The Fermilab-DESY collaboration pays off: high grades for third-harmonic module

Plot for the experts: The four cavities, prepared and first individually tested by Fermilab (blue and cyan bars), showed the same excellent performance at the test of the complete system at DESY (red and yellow bars).

The 260-metre-long free-electron laser FLASH at DESY is currently undergoing a major upgrade. One of the crucial new components in the superconducting linear accelerator of FLASH is a superconducting module operating at 3.9 GHz, three times the normal accelerating frequency. This third-harmonic module, built by Fermilab, now has passed extensive tests. Once installed and in operation, it will form narrower electron bunches within the FLASH accelerator, leading to brighter FEL light.

The RF system test with the module at DESY’s cryomodule test bench showed a very promising performance. The four superconducting cavities in the module have been operated with an average acceleration gradient of 23 MV/m, exceeding the design value of 14 MV/m by 65 percent. Furthermore, the RF stability achieved by digital control surpasses the requirements of FLASH, which are regarded as very challenging.

The module will improve significantly the performance of the FLASH facility, which is re-starting in spring 2010. The benefit is a more efficient bunch compression, which provides more flexiblity in the bunch parameters for the FEL process.

FLASH is based on the accelerator technology that will also be used for the European XFEL project and proposed projects like Project-X at Fermilab and the International Linear Collider (ILC). Since 1997, FLASH serves as a test facility for the international TESLA Technology Collaboration developing superconducting 1.3-GHz RF technology for high-performance linear accelerators. Both Fermilab and DESY are founding members of this collaboration. The 3.9-GHz module is a further example of the very fruitful collaboration between the two laboratories.

The Fermilab-built ACC39 module with the coupler vacuum and wave guide systems assembled in the Cryomodule Test Bench (CMTB) at DESY.

The 3.9-GHz module has been developed and built by Fermilab. The three times higher frequency compared to the 1.3-GHz TESLA style cavities required a complete revision of the superconducting cavity design, the production and preparation methods used so far together with some significant cavity redesign. DESY provides the RF power system consisting of a high-voltage power supply and a klystron, the RF control, the coupler vacuum, the cavity tuner motor electronics and the interlock systems for the complete RF system. Other US members of the TESLA Technology Collaboration like the Thomas Jefferson National Laboratory, Cornell University and Argonne National Laboratory contributed to the development of the 3.9-GHz module by giving advice and performing various production steps.

In April, the complete module was transported by plane and truck from Fermilab to DESY. After several checks, tests and a revision of some bearings the module was installed in DESY´s cryomodule test bench for the first complete system test. This test bench is mainly used for testing the 12.2-metre-long 1.3-GHz modules built at DESY. It required some adaptation work to be able to host the 3.9-GHz module. Further developments and adaptations were performed at DESY for the new 3.9-GHz RF system, such as setting up the RF power station and the RF control and adapt methods and software, which are usually used for testing 1.3 GHz modules.

The first tests of the whole module show that the devices developed at the two laboratories fit perfectly and have a performance well above the required design specs.