In the early hours of 23 April the Large Hadron Collider (LHC) operator team declared the first low-intensity stable beams for this year. These first fills for collisions were with individual bunches. They were delivered to the experiments during three consecutive nights, allowing the machine to continue with machine development in daytime. In order to prepare the machine to be filled with trains of bunches, the LHC operators started to undertake scrubbing. This operation is performed to remove electron clouds by circulating the beams with an increased number of bunches at injection energy. After these preparations, the machine will return to stable beams whereby their intensity will be gradually increased. This ramp up phase in the beam intensity together with further scrubbing as needed is foreseen for the next four weeks.
Event Display from a proton-proton collision at 13 TeV in ALICE showing one track in the muon arm and tracks in the central barrel. Filling scheme for a low intensity fill is Single_3b_2_2_2
Due to specific trigger conditions, ALICE processes the signals from the collisions at a certain interaction rate. From the outset the luminosity, as measured by the T0 detector, was therefore levelled to a given predefined target luminosity as defined by the known number of colliding bunches. In order to provide ALICE with a levelled luminosity, the machine simply steers the beams apart, maintaining a fixed distance.
For the past few weeks, ahead of stable beams and during the final machine development phase, ALICE performed both technical and cosmic runs. After completing its commissioning, the Time Projection Chamber (TPC) was included back in these runs in global data taking. With the first data collected during stable beams the ALICE team will be able to test the performance of the new readout cards of the TPC, as well as to check the data quality after the replacement of the gas of the TPC.
As part of closing out the machine commissioning which started in early March, another important step was to define the collision points by using so called “quiet beams”. On 11 and 12 April the machine declared quiet beams, a special beam mode whereby the beams are aligned and steered head-on without being declared stable. All four experiments were asked to provide their luminosity monitors to assist the LHC operators to find the exact collision points. ALICE benefitted from these collisions in order to set up the trigger detectors and to align most of the triggers with respect to the beam phase. These were validated two weeks later with the first collisions in stable beams.