respect to the incident ion beam direction. At room temperature a minimum in the transmitted ion intensity appears around the forward direction, which was previously observed and interpreted by a blocking of the transmission by repulsive Coulomb forces due to a uniformly charged ring shaped region in the centre part of the capillary. By heating the tapered capillary to temperatures around 100 °C and thus drastically increasing the electrical conductivity of the capillary material we no longer observe a minimum in the transmission curve but the transmission curve now has its maximum in forward direction. Since the maximum transmission at high temperature in forward direction is still smaller than the minimum in transmitted intensity at room temperature, we conclude that even at room temperature and in forward direction the focusing effect due to guiding is dominant and only partially weakened by blocking. Our experimental results are well reproduced in simulations using a theoretical model originally developed for straight nano-capillaries.