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

[Mark Slater]

Dear All,

With help from Yury, here is another try at what to tell the reviewer in 
answer to point 2. I'm going to postpone sending the paper for 48 hours to 
let people check the revision again (the paper has been updated on the 
website with the corrections). Let me know if there are any problems!

Thanks,

Mark

> ********************************
> 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.

For the cylindrical cavities, there is a potential for coupling between x- and
y-dipole modes due to imperfections in the mechanical assembly and locations
of the coupling slots. The measurements show that this is below 30 dB (CHRIS
WOULD NEED TO CHECK THIS).

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.

In addition, after further examination it seems unlikely that coupling is 
responsible for the low-amplitude effects seen (Fig. 22) as we had written 
initially. It now seems more likely that errors in relative scale 
determination are responsible and have changed the text accordingly.