Improved beam extraction for a negative hydrogen ion source for the LHC injector chain upgrade, Linac4

In the scope of an upgrade of the injector chain of CERN’s accelerator complex, a new linear accelerator, Linac4, is under construction. This accelerator will replace the existing 50 MeV proton linac, Linac2. By increasing the beam energy to 160 MeV, Linac4 makes it possible to double the brightness in the PSB, and ultimately increase the luminosity in the LHC. Linac4 will accelerate beams of negative hydrogen (H-) to be injected into the PSB by multi-turn, charge exchange injection. The ion source was initially based on the non-caesiated RF-volume source from DESY. However, the beam extraction from this source could not handle the 45 keV beam energy required by the RFQ. A new beam extraction system has therefore been designed, via IBSimu simulations [1], to extract and transport the H- ion beam respecting the Linac4 requirements. Key features of the extraction system is a tuneable puller voltage to adapt the extraction field to the ion and electron beam currents, and a magnetized Einzel lens to dump the co-extracted electrons with an energy below 10 keV. This new extraction system has been successfully commissioned at the Linac4 ion source test stand, and is currently being used during the commissioning of the components of Linac4. An H- beam peak current of 60 mA has been measured at the test stand, and long-term stable H- beams of 35 mA have been demonstrated. The extraction system is flexible for the connection of different plasma generators, and can also be used to extract a proton beam by inverting the polarities of the high voltage transformers.