[esa-t474] All corrections to T474 paper complete (fwd)

[Mark Slater]

Dear All,

After comments from Johnny and Chris, I've altered the reply to point 2 
slightly. If everyone is happy I'll submit on Saturday morning (Friday 
evening US)!

Thanks,

Mark

P.S. The paper on the website has also been updated by the way:

www.hep.phy.cam.ac.uk/~slater/esabpm.pdf

> ********************************
> 2) In several places (line 178 on p. 12, and later, in lines 671 to 674 on
> p. 42, line 587, p. 35), the paper talks
> about unwanted coupling with other modes
> in the cavity. BPM 1 is said to have rectangular cavities, and I am
> guessing that the mechanical damage brought the
> detuned orthogonal mode closer to the
> filter acceptance window.  Is the artifact in the BPM 4x signal due to the
> same issue? In that case, analysis of the
> residual vs. motion in the y-direction
> would give a much clearer indication of the source of the cross-coupling
> (if data is available).
> 

There are several sources of coupling to the additional modes in our cavities.
First, the most obvious, is the leakage of the monopole mode into the
processor bandwidth. This is significant for the old (rectangular) cavities,
and is mitigated by the reduced bandwidth of the corresponding processors (20
MHz). The monopole mode introduces an fixed offset in the beam position
derived from the DDC algorithm, and to first order does not cause significant
problems (aside from the reduced dynamic range).

The are also two ways the dipole mode in the orthogonal direction can couple.
In the rectangular cavities, the orthogonal dipole mode is separated from the
main dipole mode by over 200 MHz, so aside from the damaged cavity, the
coupling is insignificant. The main contribution is from the coupling between
the two orthogonal cavities. For BPMs 9-11, it is about -40 dB, and smaller
for BPMs 1-2.

With this level of coupling, it would be extremely difficult to differentiate
the correlation between X and Y measurements induced by the coupling from the
simple geometrical misalignment (i.e. the axes of the cavities rotated with
respect to the axes defined by the movers and/or the corrector magnets). The
size of the effect we see is consistent with the numbers above.

After this further examination, it therefore seems unlikely that other
cavity modes are responsible for the low-amplitude effects seen in Figure
22. Though the coupling for the cylindrical cavities is higher than that
for the rectangular cavities (measurements indicate -20->-30dB), as we see
the low amplitude effect in both the cylindrical and rectangular cavities,
it seems more likely that errors in the relative scale determination are
responsible and we have changed the text accordingly to reflect this.