Transcript 2012-08-16, Dan Peterson

2012-08-16, Dan Peterson
Apparent inconsistencies and other issues in the xBSM measurements of IBS Scans
We have studied the pinhole and CodedAperture fits for the C-line and the D-line.
The plot, from Mike, 2012-07-12, shows beam sizes for representative runs.
We do not have a reason to expect agreement between the C-line and D-line.
However, the data from the different optics for a given line
should show the same result, as the machine conditions were the same.
From the plot,
the C-line Coded Aperture is high;
the D-line Coded Aperture is low.
These results are based on the
2012-07-11 functions.
These functions have an error; the applied sideband is too large.
( The sideband is introduced to match the Edge model to the data.)
Correcting the sideband level will bring the D-line CA up.
It will also bring the C-line CA up.
So, a remaining discrepancy is that the C-line CA result is high.
Much effort has gone into understanding the Coded Aperture image in hopes of reconciling the Cline CA and pinhole scans.
The image modeling has become more mature.
In benchmark runs, CA and pinhole agree far better than in the C-line IBS scan.
The discrepancy is not going to be resolved with further development of the CA image modeling.
At 2.085 GeV,
CA (without diamond), CA (with diamond), and PH (with Diamond)
agree within 0.5 micron.
At 2.300 GeV,
CA (with out diamond), CA (with diamond), and PH (with Diamond)
agree within agree within 4 micron . (CA is low).
The next condition to be modeled will be 2.300 GeV, CA (with Molybdenum filter).
This is a very different image and will be an interesting test of the image understanding.
In the D-line,
At 2.085 GeV,
CA (with diamond), and PH (with Diamond)
agree within 1 micron.
While studying possible sources of the discrepancy between CA and pinhole,
I note two striking issues regarding the IBS results .
1) The C-line CA scan shows
very different behavior at low beam current,
when compared to any other scan.
This may be relevant to the failed effort to reconcile the CA and PH scans.
2) There is significant scatter in the beam size measurements
for low beam current, in all scans.
This is possibly due to real beam instabilities,
or is possibly due to detector issues.
This should be understood before presenting IBS results.
Individual IBS scans for the C-line.
The benchmark runs on the first slide are a subset
of these sets of runs.
The striking abnormality is that the
measured beam size increases with small
beam current.
Understood issue with
intermittent dead pixel.
Run 68: C-line pinhole
Run 69: C-line Coded Aperture
Individual IBS scans for the D-line.
The benchmark runs on the first slide are a subset
of these sets of runs.
Run 4F: D-line Coded Aperture
Run 4B: D-line pinhole
In this case, the D-line CA is high;
this is from using an earlier function with
zero applied sideband.
First problem:
low beam current, C-line Coded Aperture.
at 5ma,
run 25216, 38.245 μm beam size
run 25218, 38.404 μm beam size
high current
at 2ma,
run 25333, 28.229 μm beam size
low beam size
at 1 ma
run 25487, 32.217μm beam size,
normal
at 0.25 ma
run 25637, 35.209 μm beam size
run 25641, 37.102μm beam size
increase beam size at low current
at 0.10 ma
run 25663, 37.934 μm beam size
increase beam size at low current
at 0.10 ma
run 25665, 46.334 μm beam size
very large beam size
Individual IBS scans for the C-line.
Run 69: C-line Coded Aperture
Mike’s results, without any changes
ave-of-fits, latest CA function
remove flaky channels
remove flaky fits
Apparent anomalous behavior of C-line CA at low current.
There are several issues with this scan.
1) Flaky channels
a) Physical Diode 27 went dead, on the average, and may be oscillating,
for a period of runs at ~2 ma beam current. This causes the drop in measured beam size.
b) Physical diode 19 is too high (bad calibration) at low beam current, and gets worse
at lower beam current.
2) Flaky fits. This becomes a problem at low beam current because of statistical errors.
3) (Not a contribution to the apparent rising beam size. )
Beam motion increases at low current.
At high current, the beam motion is about 7 μm at the source, 0.35 pixels.
At the lowest current, beam motion increases to about 50 μm at the source, 2.5 pixels.
Addressing these issues leads to the green circled
measurements on the previous page.
But the C-line CA beam size measurements are still higher than the pinhole measurements.
CA: ~32 μm, pinhole: ~26 μm, with beam current between 1 and 2 ma.
This observed CA/pinhole difference is inconsistent with the dedicated CA/PH comparison.
The beam motion at small beam current is larger than in other scans.
Second problem:
there is significant scatter in the beam size
Run 4B: D-line pinhole
Mike’s results, “average of fits”
Not only is there significant scatter in the beam size,
the beam size is clustered around two values.
For lack of another idea, look at the beam motion.
beam size: sigma (average of fit) and
beam motion = sqrt( σ2(fit of ave) - σ2(ave of fit) )
the scan has ~ 46 measurements, spaced at 10 seconds, 460 seconds or 8 minutes
………
10 sec
1024 turns, 2.5 x 10-3 sec
For the 0.4 ma measurements, something is causing the beam size to vary with the range (23 to 30 μm) .
Individual measurements (1024 turns) appear to have random beam size within the range.
- The period of the change is less than 5 sec.
Is the period of the change as low as 0.01 sec ( i.e. 4x the measurement time of 2.5 x 10-3 sec) ?
If that is the case, we should see the size change during the 1024 turns.
0.4 ma
sigma of image at detector (pixels) vs. turn number
(poorly formatted)
Looked for variation in the image size
in the plot of (image sigma vs turn) for several runs.
This above example is the most promising for showing a change.
(The individual turn images are clean.)
The fitted change in image size over the run is +0.067 pixels,
while the total change in image size is 0.32 pixels (7 μm at the source).
(The green circled run has fitted change in size of -0.017 pixels.)
Naively, this is showing 0.067/0.32 = 21% of the change in one run.
We need much longer runs to verify if we are seeing the beam size change.
0.2 ma,
not 0.4 ma
sigma of image at detector (pixels) vs. turn number
(poorly formatted)
At 0.2 ma, again looked for variation in the image size
in the plot of (image sigma vs turn).
There appears to be
multiple nominal combinations of beam size and motion.
The above example is chosen
for having large motion at the critical beam size
(The individual turn images are clean.)
The fitted change in image size over the run is +0.053 pixels.
Beam position vs. size shows no correlation.
Beam motion vs. position shows no correlation indicating that
the apparent motion between 6 and 18 is not due to a finite pixel.
Conclusions
The up-turn at low beam current
seen only in C-line Coded Aperture
is due to a combination of
flaky channels in the detector and
flaky fits, especially with low signals.
The C-line Coded Aperture is still high compared to pinhole.
This will not be changed with further development of the
fitting function.
However, this scan shows increasing beam motion with
decreasing beam current; the beam is not getting quieter.
Run 69: C-line Coded Aperture
The spread in the beam size is seen at low beam current,
in all scans.
There appears to be two beam size central values,
with changing beam motion within these “states”.
The observed images look normal.
Beam size and beam motion are not correlated with
beam position, which might have suggested
detector calibration or pixel effects.
Beam appears to be unstable at the end of the IBS scan.
Run 4B: D-line pinhole