As part of a multi-laboratory collaboration, Jefferson Lab was engaged in the fabrication, assembly, and testing of 23 cryomodules for the superconducting linac portion of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. The superconducting linac is designed to accelerate protons to an energy of 1 GeV onto a mercury target. Eleven of the cryomodules contain 3 medium velocity ( b=0.61) superconducting RF cavities, with the remaining twelve contain 4 high velocity ( b=0.81) superconducting RF cavities. In addition to superconducting cavities, the cryomodules contain tuner mechanisms, power couplers, diagnostic
In order to achieve the required energy of 1 GeV within the limitations of the RF and cryogenic system parameters, the cavities used in the medium (high) beta cryomodules needed to achieve an average gradient of 10.2 (15.6) MV/m at a Q-value of 5 x 10 9. These gradients correspond to peak surface electric fields of 27.6 (34.2) MV/m. While these surface fields do not represent the state-of-the art achieved in R&D or prototype cavities worldwide, for the high beta cavities they represent the highest surface fields specified for a production SRF installation to date. The cavities processed and tested for the SNS cryomodules, which were fabricated by Accel Instruments Gmbh in Germany, not only met these required specifications, but in fact exceeded them in many instances, with the highest peak surface electric fields measured at 54.2 (56.5) MV/m for the medium (high) beta cavities.
For the medium beta cryomodules, of which seven were extensively tested, the average peak (accelerating) electric field of the cavities was 41 (15.1) MV/m, which exceeds the required performance by almost 50%. Similarly, for the cavities in the two high beta cryomodules tested, the average peak (accelerating) electric field of the cavities was 43 (19.6) MV/m, which exceeds the required performance by 25%.
Cryomodule performance is characterized by more than just the performance of the cavities installed in the module however, as overall performance is the product of numerous distinct elements that must work in concert to achieve the operational capabilities and stability required when the cryomodule is installed in the linac. The cryomodules fabricated and assembled for the SNS project met or exceeded required performance specifications in the areas of HOM (higher order mode) extraction, input coupling, tuner operation, and sensitivity to microphonics (environmental disturbances). For example, the FPC performance is characterized by its ability to deliver power to a cavity without arcing or vacuum problems. Only 6% of the cavities tested were limited by coupler performance (and at a level greater than that needed to achieve required cryomodule performance). In addition, extensive measurements of the cavity response to Lorentz forces were performed. In general, the cryomodules met or exceeded the requirements. It was also shown that the additional use of the piezo tuner was a simple and effective method of reducing the sensitivity of the cavity to Lorentz forces.
Jefferson Lab joined the Spallation Neutron Source collaboration in February 2000. The accepted work scope included:
| Scope Element | Status | |
|---|---|---|
| 1 |
An R&D program to determine parameters and specifications for the production of cavities and cryomodules, fabricate and test four medium-β (β = 0.61) and two high-β (β = 0.81) six-cell cavities, and fabricate and test a three-cavity prototype cryomodule. |
Completed September 2002 |
| 2 |
Design and construction of a cavity/cryomodule production facility capable of producing/repairing five cryomodules/year. |
Descoped as a cost-saving measure after design was complete (November 2000). |
| 3 |
Design and procure/construct a helium refrigerator and helium distribution system. |
Completed December 2004 |
| 4 |
Design and procure/construct 11 three-cavity medium-β cryomodules. Originally, two of these cryomodules were to be tested at JLab. This scope was subsequently expanded to the testing of seven cryomodules. |
Completed May 2004 |
| 5 |
Design and procure/construct 12 four-cavity high-β cryomodules. Originally, one of these cryomodules was to be tested at JLab. This scope was subsequently expanded to the testing of two cryomodules. |