Design of the input coupler for the ERL main linac in Japan

1. Design of the input coupler for the ERL main linac in Japan H.Sakai, K.Shinoe, (ISSP, Univ. of Tokyo) T.Furuya, K.Umemori, T.Takahashi,S.Sakanaka, T.Suwada (KEK) M.Sawamura (JAEA)

2. Motivation • KEK, JAEA, Univ. of Tokyo and other Japanese institutes just started ERL project to construct a 100-200MeV ERL test facility since last year (2006). • Superconducting cavity is one of the key component for ERL. R&D and development of components like cavity ,power coupler, HOM coupler, tuner … must be carried out. • Main target for ERL main linac: • 20MV/m gradient • 1.3GHz frequency • Reliable operation Input coupler must be suitable for these requirements.

3. Pg vs Df (Eacc and QL) 20kW Assume the frequency change by mechanical vibration of Df = 50Hz to be conservative and reliable.QL = 2×107 at 20MV/m when the power Pg is lower than 20kW

4. Parameters and philosophy ERL: QL = 2×107 at 20MV/m Other mode for example No ERL (FEL) -> 1mA,20MV/m -> QL = 5×106 We also add high power pulse conditioning -> lower QL is better Our start : 20kW input power , max: 20MV/m, QL : tunable (5×106)--2×107 • Candidates of input coupler for ERL injector • Based on TRISTAN input coupler • “antenna typed coaxial coupler” • (Max. beam power of 400kW on KEKB operation at 4K) • 2K & 1.3GHz coupler for STF-BL (pulsed 1.2MW) This type coupler is reliable for high power operation

5. TRISTAN type input coupler (KEKB) window frequency : 500MHz , temperature : 4K Test input power : 0.8MW , operation 400kW at KEKB water cooled-inner conductor Single ceramic window Candidate for the ERL injector coupler

6. Our basic design for ERL main linac (start from ILC coupler at STF) TRISTAN type ceramic window Monitor Port Beam Pipe 5K Low loss 0.2W to 5K Higher Static loss 1W to5K No Tuning Warm Window Cold Window 80K Vacuum Port Dynamic loss < static loss Qext = 2.0 x 106 Prf = 350 kW R&D for input coupler at 1.3GHz at 2K was tested on STF

7. Test and results of ILC input coupler results ILC(BL) Input coupler Power capacity of 1MW (1.5ms, 5Hz) peak power was confirmed Power test stand at STF But : Ave: 7.5kW < 20kW

8. Comparison between ILC and ERL We need to check the heat load by input power

9. L1 C-c cross section Expected heat load in CW operation L2 A B r=30mm c 51W 50.2W copper 300K 80K c 2K Dtcu=5mm SS 5K 20kW input(Dynamic loss) Dtsus=1mm (Static loss) Dynamic loss is larger than static loss and heat load of inner conductor is dominant.

10. Modification from ILC input coupler • Reduce the heat load at inner conductor。 • Change impedance 50W -> 60W • Ceramic HA95 -> HA997 (tand reduce by a factor of ten ) • Input gas cooling in inner conductor • Variable Qext • Add bellows in outer conductor of cold side • Use rod connected inner conductor • T-bar connection was used Blue items of modifications were based on the Cornell ERL input coupler

11. Design of ERL main input coupler (ver.1) Cold window f96 f35 T-bar Bellows Warm window ・20kW CW input power ・variable coupling ・gas cooling ・double ceramic of same size ・disk type ceramic window (HA997) ・60W impedance Inner rod: ・Gas cooling ・change Qext RF

12. Expected Qext L=55mm (60W) 60φ Calc by HFSS 22φ L_port1 Beam pipe 100φ 9cell case L=55mm • Qext1=5x106～2x107 •  Lport1 = 51.3～61.1mm • Variable range = ±5mm

13. R&D items • Ceramic window（impedance : 60W、ceramic HA997 used） • 80K cool-down test (check the thermal stress) • Check the heating during the 20kW power operation（measure the temperature） • Multipacting study（measure the electron by e-pick up probe.） • Bellows • Same check lists as the ceramic window, especially we’d like to check the heating during the 20kW power operation (measure the temperature) • Can we suppress the temperature rising at the bellows by air gas ？ • 80K cool-down test of copper plating of bellows • Check the distribution of the thickness of the copper plating of bellows • Mechanical design • Easy assembly ?, cool-down test , vacuum leak ? Recently we have found the large temperature rising (DT> 80 degree) on ceramic parts and inner conductor in Cornell ERL input coupler by putting the 35kW CW power. (S.Belomestnykh of TTC meeting on 24.Apr.2007) Therefore, we need to check the heating of ceramic and bellows during the 20kW operation.

14. Plan of component test of input coupler Test sample ・ ceramic ・ bellows ceramic • Test sample will be • cool down at 80K by Liq. N2 • Standing wave of max 30kW • phase will be changed

15. Test stand place(JAEA) Test stand here 1.3GHz CW 30kW IOT 4.5m circulator 1m

16. IOT & circulator IOT 30kW IOT circulator 40kW circulator IOT

17. 2007 Time schedule of input coupler (test stand) 2008 End of July End of Sep. Test stand Test stand design Production of test stand Middle of Oct. High power test (ceramic & bellows) End of June Ceramic production Cool down test End of June Bellows production End of July Cool down test bellows for high power test Thermal analysis, T-bar, Precise designing Design of Input coupler for main linac Prototype production ‘08 end of Mar.

18. Summary • Input coupler design is just started for ERL main linac. • Input power : 20kW • Gradient : max 20MV/m • QL : 5 x 106 – 2 x 107 • Design is based on ILC coupler at KEK-STF. Especially, disk type ceramic was applied. Some modifications are applied. • One issue is the large heat load of ceramic window and bellows by adding the 20kW CW power. In order to estimate the temperature rising, first we have a plan to make a test stand for the component test in this year. • Thermal analysis of input coupler is now under way.

19. Ceramic window Disk ceramic with choke is applied to avoid standing on the large electric field on the edge of ceramic window. Peak E field is near the choke By changing the length of the inner choke, reflection was reduced down to -25dB

20. Thermal analysis (pleliminary) Bellows 10W No air flow Add air flow DT = 800K DT = 10K