Feedback from structure tuning:

Effects and Compensation of length (phase) difference in

dual-feed couplers

22 May 2015

Rolf Wegner

222.05.2015 Phase difference in symmetric feeds

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

22.05.2015 Phase difference in symmetric feeds

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

22.05.2015 Phase difference in symmetric feeds

structure: TD26_CC_N1

5

typical input reflection – symmetric feed

22.05.2015 Phase difference in symmetric feeds

structure: TD26_CC_N1

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asymmetric feed

22.05.2015 Phase difference in symmetric feeds

structure: PSI-CERN N5

7

asymmetric feed

22.05.2015 Phase difference in symmetric feeds

structure: PSI-CERN N5

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asymmetric feed

22.05.2015 Phase difference in symmetric feeds

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

22.05.2015 Phase difference in symmetric feeds

structure: TD26 CLEX N4

similar behaviour seen for all 4 TD26 CLEX structures

length difference between arms is ~0.3 mm

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asymmetric feed - compensation

22.05.2015 Phase difference in symmetric feeds

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.

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Options for compensating phase difference

22.05.2015 Phase difference in symmetric feeds

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 armsa) mechanical deformationb) 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

Phase difference in symmetric feeds 12

Thank you for your attention

22.05.2015