3.9 GHz Cavity Program

As part of its role in the TESLA collaboration, Fermilab established a program for producing 3.9 GHz superconducting RF cavities. These third harmonic cavities will be used in DESY’s TTF/FLASH accelerator, a test facility for superconducting technology and free electron laser. Fermilab will ultimately deliver an assembled cryomodule containing four 3.9 GHz cavities to DESY in 2008.

A 3.9 GHz cavity ready for testing

The 3.9 GHz program marks Fermilab’s first effort to build superconducting RF cavities for use at another laboratory. Inside TTF/FLASH, the 3.9 GHz cryomodule is located immediately downstream of the first of six accelerating 1.3 GHz cryomodules. By operating at the third harmonic of the fundamental frequency of the TTF accelerating modules, the 3.9 module can reduce the longitudinal momentum distribution of each bunch, ultimately allowing the FEL to operate at low wavelengths of the order of 6 nanometers. Like the 1.3 GHz cavities for the ILC, the 3.9 GHz cavities also consist of nine elliptical cells made from fine-grain niobium. By virtue of their resonant frequency, however, they are a factor of three more compact, presenting unique engineering challenges.

In collaboration with Jefferson Laboratory, Fermilab will build a total of 10 cavities. Ultimately all of the cavities will come to Fermilab, where the 3.9 GHz cryomodule assembly will take place. As each cavity is built, Fermilab technicians and engineers measure, tune, and clean it in clean room facilities at A0 and Industrial Building 4. Buffered chemical processing takes place in the G-150 facility at Argonne National Laboratory by Fermilab staff. The cavities then come back to Fermilab for High Pressure Rinsing, preparation, and cold testing in a Vertical Test Stand located at A0. Once performance specs are met, the 3.9 GHz cavity then gets fitted in a helium vessel and proceeds to the Horizontal Test Stand, located in the Meson Detector Building. When the cavity passes all of the tests, it will be deemed worthy to join other qualified cavities inside the cryomodule. The final cryomodule assembly consisting of a string of four cavities will take place at the Cryomodule Assembly Facility located at MP9.

So far Fermilab and JLab have produced four 3.9 GHz cavities. These cavities are currently undergoing processing, treating, and testing. To date a gradient in excess of 20MV/m has been achieved with Cavity #4. Cavity #3 is expected to be the first one tested in the Horizontal facility late this spring. Fermilab will fabricate and test the necessary 3.9 GHz cavities by the end of 2007. The assembly of the cryomodule will also start late in 2007. The completed cryomodule is expected to ship to DESY in January/February 2008. The 3.9 GHz cavity program at Fermilab serves an important role both to advance superconducting technology, and to debug FNAL’s new SCRF infrastructure. The knowledge gained will contribute to the ILC effort because so called “crab” cavities operating at 3.9 GHz will be needed to maximize the luminosity at ILC. Fermilab’s experience with the DESY FLASH module has positioned FNAL to take a leadership role in this important effort.

3.9Ghz- Update (ppt, 22MB)