Crab kicks for brighter collisions

19 April 2018

Advanced radio-frequency crab cavities are to be tested for the first time in a proton beam, a vital step towards the high-luminosity LHC upgrade.

Two key parameters underpin the physics reach of a particle collider: its collision energy and its luminosity, which is the number of potential collisions per unit area per unit time at the interaction point of the colliding beams. Accelerator physicists have devised numerous ways to boost the luminosity and thus scientific reach of colliders, via innovative magnet and radio-frequency (RF) technologies. One such RF innovation is the crab cavity, which is a key feature of the high-luminosity upgrade to the Large Hadron Collider (HL-LHC) now in its construction phase at CERN.

The roots of crab cavities can be traced to 1988, when Bob Palmer at Brookhaven National Laboratory proposed the crab crossingscheme for an electron-positron linear collider. The first implementation in an accelerator was much later, in 2006, at the KEKB electron–positron collider in Japan. One crab cavity per ring operated for approximately four years and, in conjunction with other accelerator elements, helped the collider reach record luminosities. Crab-crossing was considered as one of several potential LHC upgrade paths as early as 2002 and, in 2006, the first proposal for the HL-LHC crab cavities was made. Soon afterwards, crab cavities – along with high-field niobium–tin magnets – were adopted as one of the key technologies allowing the HL-LHC to multiply the integrated luminosity of the present LHC by a factor of 10.

Crab cavities

An extensive R&D programme followed, and the first tests of the HL-LHC crab cavities took place in spring last year (CERN Courier May 2017 p7). Beginning in early 2018, two cavities were installed in CERN’s Super Proton Synchrotron (SPS) to study how they behave with real beam (image left). This will be the first time that a crab cavity has ever been used for manipulating protons, paving the way not just for the HL-LHC but a variety of other accelerator applications.

Crab-wise crossings

The advanced niobium-tin “inner-triplet quadrupole” magnets for the HL-LHC (CERN Courier March 2017 p23 and September 2017 p17) will be placed on either side of the ATLAS and CMS experiments to squeeze the incoming proton beams into smaller transverse beam sizes at the collision point than those presently obtained at the LHC. To avoid unwanted parasitic collisions in the single beam pipe either side of the interaction points, the HL-LHC will operate with a crossing angle, which has the negative effect of reducing the luminosity with respect to that obtained with head-on collisions. Smaller beam sizes at the interaction point of the HL-LHC imply larger beams in the inner triplet magnets and, consequently, a larger crossing angle is necessary to ensure sufficient separation of the beams. At the nominal HL-LHC, the large crossing angle coupled with the small bunch dimensions would result in a luminosity reduction of around 66% if not corrected.

To recover some of this potential loss of luminosity while maintaining the necessary beam separation, an elegant crab-crossing scheme has been proposed. Independent superconducting crab cavities for each beam are positioned around 160 m upstream and downstream of a given collision point. The crab cavities provide a time-dependent transverse kick to the protons in the head and tail of a bunch. As a bunch moves towards the interaction point, the kick serves to rotate the bunch in the crossing plane so that it effectively collides head-on with its incoming counterpart (figure 1). The downstream crab cavity then reverses the kick to confine the rotation to the interaction region and leave the particle orbit in the rest of the machine untouched. A total of 16 cavities – eight (two per beam per side) near ATLAS and eight near CMS – will be required for the project.

The tight spatial constraints of the HL-LHC upgrade meant a long journey of technological challenges. The large crossing angle (implying a large transverse kick) between the HL-LHC beams required a radically new RF concept for particle deflection with a novel shape and significantly smaller cavities than those used in other accelerators. In less than two years, more than 10 concurrent designs from RF experts across three continents were being discussed as potential options. By 2013, three designs stemming from a worldwide collaboration between CERN, the US and the UK were considered to be most adapted to the HL-LHC: double quarter wave (DQW), RF dipole (RFD) and four rod (4R). The results of RF tests of these designs were highly promising and, in 2014, an international panel recommended that efforts be focused on the first two (figure 2) with the aim of making a full validation with real proton beams. Both designs will be used, one around ATLAS and the other around CMS.

The cavities are made from sheets of high-purity niobium, a type II superconductor commonly used for very high-field superconductors. A sheet thickness of 4 mm is necessary to cope with the strict mechanical constraints; advanced shaping, machining and ultra-precise electron-beam welding are required to produce the complex shapes with mechanical tolerances well below 1 mm. Once “dressed”, each cavity is equipped with: a helium tank, an internal magnetic shield, a precision frequency tuning system, a fundamental RF power coupler, a field probe and two or three higher-order mode couplers (figure 3).

The helium tank serves as an enclosing body to cool the cavity surface with saturated superfluid helium at 2 K and its geometry was chosen to limit the maximum stress on the cavity. Owing to the unconventional geometries of the crab cavities, titanium was selected because it has nearly the same thermal contraction as niobium. An active frequency tuning system structurally integrated with the tank with a resolution of a few nanometres allows the cavity frequency to be precisely synchronised to the beam. Two identical cavities are inserted into a special cryomodule that serves as a high-performance thermos flask under high vacuum, thus reducing the heat load and stray magnetic fields from the outside environment and keeping the cavities stable at 2 K. This is made feasible with different layers of materials all properly thermalised in a staged fashion and under high vacuum conditions to minimise the total footprint. The most external layer of the cryomodule is the vacuum vessel. A complex cryogenic circuit with staged temperatures also allows the passage of the cold helium (2 K and 50 K) to cool the cavity ancillaries such as higher-order mode couplers and RF lines. The cryomodule also serves as the support structure and keeps the two cavities precisely aligned.

