simulation of a large beam spot in a 15cm cryotarget cell d. mack 1/26/15
TRANSCRIPT
Simulation of a Large Beam Spot in a 15cm Cryotarget Cell
D. Mack1/26/15
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https://userweb.jlab.org/~kalyan/meetings/HallA_Collab/halla_gmp_2015.pdf
Several shocks here: • Evidence of beam scraping on entrance tube• Unusually large intrinsic beam rms size (0.6-
0.75mm)
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Finding a Well-Posed Question
What the hell is going on?! We’re a national lab with over 2 decades of experience. We’re better than this.
• Does the new machine have a larger halo?• Are we more sensitive to halo because of poor Signal/Bkg conditions
(i.e., small elastic xsects at high Q2 while running two spectrometers at multi- GeV eloss)?
• Was the target alignment out of spec?• Has the target geometry been modified? No.
I’ll settle for an answer to the question:
“Given the observed large intrinsic spot size, a 2mmx2mm raster, and hopefully reasonable misalignments,
what fraction of the primary beam will hit the beam entrance tube used in Fall 2014?”
(This is the best-case scenario. Plots for 3mmx3mm FR are in the backups. Poorly constrained halo models will also make the interception worse.)
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D. Meekins, Gmp meeting, Oct 2013
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Entrance Tube Dimensions
4cm cell (note longer entrance tube)
15cm cell that was used in experiments
Inner Radius = id/2 = 8.5mmLength = 10cm
Alternative 15cm cell Inner Radius = 0.505”/2 6.4mm
Length = 10cm
Drawing numbers in backup slides
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Simulation Ingredients
Beam parameter Numerical ValueIntrinsic beam spot size (Xrms,Yrms) (0.8mm,0.8mm)
Fat beam spot. The point of this sim.
Fast Raster FRX, FRY(flat, non-diverging distribution)
(+-1mm,+-1mm)
Position of Beam rel. TgtXoffset, Yoffset
(0.mm,0.mm) up to (2mm,2mm)
Pitch of Tgt rel. Beam Pitch: 0 degrees to 1.43 degrees (2.5mm over the 10cm
entrance tube from vacuum/cool-down)Warning: pivot is about DS window
Yaw, roll of Tgt rel. Beam Yaw: ignored as redundantRoll: irrelevant by symmetry if ellipticity of
apertures is ignored
• Generate 5x10**7 electrons. (Do book-keeping to 0.1 ppm. But plot at most 500 good electrons and 500 fouls. )
• The gaussian random distribution for spot size uses the Box-Muller approximation (good out to 6.6σ, more than sufficient to look for ppm level interception)
• Fast determination of fouls is done by checking radial distance of electron to centers of US/DS apertures. (this ignores a few microns of ellipticity when the tube is pitched 3 degrees. Note cos 3° = 0.9986 )
Not explored: flow diverters, 4cm cell (has a longer entrance tube), Silviu’s cell (smaller radius).
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The following simulations have no halo so represent best-case scenarios.
The units on all the Excel plots are mm.
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No Misalignments – No Worries
-10 -8 -6 -4 -2 0 2 4 6 8 10
-10
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0
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Ytgt vs XtgtStd FR, spot; no offsets tube walltube wall Alt.
0.8mm,0.8mm spot. 2mmx2mm raster
Interception was undetectable
(below the 0.1 ppm sensitivity of my
simulation)
It was clear at this point I was going to have to be a little aggressive about misalignments.
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Spot Size Has a Major Effect Though(left hand plot – same as previous slide
right hand plot – formerly 0.8mm rms spot size turned off)
-10 -8 -6 -4 -2 0 2 4 6 8 10
-10
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0
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Ytgt vs XtgtStd FR, spot; no offsets tube walltube wall Alt.
-10 -8 -6 -4 -2 0 2 4 6 8 10
-10
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0
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Ytgt vs XtgtStd FR, no spot or offsetstube walltube wall Alt.
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Add a Significant Position Misalignment… and still no problem
-10 -8 -6 -4 -2 0 2 4 6 8 10
-10
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0
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Ytgt vs XtgtStd FR, spot; offset 2,-2mmtube walltube wall Alt.
