Beam Commissioning of Linac4 up to 12 MeV

CERN Linac4 is made of a 3 MeV front end including a 45 keV source , a 3 MeV Radio Frequency Quadrupole (RFQ) and a fast chopper, followed by a 50 MeV Drift Tube Linac (DTL), a 100 MeV Cell-Coupled Drift Tube Linac (CCDTL) and a 160 MeV Pi-Mode Structure (PIMS). The Linac4 beam commissioning is performed in 6 stages of increasing energy. Movable beam diagnostics benches, with various instruments, are used at each step to allow the detailed characterisation of operational parameters that will play a key role in the overall future performance. The first three stages of the commissioning, up to 12 MeV beam energy, have been completed at the end of 2014. The RFQ and the chopper line at 3 MeV, as well as the first tank of the DTL at 12 MeV were fully characterised, using permanent diagnostic instruments and a movable diagnostic bench equipped with a spectrometer, a slit-grid emittance meter, a Bunch Shape Monitor, Beam Position Monitors and a laser-emittance device. This paper reports on the strategy and the results of the commissioning up to 12 MeV. It also presents the validation of the set-up strategy, which is essential for the next stages of commissioning.