H- extraction systems for CERN’s Linac4 H ion source

Abstract

Linac4 is a 160 MeV linear H

accelerator at CERN. It is an essential part of the beam luminosity upgrade of the Large Hadron Collider (LHC) and will be the primary injector into the chain of circular accelerators. It aims at increasing the beam brightness by a factor of 2, when compared to the currently used 50 MeV linear proton accelerator, Linac2.

Linac4’s ion source is a cesiated RF-plasma H ion source. Several beam extraction systems were designed for H beams of 45 keV energy, 50 mA intensity and an electron to H ratio smaller than 5. The goal was to extract a beam with an rms-emittance of

mm mrad.

One of the main challenges in designing an H extraction system is dumping of the co-extracted electrons. Separating the electrons from the negative ions as early as possible reduces space-charge induced emittance-growth. However, a strong magnetic field close to the extraction might cause unnecessary strong deflection in a region of low beam energy. For this purpose a novel magnetic configuration was designed using a magnetic shield between the magnetic fields of the source and the electron dump, which conserves the filter field strength to keep the electron to H

ratio low and effectively dumps the co-extracted electrons. Magnetic configuration and beam trajectories were calculated using the TOSCA Opera 3D code and IBSimu, respectively. Three extraction systems will be discussed in terms of electron dumping efficiency, emittance and transport through the extraction system and LEBT to the RFQ and compared to the simulations.

An improved emittance conservation through the extraction system and LEBT is predicted and further design improvements are proposed. Measurements show that the novel electron dump successfully traps the co-extracted electrons.