Hi,
indeed when the beam charge was calibrated we had low beam currents. But
the calibration was done inserting a "fake" charge into the Q-loop of
the dcct so it should not matter. This fake charge was monitored externally
with an absolute, high precision device (10^-6 HP ammeter) simultneously
read out in epics (there should still be variable for it in the list).
Note: this is as good as recalibrating the dcct. You can't argue the
epics dcct is better. It's the same thing done a bit earlier (actually
the operations dcct may also walk away from true current).
Chris is right that this calibration has to be rechecked. But I do think
it did reach up to at least 150 mA at that time. I did leave a plot in the
paper logbook. I am puzzled something changed. It must be the
"extrapolation" or else something really bad happened to the scalers.
The scaler current is calibrated ""once" for both the beamgated and
ungated channels. Their ratio should still be the overall deadtime, even
if the said number of C is wrong. Just a minor point (which I guess it
is obvious to you now) in the picture you show, the dead time is not the
ratio of dots to top line" since that is off also by the ratio of "bot to
top lines"
About the calibration: it can be easiliy redone, me & chris did talk
about it before I left, it essentially amounts at taking epics data only
with the HP hooked up while injecting current into the toroid (from
the RSG building, dan cheever knows how to reproduce the set up).
Typically the data was taking controlling the beamgate by hand (with the
command beamgate_on/beamgate_off) so as to take data only once a stable
injection current is reached and not while you are stepping up. You can
then take data over any range of injected or pseudo current, at each
setting you have the scaler counts/s corresponding to injected mA
Make sure no target is flipping (!) or the beamgate is not controlled
elsewhere (it will affect the beamgated scaler...)
Scalers shoud be good to 1 count in 10^6, and you count less than 5e4 so
no need to worry here. Let me know if there are questions.
Sorry I was away last week, could have picked up this earlier. Good luck
to you all with your work.
-- tancredi
________________________________________________________________________________
Tancredi Botto, phone: +1-617-253-9204 mobile: +1-978-490-4124
research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949
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On Wed, 6 Apr 2005, Chris Crawford wrote:
>
> One possible reason for loss of luminosity, is that our scaler current is miscalibrated. See the attached figure. The top line is EPICS beam current, the line just below is the scalers current, and the fuzz is the beam-gated scalers current. I think happened is that it was calibrated at low currents, before we were so optimistic about our beam operating parameters. There was a small nonlinearity in the fit with to quadratic, and now we are extrapolating.
> --Chris
>
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