A detector to measure transverse profiles and energy of an H? beam using gas stripping and laser photo neutralization

A detector has been developed at Brookhaven National Lab (BNL) [1] and installed in the exit beam line of the BNL H− linear accelerator (linac) to measure transverse beam profiles, average beam energy and beam-energy spread. These beam properties are found by deflecting beam electrons, produced by both gas stripping and laser neutralization, into a detector. An H− ion, with a first ionization potential of 0.756 eV, can be neutralized by collisions with background gas and by absorbing the energy of a photon of wavelength shorter than 1.64 m. Free electrons produced by both mechanisms are deflected out of the H− beam by a dipole magnet and into a chamber which measures electron charge vs. energy. Ion-beam profiles are measured by scanning a laser beam across the H− beam and measuring the laser-stripped electron charge vs. laser position. Beam energy is deduced by measuring either the laser-stripped or gas-stripped electron charge which passes through a retarding-voltage grid vs. the grid voltage. Since beam electrons have the same velocities as beam protons, the beam proton energy is the electron energy multiplied by mp/me=1836, [E=(γ-1)mc2].