1. HISTORY OF SNS DESIGN AND TECHNOLOGY CHOICES PROJECT X WORKSHOP NOVEMBER 12-13, 2007 R. KUSTOM

2. BACKGROUND TO TECHNOLOGY CHOICE At time ANL/APS group came to SNS, the project had already started and a reasonably well developed RT Linac preliminary design existed to generate a 1-MW beam on target. The engineering details were still being developed. A construction contractor was on site preparing for the start of the conventional part of the project. A committee was formed to consider whether a different choice of technology offered greater scope and flexibility. The recommendation was to consider a SC Linac. A project management study determined that it was still possible to meet the cost and schedule milestones without serious breakage due to the technology switch.

3. Reasons for Choosing the Superconducting Option for the SNS Linac The higher gradient possible with SC cavities resulted in a general reduction of 140 meters in tunnel length (325m compared to 465m) The electrical power requirements were reduced by 12 Mw The SC option provided greater flexibility for future upgrades in power and for the future addition of a second target station.

4. Additional Considerations in the Choosing the SC Option for the SNS Linac Beam loss due to halo was less likely in the SC linac because of the larger beam aperture (1.75- 2.0 cm to 5.0 cm bore radius). There existed a significant industrial base for building superconducting cavities. There was not for RT cavities. Higher availability because cavity gradient was conservatively chosen allowing flexibility in operating parameters to adjust for weak or damaged cavities and still meet design energy.

5. REDUCED LENGTH AND HIGHER GRADIENT WITH SC LINAC RT LINAC Total linac tunnel length for RT Linac design was 465 meters Real-estate accelerating gradient was 2.28 MV/m for RT Linac design SC LINAC Total linac tunnel length is 324.65 meters with free space for additional cryomodules Real-estate accelerating gradient is 3.9 MV/m for SC Linac

6. RF POWER REQUIREMENTS FOR RT LINAC DESIGN Peak Linac RF power 117.2 MW, avg power 7.81 MW Peak DTL RF power 1.95 MW, avg 0.13 MW Peak DTL cu power loss 1.32 MW, avg 0.09 MW Peak CCDTL RF power 7.86 MW, avg 0.53 MW Peak CCDTL cu power loss 5.73 MW, avg 0.41 MW Peak CCL RF power 107.4 MW, avg 7.15 MW Peak CCL cu loss 74.2 MW, avg 5.21 MW

7. POWER REQUIREMENTS FOR THE SC LINAC Peak Linac RF power 50.4 MW, avg 3.63 MW Peak DTL RF power 9.7 MW, avg 0.7 MW Peak DTL cu power loss 6.7 MW, avg 0.48 MW Peak CCL RF power 11.4 MW, avg 0.82 MW Peak CCL cu power loss 7.9 MW, avg 0.57 MW Peak SC RF power 29.3 MW, avg 2.11 MW

8. REDUCTION IN PHYSICAL PLANT REQUIREMENTS DUE TO SWITCH FROM RT TO SC LINAC Switch to SC Linac saved 140 meters of tunnel construction and still left space to increase the number of cavities in the future. Switch to SC Linac resulted in a reduction of about 12 MW in operating power. Much of it due to reduction in generation of RF power, but also power requirements for chiller and cooling water pumping are reduced more than offsetting the increase due to addition of the liquid helium plant.

9. UPGRADE PATH EASIER WITH SC LINAC One of the DOE design requirements was to include a path to upgrade the facility to higher power in the future. The original RT Linac upgrade path was 1->2->4 MW by changing the bridge couplers on the CCL and adding RF. Major funding and lengthy shutdowns would have been required at each upgrade step. The SC Linac upgrade path is to increase gradient, add some extra cryomodules, and increase the accumulator ring energy. The cavities are designed with conservative accelerating gradients, 9.3 MV/m for 0.61β and 11.9 MV/m for 0.81β cavities, making it relatively easy to achieve higher energies in the future. The ring is designed to go to higher energy, ~1.3 GeV, without a major shutdown. This built-in capability make it possible to increase the facility power on target without line item upgrade budgets and installation can be done in modular steps with no long shutdown of the user program.

10. DECISION ON TRANSITION ENERGY Project schedule strongly influenced the physics design because of late switch from RT to SC Linac Decision to keep RT CCL to 185 MeV was decided by inadequate time to develop a third cavity designed for low β. Transition energy was a trade-off between having an optimized match to the SC section and insurance against weak performing cavities.

11. SNS EXPERIENCE AND PROJECT X DESIGN RELATED Project X has the advantage to develop and test SC cavities prior to the start of the project allowing a more aggressive design with greater confidence that the design goals can be met. Starting with an SC linac in the conceptual stage of the project will also Project X to develop a better optimized design.

12. SNS EXPERIENCE AND PROJECT X OPERATIONS RELATED SNS has been very successful in operating around isolated cases of weak cavity performance. This has helped enormously with maintaining availability. This option is something that should be preserved in the Project X design. SNS has 3-4 cavities per cryomodule separated by warm sections. This allows removal of cryomodules in which cavity problems need to addressed (Field emission, multipactor, HOM damper heating, stuck tuners, input coupler problems), but this reduces the real- estate accelerating gradient. The Project X design needs to allow for relatively easy access and removal of individual cavities or small cavity groups for the purpose of improving performance, or replacement, while still achieving higher real-estate accelerating gradient.