Successful operation of the crab cavities depends on their correct position and orientation, and the HL-LHC places tight constraints on the cavity alignment; in particular, the transverse displacement of one cavity with respect to another should not exceed 500 μm. To determine the cavity alignment relative to targets positioned outside the cryostat, a position monitoring system five to ten times more precise is needed. Frequency scanning interferometry was chosen for this task, whereby a laser beam is sent inside the cryomodule and reflected off several reflectors placed on the cavity interfaces to track their movements. The optical path of the measurement beam is then compared with a beam from a reference interferometer, offering absolute interferometric distance measurements with sub-micrometre precision.

Early last year, two superconducting prototype DQW-type crab cavities manufactured at CERN underwent RF tests in a superfluid helium bath at a temperature of 2 K. These first cavity tests demonstrated a maximum transverse-kick voltage exceeding 5 MV, surpassing the nominal operational voltage of 3.4 MV. The corresponding electric and magnetic fields on the cavity surfaces were 57 MV/m and 104 mT, respectively (for comparison, the KEKB cavities reached a maximum of 2.5 MV kick voltage in similar tests). Prototypes of the DQW and the RFD cavities were built in the US and reached even higher fields than the CERN prototypes, demonstrating the robustness of the RF design.

By the end of 2017, the two crab cavities were assembled at CERN into a special cryomodule to allow operation in an accelerator environment. Its design was a joint effort between CERN and the UK. The module was successfully RF-tested at 2 K in December at CERN’s SM18 facility (figure 4, lower right), validating the mechanical, cryogenic and RF functioning prior to its installation in the SPS for beam tests. The complete installation of the cryomodule and support infrastructure, requiring a new high-power RF and cryogenic system, had to be finished in a period of eight weeks during the year-end technical stop of the SPS. The rush was dictated by the operation schedule of the CERN accelerator complex: after 2018, all the accelerators of the injector complex will be stopped for two years, to undergo a major upgrade in preparation for the HL-LHC.

Ready for beam

Proton beam tests of the compact crab cavities in the SPS are considered a prerequisite before installation into the LHC itself. The aim is to demonstrate the operational performance of the cavity  and transparency throughout the energy cycle and to study long-term effects on proton beams and failure modes.

A 15 m long section of the SPS ring was identified as suitable for installation of the in-beam crab-cavity test stand. The beam line was equipped with two articulated, Y-shaped vacuum chambers to provide a bypass to the circulating beam, with highly flexible bellows allowing for a lateral displacement of approximately 51 cm (figure 5, middle). Via this articulated continuous connection of the vacuum beam pipe, the crab-cavity test module can remain parked out of the beamline during regular operation of the SPS and be transferred back into the beamline during periods dedicated to crab-cavity testing. This is essential because the entrance diameter of the crab cavities is smaller than what is needed for beam extraction to the LHC, plus there is an inherent operational risk associated with having a prototype element inserted in the beamline of the main injector to the LHC. The motorised transfer table, produced by Added Value Solutions (Spain), also supports the Y-chambers, the two RF circulators and passive loads, and a cryogenic valve box – corresponding to a total load of about 15 tonnes, which is moved in and out of the beam with a positioning precision of a few micrometres.

Due to limited space and accessibility in the underground areas of the SPS, the RF amplifiers required to power the cavities were placed in a surface building and RF power routed via coaxial lines to the cryomodule. The cold box for the cryogenic refrigeration system, by contrast, was installed underground because transporting liquid helium along a vertical pipeline increases losses by evaporation. Helium is liquefied in the cold-box and routed to the cryomodule via a 110 m long cryogenic distribution line. A comprehensive beam-test programme is now under preparation. In parallel to the installation of the prototype cryomodule in the SPS, an effort has started to fabricate at CERN two RFD prototype cavities. They will then be assembled at Daresbury (UK) into a cryomodule that will be tested in the SPS after the second LHC long shutdown. An industrial contract for the production of DQW cavities for the full crab-cavity system for the HL-LHC was finalised in December 2017 with Research Instruments in Germany, and the industrial production of the RFD cavities, which is now under the responsibility of a collaboration with the US, is also ramping up.

Superconducting crab cavities and RF deflectors have a wide range of applications other than high-energy physics. The significant contribution of the HL-LHC developments to ultra-compact and very high-field cavities are already influencing new proposals for luminosity improvements similar to the HL-LHC for electron–hadron colliders, bunch compression in light sources to produce sub-picosecond photon pulses, and ultrafast particle separators in proton linacs as a means to separate bunches of secondary particles for different experiments.

It is now also evident that crab cavities are useful beyond the HL-LHC for even higher energy colliders. In the proton-proton version of the Future Circular Collider (FCC) study, the bunches would be squeezed at the collision point by a factor of 2–4 compared with that of the HL-LHC. Without crab cavities to compensate, only 20% of the available peak luminosity would be exploited by the machine. It is clear that this advanced RF technology, taken further than before by its adaption to the HL-LHC, has an extremely bright future for high-energy physics and beyond.

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