Sur la route du Linac 4

Après deux décennies de conception et de construction, l’accélérateur linéaire Linac 4 du CERN est sur le point d’être intégré à la chaîne d’injection du LHC. Cet accélérateur, qui jouera un rôle vital pour le LHC à haute luminosité, utilise, non pas des protons, mais des ions d’hydrogène négatifs, qu’il injecte, après avoir augmenté leur énergie, dans le Booster du PS.

For the past 40 years, CERN’s accelerator complex has been served by a little-known linear accelerator called Linac2. Commissioned in 1978, the 50 MeV linac was constructed to provide a higher beam intensity to the newly built Proton Synchrotron Booster (PSB).

It superseded Linac1, which accelerated its first beam in 1958 and was the only supplier of protons to the CERN Proton Synchrotron (PS) for the following 20 years. Linac1 was sent into retirement in 1992, having spent 33 years accelerating protons as well as deuterons, alpha particles and oxygen and sulphur ions, and is now an exhibit in the CERN Microcosm. Linac3 took over CERN’s ion production in 1994, but today Linac2 is still injecting protons into the PS and SPS from where they end up in the Large Hadron Collider (LHC).

Although the construction of this workhorse of the CERN accelerator chain was an important step forward for CERN, and contributed to major physics discoveries, including the W, Z and Higgs bosons, Linac2’s relatively low energy and intensity are not compatible with the demanding requirements of the LHC luminosity upgrade (HL-LHC). Persistent vacuum problems in the accelerating vessels over the past years also raise major concerns for the performance of the LHC. For this reason, in 2007, it was decided to replace Linac2 with a more suitable injector for the LHC’s future.

A decade later, in spring 2017, the 160 MeV Linac4 was fully commissioned and entered a stand-alone operation run to assess and improve its reliability, prior to being connected to the CERN accelerator complex. The machine’s overall availability during this initial run reached 91 per cent – an amazing value for an accelerator whose beam commissioning was completed only a few months earlier. The Linac4 reliability run will continue well into 2018, sending the beam round-the-clock to a dump located at the end of the accelerating section under the supervision of the CERN Control Centre (CCC) operation team. After a consolidation phase to address any teething troubles identified during the reliability run, Linac4 will be connected to the next accelerator in the chain, the PSB, in 2019 at the beginning of the LHC Long Shutdown 2. Test beams will be made available to the PSB as soon as 2020, and from 2021 all protons at CERN will come from the new Linac4, marking the end of a 20 year-long journey of design and construction that has raised many challenges and inspired innovative solutions.

Linac4 has the privilege of being the only new accelerator built at CERN since the LHC. With an accelerating length of 86 m, plus 76 m of new transfer line, Linac4 is definitely the smallest accelerator in the LHC injection chain. Yet it plays a fundamental role in the preparation of the beam. The linac is where the beam density is generated under the influence of the strong defocusing forces coming from Coulomb repulsion (space charge), and where negative ions initially at rest (containing protons emerging from a bottle of hydrogen gas) are progressively brought close to the relativistic velocities required for acceleration in a synchrotron. This rapid increase in beam velocity requires the use of complex and differentiated mechanical designs to accelerate and focus the beam. Combined with the need for high accelerating gradients (the beam passes only once through the linac), particular demands were placed on Linac4 to achieve the high values of availability required by the first element of the acceleration chain.

The main improvements provided by Linac4 stem from the use of negative hydrogen ions instead of protons and from a higher injection energy into the PSB. Negative hydrogen ions – a proton with two electrons – are converted into protons by passing them through a thin carbon foil, after their injection into the PSB to strip them of electrons. This charge-exchange technique involves progressively injecting the negatively charged ions over the circulating proton beam to achieve a higher particle density. After injection, both beams pass through the stripping foil leaving only protons in the beam. This provides an extremely flexible way to load particles into a synchrotron, making the accumulation of many turns possible with a tight control of the beam density.

Extensive modifications

However, the use of hydrogen ions does not come without complications. It requires extensive modifications to the injection area of the synchrotron and a complex ion source in front of the linac. The other key element for generating the high-brightness beams required by the LHC upgrade is the increase of the injection energy in the PSB by more than a factor three with respect to the present Linac2, which reduces space-charge effects at the PSB injection and allows the accumulation of more intense beams.

