1. Feedback from structure tuning: Effects and Compensation of length (phase) difference in dual-feed couplers 22 May 2015 Rolf Wegner

2. 22.05.2015Phase difference in symmetric feeds2 What happens when the electric length of the 2 feeding arms into an accelerating structure differs ? 1.Effects 2.Options for Compensation

3. Symmetric feed 22.05.2015Phase difference in symmetric feeds3 structure: PSI-CERN N5 = CLIAPSI N5 typical setup, 2 symmetric arms for feeding input (and output) input – connected to VNA ports 1 and 3 (not 1 and 2 !) output

4. typical input reflection – symmetric feed 22.05.2015Phase difference in symmetric feeds4 structure: TD26_CC_N1

5. typical input reflection – symmetric feed 22.05.2015Phase difference in symmetric feeds5 structure: TD26_CC_N1

6. asymmetric feed 22.05.2015Phase difference in symmetric feeds6 structure: PSI-CERN N5

7. asymmetric feed 22.05.2015Phase difference in symmetric feeds7 structure: PSI-CERN N5

8. asymmetric feed 22.05.2015Phase difference in symmetric feeds8 structure: PSI-CERN N5 difference in electric length between the 2 input arms can be determined via phase difference in transmission from output to each input port here: port 1 ~ 1 mm longer than port 3 (2nd input port) 12° in phase [ z (11.994 GHz, N2) = 29.838 mm ] both output ports are equally long

9. asymmetric feed 22.05.2015Phase difference in symmetric feeds9 structure: TD26 CLEX N4 similar behaviour seen for all 4 TD26 CLEX structures length difference between arms is ~0.3 mm

10. asymmetric feed - compensation 22.05.2015Phase difference in symmetric feeds10 structure: PSI-CERN N5 nicely tuned structure: * phase advance from cell to cell 150°±0.5° * average advance per cell 150.06° * very smooth amplitude profile However, a peak of the electric field or a phase advance overshoot at the beginning of the structure is needed to minimise the combined input reflection.

11. Options for compensating phase difference 22.05.2015Phase difference in symmetric feeds11 Options for compensation: 1.at the moment: no direct compensation, creating artificially a reflection from the beginning of the structure (typically cell 1 and 2) which reduces the combined input reflection (in amplitude and phase) + simple - influence on the field distribution inside the structure 2.RF compensation: modification of (one of) the input arms a) mechanical deformation b) RF tuning device of wave number 3.mechanical compensation: pre-measurement, re-machining of a flange => complicated 4.future option: single feed coupler / input + robust – accidents have been seen where an arm has been bent + simpler waveguide installation (combiner have the same length problem) - dipole kick Is a compensation needed? Maximum length difference seen so far (~15 structures) is ~ 1 mm

12. Thank you for your attention 1222.05.2015Phase difference in symmetric feeds