Figure 1. Upper and lower clam shells of a TAXN unit delivered to CERN. F Thompson / CERN

By Javier Fernandez Roncal and Florence Thompson (CERN)

The first two sets of copper clam shells for the neutral beam absorber (Target Absorber for Neutrals, TAXN) have been delivered to CERN, marking a key milestone in the procurement of the absorber cores for the High-Luminosity (HiLumi) LHC upgrade 

The TAXN is the new neutral beam absorber designed to replace the existing one, TAN, after Long Shutdown 3. Four of them will be installed,  one each side of the ATLAS and CMS collision points. TAXN’s function is to intercept forward high-energy neutral particles produced in the collisions,  thereby protecting the downstream dipoles (D2 and onwards), whilst providing sufficient aperture to meet the enhanced optics requirements of the HiLumi LHC.

Figure 2. Diagram of the TAXN. Each TAXN unit will sit 130 m away from the collision point. On the left, a birds-eye view shows the Y chamber. Images courtesy of Javier Fernandez Roncal / CERN

The absorber itself is a 3.3-metre-long ETP copper block, weighing around 3200 kg, which consists of two halves (upper and lower clam shells) that clamp around a Y chamber (combination/recombination vacuum chamber). The Y chambers are the parts of the LHC beam pipe where beams change from being in separate parallel beam pipes to a single beam pipe (within which they collide at the collision points).

Different to the TAN, the new TAXN integrates an active water cooling circuit to prevent overheating , and an improved bakeout system , and also accommodates the new BRAN and ZDC detectors, to measure characteristics of neutral particles, within its structure. 

Figure 3. Two sets of two copper clam shell units. Each box contains two clam shells to be placed together around a Y chamber to form one TAXN unit.

With two sets of clam shells for the TAXN now received, two sets remain pending delivery and expected to arrive in the next couple of months. The absorbers will be assembled with Y_Chambers, cooling pipes, supports and other elements which will complete the assembly and be used to validate the cooling and bakeout cycle.

Figure 4. Layout around ATLAS and CMS collision points for HiLumi LHC. The inner triplet quadrupoles, which are the closest magnet assemblies to the collision points, are to be protected from the collision debris by a copper absorber called the Target Absorber for Secondaries, TAXS, which is a new and improved version of the current TAS. This intercepts charged and neutral particles, primarily pions and photons, leaving the collision point. After these first four quadrupoles (Q1, Q2a, Q2b and Q3), the first dipole, D1, deflects charged particles away. Neutral particles continue through and are intercepted by the TAXN. 

Read  more news about beam absorbers TAXS and TAXN for HiLumi LHC:

Collider-experiment interface: how engineers are preparing for manoeuvres in this highly restricted region of the HL-LHC

Target Absorbers for Secondary Particles produced for ATLAS