Transmission dynamics of 1 MeV H+ microbeam guided through an insulating macrocapillary

Abstract

Experimental observation of the dynamics of a micro-focused, 1 MeV proton beam guided through a single, insulating macrocapillary is presented. The micrometer sized proton beam was center injected into a poly(tetrafluoroethylene) capillary with macroscopic dimensions. The capillary was tilted to 1° with respect to the beam, i.e. no geometrical transmission was possible, but the beam first hit the inner capillary wall. After a sufficient electric charge-up of the insulator material due to the interaction with the charged particle beam, guiding appeared thanks to the beam deflection caused by the formed electrostatic field. With an increasing amount of the accumulated charge on the wall, the deflection of the beam also increased, resulting in the dynamics of the transmission. When a dynamical equilibrium between the charges being deposited and flowing away in form of leakage current was set in, the deflection of the beam saturated at a certain position. Here, the transmitted fraction of the beam reached 90% relative to the incident beam. At this position the angular distribution of the transmitted beam was determined. In the sample tilting plane focusing effect was observable, while in the perpendicular plane, the beam was defocused.