At high energy, protons can be accelerated by elliptical niobium superconducting cavities operating at the temperature of superfluid helium (< 2.17 K). This high energy part of the Linac is made up of several sections, made up of groups with multi-cell cavities of varying geometries, suitable for proton velocity relative to the speed of light (β = v/c = 0.47, 0.65, 0.85). Over the last three years, we have set up a number of “tools” allowing us to build and test these cavities.
Cavity design, fabrication and testing
The design work and first tests on mono-cells were the subject of a university thesis presented in 2001. Mechanical and electromagnetic 3D calculation codes were used to define the shape of the cavity. The accelerating electric field along the beam axis is optimised, at the same time as limiting the maxima of the electric and magnetic fields on the internal cavity surface. Because these latter are responsible of cavity performance limitations through surface electron emission and heating of the wall leading to loss of niobium superconductivity. These cavities also demand a very good quality factor so as to minimise cryogenic consumption.
After the work carried out on mono-cell cavities, development continued with the fabrication and testing of a 5-cell cavity (f = 700 MHz, = 0.65), working jointly with the IN2P3 (Institut national de physique nucléaire et de physique des particules). In each case, the performance of the cavities built was about 30% superior to that required by the specifications for their incorporation in the Linac.
At the moment, R&D is centred around the construction and testing of mono and multi-cell cavities with a value of β = 0.47 as well as follow-up of technological developments needed.
Given the length (1 m) and mass (~100 kg) of one 5-cell cavity, special commissioning tools were needed, in particular handling trolleys, one of which is specially designed for dust-free clean room environments.
So as to fully fit out the cavity, we are developing with IN2P3 a cold tuning system which will allow adjustment of cavity frequency by mechanical deformation, as well as a power coupler and its associated test bench allowing us to inject the radio frequency field into the cavity.
Cryholab is a horizontal laboratory cryostat for testing mono and multi-cell superconducting cavities at liquid helium temperature (4.2 K) or superfluid helium temperature (1.8 K), in conditions close to those of an accelerator. A helium tank is welded around the cavity and this latter is sited horizontally along with its power coupler and cold tuning system. So that Cryholab can operate alone, a liquefier, supplying the necessary liquid helium, is combined with it.
Cryholab has been designed and built in cooperation with IN2P3 and is partly financed by the Ile de France region.
Because of thermal insulation, the cryostat is first cooled to the temperature of liquid nitrogen (77 K): this kind of constraint is also present in accelerators. The cavity cooling process is therefore slow and reduces cavity performance, since niobium hydrides form at around 100 K. To prevent this happening, the cavity must first be annealed to eliminate the hydrogen trapped in the material: this operation is carried out at 800°C in a vacuum oven.
Installation and commissioning of Cryholab took place in 2000 and 2001. First tests on mono-cell cavities took place at the end of 2001 and the beginning of 2002. A 5-cell cavity test was carried out at the beginning of 2003 with an operational helium liquefier. The coupler, which will allow us to continuously inject the 700 MHz radio frequency wave (with a power of 80 kW), or inject it in pulsed mode (1 MW), has been the subject of studies and is under construction.
Cryholab will be used to test other types of superconducting cavities as part of several European programme for development of accelerators.