Thermal and mechanical requirement N. Solyak (FNAL) Coupler worshop . LCWS2013

1. Thermal and mechanical requirement N. Solyak (FNAL) Coupler worshop. LCWS2013

2. Outline • ILC TDR specification • Power requirements for Eacc=31.5 MV/m ± 20% • QL tuning range • Power overhead • Effect of gradient distribution • Requirements for cryogenic losses • Mechanical requirements • Conclusion

3. TDR specifications TDR v3-II. Table 3.7 Main specifications of the input coupler. The parameters represent the approximate maximum expected values during operation, including possible high L upgrades. Average power ~ 3.3/6.6 kW 400kW x 5/10 Hz x 1.65ms • Low energy regime requires 10 Hz operation for electron linac at reduced gradient. • Bunch compressor at lower gradient will not compress beam to nominal size. Is it OK?

4. NW & WDM, Aug.28, 2013

6. Impact of 20% gradient spread Flat-top operation (Pk,QL solution) for 8.8 mA; Gradient = 31.5 MV/m ± 20% Loaded Q (left) and required power vs. cavity gradient for different matching gradient • Constraint : QL ≤ 7∙106  Matched Gradient ≤ 30.2 MV/m • - smaller QL tuning range (2÷7) ∙106 • - Maximum RF power : 390 kW (@38MV/m) For Gmatch= 30.2 MV/m:

7. Reflected power (@ flat-top) and power losses in coupler 430 kW 410 kW Maximum power losses in coupler corresponds to ~430kW transmitted power (blue curve). Power reflection ~11%

8. Nick Walker and Wolf-Dietrich Moeller presentation

9. ~410kW ~430kW Conclusion: In case of large gradient spread ±20% it is beneficial to chose lower matched gradient (~30 MV/m , -4%). It will reduce filling time and required QL range. Drawback is higher forward power and higher losses in coupler, partially compensated by shorter RF pulse.

10. Effect of accelerating gradient distribution Ncav= 10,000 Power losses in coupler normalized to losses for the case without gradient spread. (need small correction for filling time). Cryogenic losses are scaled as power losses (max increase ≤ 10%)

11. Cryogenic requirements

12. TTF3/XFEL coupler STF-2 coupler

13. TTF3/XFEL STF-2 coupler Denis Kostin /DESY Thermal analysis done for 2 designs for different power levels, thickness of Cu and RRR

14. Summary of cryogenic losses simulation *Increase internal conductor coating h 1050 um; increase total losses ~0.8W @ 2kW • RF power simulated up to 5 kW (40 MV/m, 10 Hz, 1.65 ms, 6 mA ; margin 25%) • KEK S1-G and STF-2 couplers are compared to TTF3 / XFEL coupler. • - Designs are very close by cryogenic losses. • - STF-2 coupler has higher static losses • - STF-2 design has 5% less total RF losses (~25% less RF losses on the ceramic window ) compared to TTF3 DESY design.

15. Cryogenic losses in TDR Average power ~3kW 26/3 = 8.67 cavities per CM an average Coupler contribution 5% 30% 43% @7 kW 6% 30% 57% Both designs of coupler meet requirements. For 7 kW power requirement should be corrected

16. Modification TTF3 coupler for operation at 7 kW average power Power limited by overheating of the inner bellows >130ºC Cure: increase thickness of copper plating of inner conductor to ≥50 um Proposed for Project X, LCLS-II Effect of increase thickness of Cu (only in warn inner part): increase static losses and decrease of dynamic losses at 70 K

17. ILC RF Power Coupler design criteria comparison

18. QL tuning range Acceptable range of antenna longitudinal movements > 10 mm, taking into account lateral displacement 7 mm Ideal design Antenna depth, mm

19. Effect of antenna transverse position in TTF3 coupler

20. ILC-structure Q-external versus antenna displacements • Conclusion • For TTF3 coupler the most sensitive parameter is a horizontal antenna shift/tilt. 3mm shift change QL by ~20%. Vertical tolerances are relaxed. • For STF-2 coupler this is not issue, mechanical design guarantee small shift.

21. Coupler alignment tolerances in CM2 X- Horizontal – along CM Y- Vertical Real position of the antenna is not known. Measurements are done for position of center of 70 K flange.

22. Lateral movements With invar rod the expected thermal movement is reduced to 2mm for cavities at the end of CM. Taking into account assembly errors requirements ±(5-7)mm should be acceptable.

23. Conclusion • Requirements for 10 Hz operation with beam current = 8.8 mA and Eacc=31.5 MV/m ±20% is tight: • Max coupler power ~400  450 kW • QL tuning range: (2÷7)∙106  (1÷10)∙106 • Matching gradient ~30 MV/m is beneficial for reduction of tuning range and power overhead. • Coupler contribution to cryogenic losses at 2K is ~5%. Increasing of that value is not critical. 10 Hz operation at full accelerating gradient requires more cryogenic. Major contribution from coupler is 70K. • Mechanical requirements can be relaxed for lateral movements and longitudinal tuning range. But assembly procedure should guarantee level of alignment errors < (2-3) mm.

24. Back-up slides