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

Phase 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

Symmetric feed Phase 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

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

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

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

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

asymmetric feed Phase 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 ( GHz, N2) = mm ] both output ports are equally long

asymmetric feed Phase 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

asymmetric feed - compensation Phase difference in symmetric feeds10 structure: PSI-CERN N5 nicely tuned structure: * phase advance from cell to cell 150°±0.5° * average advance per cell ° * 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.

Options for compensating phase difference Phase 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

Thank you for your attention Phase difference in symmetric feeds