LHCb connects its pipes
Two weeks ago the first beryllium section of the LHCb beam vacuum chamber was installed. This three-day operation, after requiring lengthy preparation work, demanded patience and precision as the first of four sections of the beampipe was connected to the vertex locator (VeLo) vacuum vessel.
This first installed section is composed of a nearly two-metre long conical tube of one-millimetre thick beryllium and of a thin spherical-shaped window, 800 millimeter diameter, made of an aluminum alloy, and has the appearance of a mushroom lying on its side. The window is connected to the conical part of the beampipe through an aluminum alloy bellow, which is needed to allow for mechanical alignment once the assembly is installed.
Beryllium was chosen as the material for 12 m of the 19-m-long LHCb beampipe, to minimize the level of background in the experiment because of its high transparency to the particles produced in the collisions (see Bulletin No. 05/2005). Although it is the best material for this purpose, it is very fragile and toxic if inhaled or ingested, so any accident that could create dust particles, like surface scratching or fracture must be prevented. It was thus with great care and precaution that the first section of the vacuum chamber was transported in a robust box, surrounded with padding and various tooling to ensure its safe journey to Point 8 and its connection to the VeLo vacuum vessel. High resistance aluminum alloys were also used in the production of this beampipe section. Although not as good as beryllium, aluminum provides a good compromise between performance, risk and manufacturing cost. Since it could be welded to the beryllium parts, aluminum was used in the flanges, the window and the bellow.
While the beryllium cone was manufactured by a specialized company, the VeLo window and the final assembly were done in the CERN main workshop involving high-precision machining and electron beam welding.
Once the beampipe, inside its special tooling, was removed from the transport crate, it was placed on a frame which slides over rails to gently move it through the RICH1 detector and minimize the chances of damage occuring during installation. A wakefield suppressor was then inserted. This delicate procedure involves the electrical connections between the Velo vacuum vessel and the beampipe. Finally, the spherical window was connected to the VeLo vessel using a metal seal.
After the installation was completed, the system was pumped down and a leak test was conducted.
Following the installation of the four beampipes, the commissioning phase will include a bakeout cycle consisting of heating the vacuum chamber surfaces for 24 hours to a temperature that may vary up to 250 °C, in order to activate the non-evaporable getter coating (NEG) that will act as a distributed vacuum pump during the machine running. Once activated, the NEG coating applied on the beampipe's inner surface before installation, along with ion pumps, will provide the pumping capacity to obtain the required ultra-high vacuum level. 'We aim to reach an average dynamic pressure of 10-9 millibar,'explained Delio Ramos, engineer in the AT-VAC group responsible for the LHCb beampipe. 'Dynamic, as it corresponds to the vacuum pressure with the beam passing through the beampipe.'
During the coming months, LHCb will install another beryllium section of the beam pipe, sliding it through the centre sheath of RICH2. Given its length of 6 metres, this will require the greatest care and involve some difficult manipulations. The stainless steel section of the beampipe will then be put in place before the last beryllium section is installed in the dipole magnet.
LHCb is a forward experiment, measuring only one side of the collisions. The focus of the experiment is on the precision measurement of CP violation and rare decays of b quarks.
