feedback from structure tuning: effects and compensation of length (phase) difference in dual-feed...

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

6

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

8

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

10

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.

11

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