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LS2 Report: a technological leap for SPS acceleration

The SPS radiofrequency acceleration system is being enhanced with a new technology: solid-state amplifiers


New solid-state amplifier system for the SPS accelerating cavities
The new solid-state amplifier system developed by CERN with the Thales Gérac company comprises 32 towers, in which 2560 RF modules, each containing four transistors, will be installed. (Image: Maximilien Brice/CERN)

Big changes are under way at the Super Proton Synchrotron (SPS). One of the major operations is the upgrade of the machine’s acceleration system. "The beams in the High-Luminosity LHC will be twice as intense, which requires an increase in radiofrequency power," explains Erk Jensen, leader of the Radiofrequency (BE-RF) group. One aspect of the LHC Injectors Upgrade (LIU) project is therefore bringing the SPS acceleration system up to standard.

Erk Jensen shows us around the huge Building 870, just behind the CERN Control Centre on the Prévessin site, which is a hive of activity. Everywhere you look, teams are pulling out cables, unscrewing components and removing electronic modules. Dismantling is one of the main activities of this first phase of the Long Shutdown. No fewer than 400 km of cables are being removed at Points 3 and 5 of the SPS, for example.

In the large halls, we can see the huge power converter and amplifier installations that supply the radiofrequency (RF) accelerator cavities of the SPS. The amplifiers use an electronic tube technology dating back to the 1970s and 80s, as the SPS was commissioned in 1976 and transformed into a proton-antiproton collider in 1981. Two tube systems exist alongside each other, each producing 2 megawatts of power.

To supply the power needed for the High-Luminosity LHC, a team from the RF group, headed by Eric Montesinos, working with the firm Thales Gérac, has developed a new system that uses solid-state amplifiers, similar to those that were recently developed for the SOLEIL and ESRF synchrotrons. The transistors for these amplifiers are assembled in sets of four on modules that supply 2 kilowatts, much less power than was delivered by the electronic tubes (between 35 and 135 kilowatts). But a total of 2560 modules, i.e. 10 240 transistors, will be spread across 32 towers. The power from 16 towers will be combined via an RF power combiner. The whole system will be able to provide RF power of two times 1.6 megawatt to the cavities.

"This system is much more flexible, since the power is distributed across thousands of transistors," observes Eric Montesinos. "If a few transistors stop working, the RF system will not stop completely, whereas if one of the tubes failed, we had to intervene quickly." In addition, it’s much easier to change a module, especially since electronic tubes in this frequency range are an endangered species, accelerators being among the last applications of the technology.

SPS,LS2, SPS,RF cavity
The four 200 MHz accelerating RF cavities of the SPS are removed from their tunnel to be upgraded during the Long shutdown 2 (LS2). (Image: Maximilien Brice/CERN)

Development of the solid-state amplifier system began in 2016. A team from the RF group worked in collaboration with scientists from Thales Gérac, and many tests and adjustments had to be carried out. Power electronics are subject to significant thermomechanical effects, so the technique for fitting the transistors onto the plate of the module, to take one example, turned out to be a particularly tricky aspect to get right. After several dozen complex prototypes had been produced, the work finally came to a successful conclusion last year: the first tower housing 80 transistor modules operated for 1000 hours, passing the validation tests in August. This was a great success that allowed series production to begin while the tests continued.

The structures, i.e. the 32 towers, have already been installed in a new room, giving it the air of a science-fiction movie set. Only one of them so far is equipped with its RF power modules, offering a taste of the even more futuristic look that the room will have in a few months’ time. The modules will be delivered as of May, continuing through to the end of the year; all of them will be tested on a specially designed test bench before being installed in the towers. Some painstaking work faces the teams that will install all the modules.

In parallel, the cavities have been removed from the tunnel. The SPS has four 200 MHz cavities: two formed of four sections, and two of five sections, each section measuring four metres. "To accelerate more intense beams, we need to reduce the length of the cavities in order to maintain a sufficiently strong electromagnetic field along their whole length," explains Erk Jensen. The teams will therefore reassemble the sections in order to form a total of six cavities: two of four sections and four of three sections.

At the same time, the beam control system is being replaced. The Faraday cage, which houses the electronic racks for the beam control system, has been completely emptied, ready to be fitted with the latest electronics and new infrastructure (lighting, cooling and ventilation systems, among others). Finally, an improved system for eliminating parasitic resonances will be installed, based on HOM (higher order mode) couplers, which were tested during the last run.

The teams must stick to a tight schedule, comprising all the dismantling work, the start of installation later in 2019, and numerous tests and commissioning tasks in 2020.

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