Progetto Speciale Nuove Tecniche di Accelerazione

The new generation colliders require mass production of multicell superconducting cavities. Just 20.000 ninecell resonators would be required for TESLA. As a Institution member of the international TESLA collaboration, the INFN has started the Special Project on New Acceleration Techniques in order to promote the R&D on new fabrication techniques for superconducting resonant cavities for particle accelerators.

The standard fabrication technique consists in cutting circular blanks from bulk Niobium sheets, deep-drawing halfcells and Electron Beam (EB) welding them at the equator and at the iris under U.H.V. In parallel to the traditional way, alternative fabrication technologies, as spinning, hydroforming and explosive forming, have come into the limelight in recent years due to the possible mass production requirement for TESLA.. If the EB weld is costly and it is a possible source of contamination, the fabrication of seamless resonators could in theory solve the problem of a possible mass production at lower costs than those resulting from the traditional approach.

The idea of manufacturing seamless cavities is not new, because such a technology offers potentiallly several benefits: the elimination of electron beam welds, the significant reduction in manufacturing costs, the reduction of necessary infrastructure for mass production because of "speedy" manufacturing

In this context, the LNL of the INFN have proposed an original approach to the scientific community: the cold forming by spinning of a full size seamless resonator.

The cavity is straightforwardly spun from a circular blank or from a tube by a cold forming process and no intermediate annealing is required. The nine-cell fabrication rate of is at moment of the order of one resonator per day, but it can be increased of several times. Tests at DESY and at Jlab on mono-cells have demonstrated that accelerating fields over the TESLA specification can be reliably achieved. Moreover, it has been also seen that spinning perfectly combines with the CERN sputtered Nb/Cu and the KEK clad Nb/Cu.

 

The spinning process of a Copper monocell cavity is displayed in fig. 1.

Fig. 1 - Spinning of a seamless monocell resonator from a circular blank. No intermediate annealing is required during forming.

 

If a larger blank is chosen, a multicell cavity can be easily spun. Also in this case no intermediate annealing is required. Figure 2 refers to the spinning of an aluminum 1.5 GHz 10-cell resonator spun form a planar blank of 3 mm thickness and 1 meter in diameter. At the current state of technology we have successfully spun already copper nine-cell resonators, and niobium five-cells. The limitation however is not conceptual but only due to the tooling adopted.

Fig. 2 - Spinning of a seamless multicell resonator from a circular blank.

The optics followed in this research program on spun cavities tries to make use of an international collaboration in the most fruitful form. The cavities spun at LNL indeed have been sent to different groups abroad.

In this framework, the cavity of fig. 3 is a 1.5 GHz monocell, treated and tested by P. Kneisel at Jefferson Lab. The cavity has undergone several treatments. However only after the mechanical grinding of the internal surface plus an in situ baking at 115 C, the cavity has reached a gradient of 33 MV/m at a Q₀ of 5e+9.

1. 

Fig. 3 – The cavity P5 spun at LNL and treated and tested at Jefferson Lab. The test 3 was performed after removing 230 microns of BCP; The test 6b was performed after 100 hrs of tumbling, 80 microns BCP, 90 minutes of heat treatment, 60 microns of BCP, a further grinding of cracks at the beam pipe and last 50 microns of BCP; The test 7c was performed after an additional 20 microns BCP followed by 40 hrs of baking in situ at 115 C.

 

 

For further info, contact the Project Leader: Vincenzo Palmieri