Long-Shutdown 2

Nearly one year after the start of the assembly activities the first 3-module-device Target Dump Injection Segmented (TDIS) unit is ready to be installed. This time span has allowed the project team to troubleshoot and ultimately validate the equipment assembly procedure and its overall design.

One major milestone achieved during this process was the completion of an irradiation experiment at CERN’s HiRadMat facility in 2018. A complete prototype TDIS module was exposed to several high-intensity proton beam pulses, reproducing in its core (two movable assemblies named jaws) the thermomechanical response expected by HL-LHC beam impacts. As anticipated/expected by previous simulations , the jaw assembly, made up mainly of isostatic graphite absorbing blocks supported by an advanced Titanium-Zirconium-Molybdenum (TZM) stiffener, did not show any beam-induced degradation, which was corroborated by online instrumentation and post-irradiation examinations. The outstanding strength at high temperatures along with the remarkable thermal conductivity plus an average specific heat make the TZM the most suitable material to cope with the challenging energy level at the TDIS back-stiffener’s spot. That is why it was chosen over many other candidates such as Glidcop® (used in back-stiffeners of most LHC collimators).

The on-line monitoring of the HiRadMat module also provided an insight into the performance of the built-in cooling system, which was able to cool the jaws down to room temperature within about 10-15 minutes after beam shots (when TZM’s temperature exceeded 200°C). The cooling system was subject of another test aiming at evaluating its capacity to keep the jaw temperature below 50°C while beams are being injected into the HL-LHC. During LHC filling, continuous resistive-wall heating of the TDIS jaws is present, mostly on surfaces directly exposed to particle beams. The testing set-up included four 1000-watt lamps to get the jaws heated up in a manner similar to real-time operation.

Further validation tests were conducted on another essential system of the injection absorber: a complex mechanism containing almost 500 components denominated “mechanical table”, responsible of the 0.01 mm-accurate motion of the jaws inside the vacuum chamber. Two mechanical tables endured, without any hint of fatigue/deteriotation, 15000 full-stroke cycles, pessimistically emulating the expected displacements over the TDIS’s lifetime. Moreover, prior to running the mechanical table cycling test, the lubricated lead screws (by far the most sensitive components of this sub-assembly) were gamma irradiated up to 5 MGy, several factors higher compared to the calculated cumulated dose at the TDIS location in the LHC tunnel. Unlike the other testing activities, the irradiation campaign of the leadscrews took place/was performed at BGS Beta-Gamma-Service GmbH (Wiehl, Germany).

As for the assembly procedure of the injection absorber, specific tools and custom-made benches were developed to ease certain critical operations, namely the mounting of jaws - which requires compressing springs that deliver a total force of 3 tons - and the insertion of the assembled jaws into the chamber. For the latter it is necessary a combination of guided translational and rotational movements of the jaw whilst being significantly close to the delicate surfaces of the jaw and the chamber. Failing to perform it correctly could result in collisions between the two components and hence, for instance, in a leaking vacuum tank or a damaged absorbing surface of the jaw.

Time-lapse of the last activities conducted on the TDIS. (Video: CERN)

In terms of the design, the single major modification that the TDIS has undergone in the last year is the reinforcement of the girder that supports the three modules. A truss was added to the structure in order to increase the girder and stiffness and to avoid buckling or torsional deformations that could be produced by axial forces (of approximately 1.5 tons) arising when the device is under vacuum.

Today, the first TDIS unit is going through conditioning to meet ultra-high vacuum specs after successfully passing the rest of the required verifications before installation in point P2 of the LHC. These checks comprise metrological and survey checks and impedance measurements, among others. In parallel, the ongoing assembly of the second unit is planned to finish before the end of August 2020, heading for installation in point P8 by the last quarter of this year. Two additional devices will be constructed and stored as spares, completing the entire TDIS series.

A mobile treatment unit has been developed that operates directly in the tunnel to apply the carbon coating on the vacuum chambers of the SPS quadrupoles. The unit includes a cathode in graphite (black structure in the photo), which is inserted into the beam tube (Image: Julien Ordan)

Ridding the accelerators of clouds is one of the major challenges of the vacuum teams. The clouds in question are electron clouds, which can disrupt the beams. Electrons are produced when the beams ionise the residual molecules in the vacuum chamber. They are then accelerated, hit the surface of the vacuum chamber and scrub off other electrons. An ‘avalanche’ phenomenon ensues, in which electrons multiply and form the dreaded electron clouds. At high beam intensities, these clouds can start to degrade seriously the proton beam quality. The problem therefore becomes critical when preparing the LHC for operation at high luminosity, which will require beams twice as intense.

Since 2007, the Vacuum, Surfaces and Coatings group has been developing a solution, comprising a fine layer of amorphous carbon applied to the internal walls of the vacuum chambers. When struck by electrons, amorphous carbon emits a lot fewer secondary electrons than the metallic surface of the vacuum chambers. This amorphous carbon coating technique has proved to be effective, having been applied to sixteen SPS magnets during the first long shutdown, and then to more magnets during year-end technical stops.

As part of the LHC Injector Upgrade (LIU) project, this innovative solution is now being implemented on all the focusing quadrupoles and the short straight sections of the SPS, which comprise corrector magnets and beam instrumentation. “The priority magnets have been identified in order to achieve the best compromise between efficiency and the cost of the operation,” explains Wil Vollenberg, in charge of the carbon coating project in the SPS. In total, the vacuum chambers of 99 quadrupole magnets and as many short straight sections will receive an amorphous carbon coating.

A mobile treatment unit has been developed to apply the coating directly in the SPS tunnel and to reduce the handling of such heavy items as the magnets. A cathode, a long graphite structure, is introduced into the vacuum chamber. The air in the chamber is pumped out and very pure argon is injected. A voltage of 900 volts is applied in order to ionise the argon atoms. The argon ions that are thus released bombard the graphite cathode, creating a shower of carbon atoms, which then stick to the walls of the vacuum chamber. The cathode is fitted onto an ingenious motorised system, which moves backwards and forwards to enable a uniform coating 400 nanometres thick to be applied.

This mobile coating system was developed and tested for the first time in 2016, and has since been improved. From the beginning of the long technical shutdown, a team of three has been working its way along the SPS tunnel to treat half of the quadrupole magnets. However, accessing the vacuum chambers of these magnets requires removing the nearby components. This is why the short straight sections, adjacent to the quadrupoles but smaller in size, are removed and treated above ground. Furthermore, parts of the vacuum chambers are replaced by treated sections: this is the case for certain chambers such as those in the vicinity of the beam dump system that is currently being replaced.

From the handling teams who move the magnets, to the surveyors who reposition them to within a tenth of a millimetre, through to the electricians, surface analysis QA experts and specialists in vacuums and magnets, many teams are hard at work and have to carry out their tasks in a precise sequence. “It takes four days to treat each magnet, but many operations are necessary before and after, which results in complicated logistics,” says Wil Vollenberg, orchestrator of the operations.

Underground, the teams are already halfway there, working in collaboration with their colleagues on the surface. A new round of operations is envisaged during the third long shutdown, not only in the SPS but also in the LHC. The future will be less cloudy for the two large accelerators at CERN.

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