by Virginia Greco. Published: 16 April 2018

…go! During the recent weeks, test, calibration and configuration activities were carried on to prepare the ALICE detector for the imminent restart of beam. In advance with respect to the original plan, the LHC is expected to deliver collisions with stable beams on 17 April.

The LHC is getting ready for injecting physics beam. The first collisions with stable beams of 2018 will be delivered between 16 and 17 of April.

 

Thanks to the excellent performance exhibited by the LHC during the latest weeks, the schedule of the accelerator restarting has been compressed and collisions with stable beams will be delivered beforehand. As a consequence, the ALICE experiment has sped up its commissioning as well to be ready to take data as soon as possible.

The operations in the experimental cavern, which included some minor repairing and interventions on the muon arm and the TPC, are concluded. Technical runs were started at the beginning of March. During them, the various detectors and systems were left working for many hours in a row – without beams in the accelerator – to check their correct functioning and their stability over time. In the first two weeks, these tests were performed only between 7 am and 11 pm and then the detector switched off, so that no crew was required to stay in the control room at night. Following this, full shifts (24 hours a day) were started and tests continued. Dry runs like these are key to the preparation for data taking, since they allow the experts to identify possible issues and glitches and to fix them in time for the restart.

The detectors were gradually included in these common coordinated runs but only after successfully completing a reintegration procedure of their detector control system (DCS), necessary to ensure proper transitioning of detectors from normal to beam-safe running conditions.

The TPC and the TRD also went through an energy calibration (called “krypton calibration”) performed using a solid rubidium source, which decays into a gaseous excited state of krypton that mixes with the gas volumes of the detectors. This excited state returns to its ground state inside the detector with a known energy spectrum.

The Data Acquisition System (DAQ) together with the Central Trigger Processor (CTP) and the High-Level Trigger (HLT), in turn, worked to get prepared for the Pb-Pb collisions that will be delivered at the end of the year, from November on. In particular, various tests have been carried out – and will be continued throughout the year whenever possible – to check, tune and improve how the full chain (from the detector signals to the final data storage) behaves when pushed to the limit of its capacity. Specifically, ‘fake’ events, carrying no meaningful information but having rates and sizes similar to those of the events expected in the future Pb-Pb collisions, were generated to put a significant load on the data channels and - partially - on the processing stages.

After completion of two weeks of dry runs, the ALICE magnet was switched on and data from cosmic ray interactions were taken with many of the detectors until the accelerator team was ready to start test injection in the beam pipe. During this beam operation time, when the experts of the machine put in place their commissioning procedure, the ALICE detector has been put in a beam-safe state. In practice, only the systems that have minimal risk of being damaged when hit by the beam when switched on can actually run. The others have to stay in standby mode or run in a non-standard configuration (for example, no high voltage is applied to the detectors that normally require it).

Collisions with unstable beams were delivered on April 12 and stable beams will be declared at some point between 16 and 17 of April. The ALICE experiment is all set and ready to start its 2018 race.

The first collisions with unstable beams were delivered on April 12, in advance on the previous schedule.