Negative ion beam space charge compensation by residual gas

The space charge of intense unbunched ion beams can be compensated by the ions created when the beam ionizes the residual gas, which creates a source of secondary particles inside the beam pipe. For negative ion beams, the effect of the beam electric field is to expel the electrons to the beam pipe walls, while the positive ions are trapped and start to be accumulated. In this paper, we report on experiments to study this space charge compensation (SCC) in a 45 keV ${\mathrm{H}}^{\ensuremath{-}}$ unbunched beam in the CERN Linac4 low-energy beam transport. Beam size and emittance were measured for different gases injected into the beam region to control the degree and speed of the SCC. These results are compared with beam simulations that include the generation and tracking of secondary ions leading to a unique understanding of the transport of the ion beam in some specific cases.