[esa-t474] Another Paper Update

[Johnny Ng]

Mark,

I'm just starting on this project from the SLAC side.

I've been reading Chris' comments.  I have a few of my own:

- Zygo measurement: what are the systematics?  is it sensitive to
   thermal/air turbulence?

- What caused the drifts?
    - was temperature variation monitored?  does it correlate to the drift?
    - are the drifts in BPM 4, shown in the bottom two plots of Fig 27, and
      the bottom plot of Fig. 28, consistent?  If I understand the plots
      correctly, I'd expect the bottom two plots in Fig. 27 to show a
      general drift (caused by the mechanical drift).

I don't know how much access to the tunnel we still have.  But if these
issues are important, it might be worth the while to do some tests in the
tunnel before the run.

regards,
Johnny

-----------------------------------------------------------------------------
Johnny S.T. Ng
Stanford Linear Accelerator Center              Office: Bldg. 51, Room 311
MS 29, P.O. Box 20450                           Phone: 650-926-4490
Stanford, CA. 94309-2010, USA.			FAX: 650-926-5566
 						E-mail: jng at SLAC.stanford.edu




On Sun, 17 Jun 2007, Adolphsen, Chris wrote:

>
> The paper is getting better but still has many problems - some comments:
>
> Table 2: the aperture of bpms 3,4 and 5 is 36 mm, not 30 mm - it is listed correctly in line 144
>
> Line 119: Include a sentence on the beam size - a general rule thumb is that systematics on position measurements start to occur at 1/100 of the beam size
>
> Line 148: A lower Q does not mean a lower resolution as the same rf energy is generated independent of Q (i.e. at low Q, the pulse is shorter but the amplitude larger). However, a shorter pulse requires wider bandwidth electronics which may be more susceptible to noise if not implemented properly - so you might say the shorter pulses are harder to measure with high precision.
>
> Line 167: I believe it is aliased not aliaced (in line 151, the more common 'realize' than 'realise' should probably be used)
>
> Line 193: I believe the 'Gaussian filter was applied to remove both out of band noise and the additional 2*omega component' before of the IF conversion to baseband, not afterward.
>
> Figure 7: the phase plot should be much flatter - in February, I noted this and Yury agreed, i.e.,
>
> "> Finally, why is the phase not constant in Figure 5 - it this waveform
>> saturated.
>>
> I am not sure where this plot comes from, but it's not what we use in the analysis :) We have tuned the LO frequencies in the DDC to make the phase flat. Mark: I can give you a better plot."
>
> Line 209: Instead of 'the position and the angle signals are exited with a 900 phase offset [?]' say, the rf signals from pure position offset (zero angle) and pure angle offset (zero mean position) differ in phase by 90 degrees. - you do not need a reference for this as it is common knowledge
>
> Figure 9: how are these two plots related - is the right one the data near the center after rotating and applying only the position calibration. Also, the reader probably does not need to know what run the data are from.
>
> Table 3: It seems strange that the one bpm that is moveable (#4) has the biggest offset from the others
>
> Table 5: Is this bandwidth or 2*pi*bandwidth (i.e. cavity 3-5 has a Q~500 so f/2Q ~ 3 MHz, not 20 MHz)
>
> Section 3.4.2 - earlier you say 'The bandwidth of the Gaussian filter used in the algorithm. This was set as
> approximately two times the predicted decay width of the BPM signal' so what do you use the fitted decay constants for ? Also, the two methods yield quite different results, over a factor of 2 for some of the widths! (i.e. compare table 6 and 7) - which one, if any, is correct (the argument in 346-356 is not convincing - my guess is that one of the analyses was done incorrectly - likely the fft, which has subtleties due to the finite sampling time).
>
> Table 11 and 12: could you put all these results in exponential notation with 2 digit accuracy (also use fractional variations instead of %) For bpm 3-5, the decay x variation is due to what looks like one bad measurement - you need a better check of data quality. Also bpm 11 has some obvious systematic effect so computing the rms is not meaningful.
>
> Figure 17+18 and Table 13 - clearly the scale info is garbage, so why not show the jitter corrected data (i.e. use bpm 4 and one other bpm to provide a measure of the beam position at each bpm on each pulse that is used to calibrate these bpms. That is, bpm 4 would be separately calibrated with the mover only once, and the other bpm would be calibrated against the corrector or bpm 4 (assuming parallel orbits) only once - even with some variation in the scales of these reference bpms over time, it would allow the relative calibration stability of the other bpms to be studied on a much finer scale than what you show here.
>
> Figure 21-24: it is not clear which spectator bpms are used for these analyses - only the last figure mentions this - one would need at least two other bpms to do this, so how was it done for bpm 2 if only local bpms are used. Also bpm 1x has serious problems so should not be used in any analysis.
>
> Figure 25 - data is garbage so why plot it without some explanation
>
> Conclusion - this paper needs a conclusion, which should state that while a good precision appears achievable, there are as yet understood systematic effects that would limit the usefulness of these bpms to provide a stable beam energy reference (either absolute or relative).
>
> It would be good to have a meeting at SLAC to discuss the paper again prior to the next run.
>
> -----Original Message-----
> From: Mark Slater [mailto:slater at hep.phy.cam.ac.uk]
> Sent: Saturday, June 16, 2007 10:20 AM
> To: esa-t474 at hep.ucl.ac.uk
> Subject: [esa-t474] Another Paper Update
>
> Dear All,
>
> 	First, apologies for this update being a day late - I lost a day last week through illness and wanted to get some more finished before submitting. Anyway, the latest update contains major overhauls on the first and second sections, including many of the comments I've already recieved. Still several to go, but I think I'm getting there :)
>
> Thanks,
>
> Mark
>
> P.S. As always, you can find the paper at:
>
> http://cvs.hep.ucl.ac.uk/viewcvs/esaBpmNote/built/?cvsroot=LC+Energy+Spectrometer
>
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