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X to Y and monopole modes coupling in the KEK BPMs

X to Y and monopole modes coupling in the KEK BPMs . A. Lyapin, UCL. Motivation. Yes, offset of the beam in x will be always seen in y channel because of the finite accuracy of the alignment of the cavity axes to the “beam” (say, magnets etc) axes

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X to Y and monopole modes coupling in the KEK BPMs

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  1. X to Y and monopole modes coupling in the KEK BPMs A. Lyapin, UCL

  2. Motivation • Yes, offset of the beam in x will be always seen in y channel because of the finite accuracy of the alignment of the cavity axes to the “beam” (say, magnets etc) axes • Yes, we probably can reduce the x-y coupling to some fraction calibrating the system and doing a regression analysis • But – we still need to have an acceptable level of x-y coupling initially, need to know how to measure it and – how to reduce it nanoBPM group meeting

  3. Structure – KEK BPM We are simulating the KEK structure putting a relativistic 1 pC bunch into it, which has an offset of 0.25 mm in x direction. We are expecting signals to come from y ports of the cavity. X2 Y1 Y2 X1 nanoBPM group meeting

  4. Dipole mode signals for a 0.25 mm offset in x (t=113 ns, q=1pC) Y1 X1 Y2 X2 nanoBPM group meeting

  5. Dipole mode signals for a 0.25 mm offset in x (t=113 ns, q=1pC) Y1 X1 Y2 X2 nanoBPM group meeting

  6. So, we see X-Y coupling… • X-Y coupling is built in – it is a design issue! Seemingly the asymmetric coupler with the loooong feedthrough again • We need to simulate the measurement – cavities are checked with a NA measuring the transmission from x to y ports • We also need to see if a simple slight distortion like dents proposed for the ATF2 cavity is a remedy nanoBPM group meeting

  7. Simulation of the measurement X in X out Y1 Y2 nanoBPM group meeting

  8. From inside nanoBPM group meeting

  9. Test vs. reality • Results of the two simulations – beam excitation and transmission – seem to coincide, no surprise, but it’s always better to check… • ~10% x-y coupling is due to the coupler addressing itself to both x and y; x and y axes of the excited field are rotated with respect to cavity’s x and y nanoBPM group meeting

  10. Introducing asymmetry We are introducing 6 mm in Ø, 1 mm in depth and 10 mm long dents on the cavity rim. Similar dents were proposed for ATF2 QBPMs. They are supposed to align the dipole mode polarizations to x and y axes and simplify the BPM’s operation. Dent nanoBPM group meeting

  11. Measurement with dents X in X out Y1 Y2 nanoBPM group meeting

  12. Dipole mode signals for a 0.25 mm offset in x (t=113 ns, q=1pC) with dents Y1 X1 Y2 X2 nanoBPM group meeting

  13. Monopole and HO modes signals(t=113 ns, q=1 pC, x=0.25 mm) TM010 Y1 X1 TM020 TM110 TE011 TM210 Y2 X2 nanoBPM group meeting

  14. Summary • X to Y coupling is high for the KEK structure, it seems like x and y of the cavity are rotated due to an asymmetric coupler • Measurement of x-y coupling by means of a NA gives realistic results • Artificial asymmetry doesn’t seem to work properly, although need to dig a bit here • Average common mode coupling is in the order of 60 μm at the resonance (has to be improved with a spectral density function for real Qs; also need to look into the time structure of the signal applying filtering) nanoBPM group meeting

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