On top of these crucial advantages for the HL-LHC, Linac4 is designed to be more flexible and more environmentally clean than Linac2. Modulation at low energy of the beam-pulse structure, the option of varying beam energy during injection and a useful margin in the peak beam current will help prepare the large variety of beams required by the injector complex, at the same time reducing beam loss and activation in the PSB. Linac4 is also designed for the long term. Having originated from studies at the end of the 1990s, the goal was to progressively replace the PS complex (Linac2, PSB and PS) with more modern accelerators capable of higher intensities for the future needs of the LHC and other non-LHC programmes. Alas, this ambitious staged approach was later discarded to give priority to the consolidation of CERN’s older synchrotrons, but Linac4 retains features related to the old staged programme that could be exploited to adapt the CERN injector complex to future physics programmes. Examples are the orientation of the Linac4 tunnel, which leaves space for future extensions to higher energies, and its pulse-repetition frequency. The latter is currently limited by the rise time of the PSB magnets to about 1 Hz, but this could be upgraded up to 50 Hz were the PSB to be replaced one day by another accelerator.

Last but not least, Linac4 is a model for the successful reuse of old equipment. All its accelerating structures operate at a frequency of 352 MHz, which is precisely that of the old Large Electron Positron (LEP) collider. Linac4 reuses a large quantity of LEP’s RF components, such as klystrons and waveguides, which were carefully stored and maintained following LEP’s closure in 2000. However, the LEP klystrons installed in Linac4 will gradually be replaced in pairs by modern klystrons with twice the power.

Reaching Linac4’s required performance and reliability posed several problems in the design and construction of the new linac. The first challenge was to build a reliable source of negative hydrogen ions, starting from a new design developed at CERN that profited from the experience of other laboratories such as DESY and Brookhaven National Laboratory. The ion source is a complex device that starts from a bottle of hydrogen similar to the one used in Linac2 and generates ions in a plasma heated by a high-frequency wave of several dozen kilowatts. Following some initial difficulties, the new ion source is now steadily providing the minimum beam intensity required by the LHC, while improvements are still ongoing.

Accelerating elements

After the ion source, the challenge for the main Linac4 accelerating section has been to integrate focusing and accelerating elements in the small linac cells, achieving a good power efficiency at the same time. These requirements motivated the use of four different types of accelerating structure: an RF quadrupole (RFQ) to take the energy to 3 MeV; a drift-tube linac (DTL) of the Alvarez type to 50 MeV; a cell-coupled drift-tube linac (CCDTL) to 102 MeV; and finally a Pi-mode structure (PIMS) to the final energy of 160 MeV. Most of these accelerating sections include important innovations. The CCDTL and PIMS structures are a world-first developed specifically for Linac4 and used for the first time to accelerate a beam. The DTL includes a novel patented mechanism to support and adjust the drift tubes and makes use for the first time at CERN of a long focusing section made of 108 permanent magnet quadrupoles. To these innovations we had to add a novel scheme to “chop” the beam pulse at low energy, a simplified RFQ mechanical design, and finally the flexible and upgradeable beam optics design.

In spite of a general trend towards superconducting accelerators, Linac4 is entirely normal-conducting. This is a logical choice for a low-energy linear accelerator injecting into a synchrotron and operating at low duty cycle. Linac4 is pulsed, and the short particle beam is in the linac only for a tiny fraction of time. Although as much as 24 MW of RF power are needed for acceleration during the beam pulse, the average power to the accelerating structures will be only 8 kW, out of which only about 6 kW are dissipated in the copper, the rest going to the beam. The power required to cool Linac4 to cryogenic temperatures would be much higher than the power lost into the copper structures.

The construction of Linac4 is a great example of international collaboration, expanding well beyond the boundaries of CERN. Already in the R&D phase between 2004–2008, Linac4 collected support from the European Commission and a group of Russian institutes supported by the ISTC international organisation. The construction of the accelerator received important contributions from a large number of collaborating institutes. These included CEA and CNRS in France, BINP and VNIITF in Russia, NCBJ in Poland, ESS Bilbao in Spain, INFN in Italy and RRCAT in India. Organising this wide network of collaborations was a great challenge, but the results were excellent both in terms of technical quality of the components and in terms of developing a common working culture.

Linac4 brought proton-linac technology back to Europe. Since the construction of Linac2 in 1978 and of the HERA injector at DESY a few years later, all new proton linac developments took place in the US and in Japan. The development effort coordinated by CERN for the construction of Linac4 allowed bringing back to Europe the latest developments in linac technology described above, with a strong involvement of European companies. A measure of the success of this endeavour is the fact that many technical solutions developed for Linac4 will be now adopted by the normal-conducting section of the new European Spallation Source linac under construction at Lund, Sweden.

The inauguration of Linac4 on 9 May 2017 marked the coronation of a long project. The ground-breaking on so-called “Mount Citron” (made in the 1950s with the spoil from the construction of the PS ring) took place in October 2008 and the new linac building started to take shape. Construction extended over the mandate of three CERN Directors General. It’s expected that Linac4 will have a long life – at least as long as Linac2 – and play a vital role at the high-luminosity LHC and beyond.