A general comment first: The questions ITRP has posed to the proponents focus largely upon technical issues for the linear collider. These are important in establishing technical and cost risk, and in potentially uncovering show-stoppers. We are not the ideal technical evaluation panel. However, the ultimate criterion for choosing a technology is that the collider should make the best physics measurements possible. This is impossible to quantify, as we cannot know the nature of the physics at the TeV scale. We don’t have questions to address this, but in the end, the physics potential will have to be our primary guideline.

Question 4: Describe the tests and simulations needed to demonstrate that the couplers between waveguides to the linac vacuum within structures or cavities will be sufficiently robust. There are about 20,000 couplers to cavities or structures in either technology. In each case, they transmit and launch the rf pulse into the accelerating structure. However, the constraints on the couplers are quite different in the two technologies. Paul Grannis June 2004

Cold LC: The cold machine couplers must: Transmit the rf pulse, Isolate the cavity vacuum from the external world and protect the Nb surfaces from contamination. Make the transition from atmosphere (in external waveguides) to vacuum. Make the transition from room temperature to 2K. Reduce multipactoring, seen in previous SC couplers (multipactoring is resonant emission of electrons in standing waves between two surfaces, driven by the rf field).

The cold coupler provides for :  Two ceramic windows; one between room temperature atmospheric pressure and vacuum and the second between vacuum and the vacuum in the cavity at 2K  Coaxial transmission line through the coupler  DC bias on inner coax to reduce multipactoring  TiN coating of surfaces to reduce multipactoring (developed at DESY and ready for industrialization)  2 bellows to permit the 15mm motion on cryomodule cooldown  Ports for monitoring light and electron density, pumping between windows, introducing the DC bias.  External Q tuning

Cold Machine Input Coupler

Cold Machine Input Coupler

A particular problem in the cold coupler is desorption of gas at the warm window and recondensation on the cold window. In the presence of multipactoring, this gas can be released causing rf breakdown. This process occurs on the timescale of a year. Time to repair a major failure is ~4 weeks, so must wait for long shutdown. The cold coupler contributes a significant heat load for the cryogenic system: 44% of load at 2K 18% of load at 5-8K 24% of load at 40-50K So, attention to the thermal properties of the coupler and insertions is essential. Cold couplers:

MTBF spec (US cold/warm study) is 10 7 hrs. Couplers must be robust. TRC said that 1 trip/cavity in 30 hrs would be acceptable to keep the energy overhead. US cold/warm study assigned a combine risk factor of 36 Concern that industrialized coupler fails to meet specs (2) Effect on energy is linear (3) Problem not detected until project engineering done (2) Major redesign to mitigate problem (3) Industrial vendor (CPI) cites challenges: Quality of copper plating; hard to plate on bellows effectively Maintaining tolerances of welded assemblies Preventing contamination of windows during e-beam welds Cold couplers:

Previous TTF II coupler showed breakdowns (> 17/hr/cavity) at gradients of ~20 MV/m under somewhat abnormal operating conditions with cavities operated near the maximum gradient. About 10 6 hrs operation. No damage observed. Recent TTF III design operated satisfactorily for 25,000 hrs with no (few) breakdowns. This design introduced the DC bias on the central conductor. Tests were up to 2MW (2X operation) and 30 MV/m. US cold/warm study cites trip rates in CHECHIA at less than 1/1000 hrs. Cold couplers:

Couplers need fairly extensive conditioning – first independent of the cavities, then after installed on the cavities. This takes between 1 and several days at present. Controls system to detect and respond to faults is moderately complex. New bilateral agreement with Orsay/Saclay to study couplers, test performance, streamline conditioning, optimize interlock and controls. Cold couplers:

Warm LC couplers: X-band couplers have no windows (last window is at klystron exit) Input waveguide system operates at linac vacuum as there is no special contamination problem for structures. Entire system at room temperature rf wave enters from two sides in rectangular waveguides at front of structure, launched into the circular disk-loaded structures. Earlier designs showed some breakdowns in the couplers (as well as structures); new design with overheight waveguides shows no breakdown at SLAC for ~10 structures. At GLCTA, we heard preliminary suggestion of a few coupler breakdowns. Recovery from breakdown is the same ( <10s) as for the structures.

RF Accelerator Structure Warm Machine Input Coupler with Upstream Cover Removed

Assessment: Warm couplers seem relatively mature and low risk. Have not yet demonstrated full MTBF, but experience suggests that a robust design has been found. Cold couplers are complex, so there are more avenues for risk (vacuum, multipactoring & rf breakdown, thermal loads). Risk of failure is severe if vacuum failure allows contamination of Nb cavity surfaces. Conditioning time adds complexity to QA/certification process. In event of major failure (gas desorption and rf failures), repair time is large. Considerable risk will remain through engineering and into pre- operation phase. I imagine that the cold couplers can be successfully designed and operated, but there is risk of some delay or impact upon operation.