[esa-t474] Energy resolution for the ESA spectrometer

[Alexey Lyapin]

Gentlemen,

Many apologies -- since we are not doing any sort of climate change 
research, I have to withdraw my analysis! ;) I thought the results 
looked too good to be true, and the reason for that is that I mistook a 
mover scan for an energy scan!!! :( At least I had something to laugh 
about for at least 5 minutes :)

Really sorry for these misleading results, but in any case I think we do 
want to publish a paper soonish, as Michele has done some really good 
work. I will try my approach to the right piece of data and see if it 
still works and cross-compare. By the way, Michele, which data set did 
you use to generate fig. 12 in your note?

Best regards,
Very embarrassed Alex




On 03/12/2010 11:51 AM, Michele Viti wrote:
> Hello everybody
> after processing data in my head, here other comments from my side...:-))))
>> What is the expected noise in the NMR probes ? I would be very
>> surprised if it was smaller than 1e-4 (that's about 0.1 G). So I bet
>> the observed fluctuations of the field are representative of the
>> measurement noise, not the actual variation of the B field.
>>
>> One way to check if the B field variation is significant is to compute
>> the pulse-pair energy resolution. In other words, do the analysis like
>> Alexey did, and write out the resolution values to a file (i.e. values
>> from top-right plot in Alexey's enePlot.pdf). Then divide this file
>> into a set of consecutive pairs, and compute the pair difference of
>> resolutions E_i-E_{i+1}. This difference will subtract out the
>> long-timescale drifts (e.g. B field drifts), and will leave you true
>> pulse-to-pulse resolution representative of the electronics noise (the
>> RMS of E_i-E_{i+1} is sqrt(2) times the single pulse resolution).
>>
>> Looking at Alexey's plot, it's already clear that the pulse-pair
>> resolution would be better -- probably by a factor of 2 or so -- than
>> the RMS displayed in bottom-right plot. In the top-right plot, you can
>> see some saw-tooth oscillations with a timescale of about 200 pulses
>> (20 sec) -- my guess this is the effect of the current feedback on the
>> magnet power supplies. There is also a long-term drift with a
>> timescale of a min or so -- probably temperature related (BPM position
>> or magnet current readout).
> For the first comments I already answered...(see the other mail). I
> think what we observed are real magnetic field fluctuations...we have
> data also from the current but they have not a good resolution...but
> they suggest the same thing....I attached also a picture with relative
> fluctuation for magnet 2 and 4 (NMR.eps).
>
> Another point to consider is the beam energy jitter: if you look figure
> 5 (right) in my note (energy BPM signal during energy scan, normalized)
> you see that for each step we have a "blob" of points (sorry I dont know
> if this is the right word) and not a line like in bottom-right figure in
> file bpmPlot.pdf
> This blob is caused, I guess, from beam energy jitter which is around
> 6*10^-4 (I'm not sure of this number, can anyone check!?!??). It looks
> like from Alexey's figure that we have a real small energy jitter...I
> think this must be checked.
> Alexey, is it the reason why when you regress BPM 4 to BPM 12/24 during
> an energy scan, you got not good results?
> still thinking....
>
> Last point: in the first strep, when we regress BPM 4 to BPMs 1,2,3,5 I
> suggest to use a run with magnets off....which is actually what I did in
> my note. What do you think?
>
> cheers
>
> Michele
>
>
>
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-- 
Dr. Alexey Lyapin

Applied Physicist
Physics and Astronomy
University College London
Gower Street
London WC1E 6BT
UK

Tel. +44 (0)20 7679 3454
Fax  +44 (0)20 7679 7145
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