Scattering of H- Stripped Electrons from SEM Grids and Wire Scanners at the CERN LINAC4

At the CERN LINAC4, wire grids and scanners will be used to characterize the H− beam transverse profiles from 45 keV to 160 MeV. The wire signal will be determined by the balance between secondary emission and number of charges stopped in the wire, which will depend on the wire material and diameter, the wire polarization and the beam energy. The outermost electrons of H− ions impinging on a wire are stripped in the first nanometers of material. A portion of such electrons are scattered away from the wire and can reach the neighboring wires. In addition, scattered electrons hitting the surrounding beam pipe generate secondary electrons that can also perturb the measurement. Monte Carlo simulations, analytical calculations and a laboratory experiment allowed quantifying the amount of scattering and the scattered particles distributions. The experiment was based on 70 keV electrons, well reproducing the case of 128 MeV H− ions. For all the LINAC4 simulated cases the predicted effect on the beam size reconstruction results in a relative error of less than 5%.