Optimization of a Traveling Wave SRF Cavity for Upgrading the International Linear Collider
Valery D Shemelin, Freeville, USA
Cornell University, Ithaca, New York, USA
H. Padamsee, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA
The Standing Wave TESLA Niobium-based structure is limited to a gradient of about 50 MV/m by the critical RF magnetic field. To break through this barrier, we explore the option of Niobium-based traveling wave (TW) structures. Optimization of TW structures was done taking into account experimentally known limiting electric and magnetic fields. It is shown that a TW structure can have an accelerating gradient above 70 MeV/m that is about 1.5 times higher than contemporary standing wave structures with the same critical magnetic field. The other benefit of TW structures shown is R/Q about 2 times higher than TESLA structure that reduces 2 times the dynamic heat load. A method is proposed how to make TW structures multipactor-free. Some design proposals can be realized to facilitate fabrication. Further increase of the real-estate gradient (equivalent to 80 MV/m active gradient) is also possible by increasing the length of the accelerating structure because of higher group velocity and cell-to-cell coupling. Realization of this work opens paths to ILC energy upgrades beyond 1 TeV to 3 TeV in competition with CLIC. The paper will discuss corresponding opportunities and challenges.
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We consider ILC upgrade paths beyond 1 TeV: (1) to 2 TeV and (2) to 3 TeV depending on the needs of high energy physics. Parameters for four scenarios will be presented and challenges discussed. 1. From 1 TeV to 2 TeV based on: a. Gradient advances of Nb cavities to 55 MV/m anticipated from on-going SRF R&D on Nb structures discussed in Section 4.3.x. b. Radically new travelling wave (TW) superconducting structures [1,2] optimized for effective gradients of 70+ MV/m, along with 100% increase in R/Q (discussed in more detail in Section 4.3.x). The large gain in R/Q has a major beneficial impact on the refrigerator heat load, the RF power, and the AC operating power. OR 2. From 1 TeV to 3 TeV based on a. Radically new travelling wave (TW) superconducting structures [1,2] optimized for effective gradients of 70+ MV/m, along with 100% increase in R/Q. The large gain in R/Q has a major beneficial impact on heat load, RF power, and the AC operating power. b. 80 MV/m gradient potential for Nb3Sn  with Q of 1x1010, based on extrapolations from high power pulsed measurements on single cell Nb3Sn cavities. Further, the operating temperature is 4.2 K instead of 2K.