Transcript Cavity BPM Modelling/Design Progress

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
► 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
10.08.2005
nanoBPM group meeting
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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
10.08.2005
nanoBPM group meeting
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Dipole mode signals for a 0.25 mm
offset in x (t=113 ns, q=1pC)
Y1
X1
Y2
X2
10.08.2005
nanoBPM group meeting
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Dipole mode signals for a 0.25 mm
offset in x (t=113 ns, q=1pC)
Y1
X1
Y2
X2
10.08.2005
nanoBPM group meeting
5
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
10.08.2005
nanoBPM group meeting
6
Simulation of the measurement
X in
Y1
10.08.2005
X out
Y2
nanoBPM group meeting
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From inside
10.08.2005
nanoBPM group meeting
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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
10.08.2005
nanoBPM group meeting
9
Introducing asymmetry
Dent
10.08.2005
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.
nanoBPM group meeting
10
Measurement with dents
X in
Y1
10.08.2005
X out
Y2
nanoBPM group meeting
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Dipole mode signals for a 0.25 mm offset
in x (t=113 ns, q=1pC) with dents
Y1
X1
Y2
X2
10.08.2005
nanoBPM group meeting
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Monopole and HO modes signals
(t=113 ns, q=1 pC, x=0.25 mm)
Y1
X1
TM010
TM020
TM110
Y2
10.08.2005
TE011
TM210
X2
nanoBPM group meeting
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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)
10.08.2005
nanoBPM group meeting
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