LAMFI-USP uses an Pelletron-Tandem type 5SDH electrostatic accelerator with a charge stripper (using N2 gas) in the center terminal, for ion charge exchange. The accelerator, built by NEC, National Electrostatic Corporation - USA, can reach up to 1.7 million volts at the terminal. It is possible to accelerate various types of ions in a wide range of energies for use in backscattering and low energy nuclear reactions experiments, X-ray Emission Analysis, ion implantation and nuclear physics.
The accelerator bean inlet and outlet are grounded while the center teminal is kept at high positive voltage. The terminal high voltage is generated by a chain of charges, where each pellet (thus the name Pelletron) is positively charged and mechanically transported, against the electric field to the terminal. The gaseous stripper, inside the terminal, reverses the charge of the beam by removing two or more electrons from the ions. The exchange efficiency is a function of the acceleration voltage and the gas pressure. For protons and He++ the working high vacuum reading at the High Energy side of the accelerator is about 2.0x10-5 torr. To generate a beam of He+ ions the HE high vacuum reading is about 2.0x10-6 torr.
At the maximum accelerator voltage, a proton beam will consist of 3.4 MeV ions and a helium beam will have an energy of 3.4 MeV, or 5.1 MeV for He++ ions. To avoid sparks from the terminal, the accelerator is maintained in tank with sulfur hexafluoride (SF6) dielectric gas, at 80 psi.
Cut view of the NEC 5SDH accelerator. In the center is the positive terminal which is electrically charged by the current of pellets which carry the induced charge from the base to the terminal. See a video of a working Pelletron accelerator (thanks to NEC).