A delicate intervention is being performed on the Time Projection Chamber in preparation for the Run3 of LHC, when the detector will have to cope with higher luminosities.
In these days, at the ALICE experimental site a peculiar and delicate procedure is ongoing: an endoscopic “cleaning” of the Time Projection Chamber (TPC). This operation, which is done in preparation of and as a test for the future Run 3 of LHC, will allow the TPC to be more efficient and afford higher collision rates. In particular, it is believed that it will avoid the parasitic continuous currents which have been observed in the TPC during some tests performed last year.
But… what does it mean cleaning the TPC and why are we doing it?
Let’s remind us that the ALICE TPC is composed of a central barrel full of a gas mixture – which is the actual detector volume –, a high voltage electrode sitting transversally in the middle of the chamber and two end plates – which collect the electric signal generated by crossing particles. The central barrel, in turn, is made of a field cage, where high voltage (100 kV) is applied between the central electrode and the end plates, and a containment vessel. The isolation of the high voltage field cage from the rest of the experiment is ensured by filling the gaps between the containment vessel and the field cage with CO2. On the side of the containment vessel, there are small holes of 8 mm diameter, which can be opened to access the volume filled with CO2.
Using endoscopic cameras, this volume was explored from the inside and the presence of filaments of dust was detected. Most likely, they are the cause of the parasitic currents observed in the TPC during high-luminosity tests. Because of the high voltage between the internal and external walls of this containment vessel, flecks of dust that entered inadvertently get polarized and tend to form filaments standing on one wall – the cathode. When there are collisions, these filaments receive radiation and as a result emit electrons (in a continuous way), which cause additional (parasitic) currents that add to the normal currents through the resistor network of the central barrel.
By inserting small extractor tubes in the side holes, it is possible to suck out the dust, which is mainly accumulated in the lower part of the vessel. This procedure is being performed in these days in subsequent steps, interleaved with tests.
Since at the moment the accelerator is not operating, in order to test the TPC with radiation, an X-ray tube is used. The amount of X-rays needed to simulate the effect of the accelerator running at high luminosity is estimated by calculations. More “realistic” tests will be done this year, after the restart of the LHC.
At the end of 2018 the accelerator and the experiments will be stopped and will enter a two-year-long phase of shutdown, during which upgrades and improvements will be done. The ALICE collaboration is planning an upgrade of the TPC, which will be equipped with new readout chambers and electronics, meant to allow the experiment to cope with the higher rate of events foreseen. The central barrel of the TPC, though, will stay as it is. Thus, this effort to avoid any parasitic current is key to guarantee the correct functioning of the detector and its high efficiency.