Abstract
Linac4 is a 160MeV 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 50MeV 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 45keV 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 0.25πmmmrad.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.
Highlights
The extraction system of an ion source is the first stage of a particle accelerator
The challenge presented in this paper is to design a high current, low emittance H− beam extraction system that will be used at Linac4
Each iteration aimed at improving the electron dumping efficiency and the beam properties for increased transmission through the Radio-Frequency Quadrupole (RFQ) over the previous version
Summary
The extraction system of an ion source is the first stage of a particle accelerator. It is responsible for early beam formation and defines the initial intensity and beam quality that is passed down the chain of accelerating structures. The challenge presented in this paper is to design a high current, low emittance H− beam extraction system that will be used at Linac. The Linac ion source is a cesiated molybdenum-surface radiofrequency-plasma H− ion source It produces H− through the volume and surface production mechanisms [3,4,5]. Separated from the H− beam and dumped This requires additional magnetic dipole fields in the vicinity of the extraction region, which are superimposed with the magnetic fields of the source. There is a multicusp field for confinement and a dipole filter field [3,9] for suppressing the high energy electrons in the vicinity of the extraction aperture and reducing the destruction of the fragile H− ions. Possible improvements to the existing setups are discussed at the end of this article
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