[esa-t474] Energy resolution for the ESA spectrometer

[Michele Viti]

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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