Gassing up the LHC experiments
ALICE TPC gas system is the first of the LHC experiments to be put on line.
With a series of gas distribution equipment put on line, the ALICE Time Projection Chamber is the first detector of the LHC experiments to receive the mixture it needs for particle tracking. After extensive commissioning starting in 2004 by the Physics Department's DT1 gas team, it took about eight days to fill the detector with 90 m3 of neon, carbon dioxide and a small amount of nitrogen.
'This is the first system that we really completed from A to Z,'said physicist Stefan Haider (PH-DT1-GS), who is responsible for the gas control systems and commissioning. Unlike in the LEP era, when gas systems were individually prepared and maintained by each experiment, the PH Department has rationalized the efforts and given the responsibility for building and operating the five LHC experiments' gas systems to a single team in the DT1 support group. This provides gas systems made from common technology and building blocks and offers a more efficient approach to long-term operation and maintenance.
The gas lab provides 23 mixtures that include argon, carbon dioxide, neon, xenon, methane, isobutene and the freons C2H2F6 and SF6. Experiments use the different mixtures for tracking, triggering or just keeping a good atmosphere in a certain region of a detector.
The basic machinery of gas distribution systems are refrigerator-sized racks that mix the needed gases. The gas lab is making 220 of these racks and 40 control racks, which send the mixture through one of two systems: Open-loop systems that replace the mixture after it's been used by the detector, or closed-loop systems that recirculate the gas, replacing only 1-5 % of the mixture with fresh gas as it goes around. Closed-loop systems are used with gases that are too expensive to continuously replace, such as Xenon or Neon. Unlike the open-loop, closed-loop systems need pumps and purification systems to remove oxygen and moisture accumulated during circulation.
Regardless of the technology used, gaseous detectors must be kept at constant pressure. 'It's like a balloon,'Haider explains. 'If you have too much gas coming in from one end, the balloon expands.'The last thing gas lab technicians want is the 'balloon'to pop or shrivel. To prevent this, a diaphragm-like device is placed on the output line that closes or opens according to the pressure inside. The software to control this process and monitor the basic functioning of the system was developed by the IT-CO team led by Renaud Barillère.Because the smallest contaminations of silicon in the gas can destroy a detector, each gas system component undergoes various tests and checks to guarantee its cleanliness. The distribution racks are also extensively tested for leaks before being sent to the experiments.
Although the ALICE TPC gas system is currently operating, it is still on the surface and will be lowered into the cavern in September. The next gas lab project is the commissioning of the ATLAS Resistive Plate Chambers system.
