Giving Protons a Boost

The first of LHC's superconducting radio-frequency cavity modules has passed its final test at full power in the test area of building SM18. These modules carry an oscillating electric field that will accelerate protons around the LHC ring and help maintain the stability of the proton beams.

The LHC project marked a milestone in October, finishing two months of testing on the first of four radio-frequency (RF) cavity modules for the accelerator, two modules for each of the proton beams. Each module, containing four single-cell superconducting cavities that carry an electric field oscillating at 400.8 megahertz, was specially designed to meet the requirements of high beam intensity in the LHC. In the tests, the electric field in all four of the module's cavities reached eight million volts per metre, well above the 5.5 million volts per metre needed to capture, accelerate, and hold the LHC beams.

The cavities' construction, copper with a thin layer of sputtered niobium, is the same as that used so successfully for the LEP superconducting cavities. Following technology transfer from CERN to European industry for large-scale production of LEP's modules, CERN could order from industry the LHC's bare cavities and several other parts. Then CERN is assembling the modules. Here the experience gained from LEP was indispensable, especially in the vital cleaning and rinsing processes and in the critical final assembly of the modules in a clean-room environment.

However, the LHC's high-intensity proton beams will be more difficult to stabilize than were LEP's electron and positron beams, so various elements of the LHC's modules presented new technical challenges. One particular challenge was building the RF power couplers, which deliver electromagnetic waves into the cavities. The LHC beam requires a wide-range movable coupler to adjust to differing beam requirements at injection and in coast. The new design allows movement of the inner antenna over a wide range without using troublesome sliding contacts, which can shed material and compromise the vacuum inside.

Once built but before being fitted to the cavities, each power coupler must undergo time-consuming conditioning over several weeks in a special test stand to make sure it stands up under varied conditions.

The modules' new design also allows polarization of the inner and outer parts of the coaxial line that carries electric waves to the cavity; the polarization prevents destructive multipactoring effects that would make the beam fail. In addition, the cavities need high power, up to 300 kilowatts-more than three times that normally used in LEP's cavities. On the RF window ceramic, which passes the electric field into the cavities while keeping the high vacuum inside the LHC, optimum design called for copper seals to maintain good heat conduction during sustained operation at high power. The seals' manufacture proved to be difficult technically, but close collaboration between CERN experts and industry met the challenge.

The success of the LHC's first RF cavity module is the result of successful collaboration with industry, together with considerable effort, not only from the AB-RF and AB-ATB groups but also from specialists in cryogenics, vacuum, power conversion, radio-protection, chemistry, surface treatment, material engineering and other disciplines in a large number of other groups in the AB, AT and TS departments.

Four remaining modules, three for LHC installation and one spare, now have to be completed as production and conditioning of the power couplers proceeds. Construction and testing of the couplers and the final assembly of the modules remain delicate operations. But if all goes as expected, the work will be complete by end of the first quarter in 2006, well before the planned installation date in the RUX45 tunnel of LHC.