x to y and monopole modes coupling in the kek bpms n.
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X to Y and monopole modes coupling in the KEK BPMs - PowerPoint PPT Presentation


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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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Presentation Transcript
motivation
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

structure kek bpm
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

so we see x y coupling
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

simulation of the measurement
Simulation of the measurement

X in

X out

Y1

Y2

nanoBPM group meeting

from inside
From inside

nanoBPM group meeting

test vs reality
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

introducing asymmetry
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

measurement with dents
Measurement with dents

X in

X out

Y1

Y2

nanoBPM group meeting

dipole mode signals for a 0 25 mm offset in x t 113 ns q 1pc with dents
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

monopole and ho modes signals t 113 ns q 1 pc x 0 25 mm
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

summary
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