0.8mm,0.8mm spot. 2mmx2mm rasterOffset 2mm,-2mm
I thought this was fairly aggressive but
credible.
Interception was still undetectable
(below the 0.1 ppm sensitivity of my
simulation)
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Add a Significant Pitchfinally get fouls
-10 -8 -6 -4 -2 0 2 4 6 8 10
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0
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Ytgt vs XtgtStd FR, spot; offset 2,-2mm tube wall UStube wall DS fouls
0.8mm,0.8mm spot. 2mmx2mm rasterOffset 2mm,-2mm
Pitch 2.5mm over 10cm (1.4°)
This pitch is a little extreme. It is certainly conspiratorial as far as the direction I chose.
Interception was 114 ppm.
Probably enough to ruin a small xsect measurement in a spectrometer set to several
GeV of eloss. Window dummy subtraction starts to
become meaningless.
fouls
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One Way to Fix – Reduce RMS Spot Size to 0.2mm,0.2mm
-10 -8 -6 -4 -2 0 2 4 6 8 10
-10
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0
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Ytgt vs XtgtStd FR, spot; offset 2,-2mm tube wall UStube wall DS fouls
-10 -8 -6 -4 -2 0 2 4 6 8 10
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0
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Ytgt vs XtgtStd FR, spot 0.2mm; offset 2,-2mmtube wall UStube wall DS
Clean transportfouls
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Scraping vs Spot Size
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90
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40
60
80
100
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Scraping vs Spot SizeX,Y = (2mm,-2mm)
RMS spot size (mm)
Scra
ping
(ppm
)
Very sensitive in this example: scraping dropped by a factor of 5-10 for every 0.1mm reduction in spot size.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.0001
0.001
0.01
0.1
1
10
100
1000
Scraping vs Spot SizeX,Y = (2mm,-2mm)
RMS spot size (mm)
Scra
ping
(ppm
)?
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Note: With a Small Spot, There’d Even Be Room for a 4mmx4mm Raster!
-10 -8 -6 -4 -2 0 2 4 6 8 10
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0
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Ytgt vs XtgtStd FR, spot; offset 2,-2mm tube wall UStube wall DS fouls
-10 -8 -6 -4 -2 0 2 4 6 8 10
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0
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Ytgt vs XtgtFR 4x4mm, spot 0.2mm; offset 2,-2mmtube UStube DS
fouls
Clean transport
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Another Way to Fix – Move Target or Beam (if you can figure out which direction)
-10 -8 -6 -4 -2 0 2 4 6 8 10
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Ytgt vs XtgtStd FR, spot; offset 2,-2mm tube wall UStube wall DS fouls
-10 -8 -6 -4 -2 0 2 4 6 8 10
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0
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Ytgt vs XtgtStd FR, spot; offset 2,2mm tube wall UStube wall DS
Clean transport
fouls
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Scraping vs Position
-2.5 -2 -1.5 -1 -0.5 0 0.50.0001
0.001
0.01
0.1
1
10
100
1000
Scraping vs Y Positionrms spot size 0.8mm,0.8mm
Y Position (mm)
Scra
ping
(ppm
)
-2.5 -2 -1.5 -1 -0.5 0 0.50
20
40
60
80
100
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Scraping vs Y Positionrms spot size 0.8mm,0.8mm
Y Position (mm)
Scra
ping
(ppm
)
?
Relative to the misaligned scraping scenario on the previous slide: • The scraping dropped by a factor of 5-10 for every 0.5mm shift in Y position.
• Shifting beam by 1.5mm had same reduction in scraping as reducing spot size 0.8mm0.5mm.
• Shifting beam in the “bad” direction by 0.5mm yielded a factor of 6.5 increase in scraping, to
737 ppm. So one could take small steps to figure out the non-scraping direction. Warning: the extra cryo heat lead may become noticeable (100+Watts).
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Conclusions• Beam spots larger than say 0.5mm rms are bad. Combining the observed large spot with
slightly pessimistic misalignments can lead to scraping.
• A small spot leads to a bkg situation which is much less sensitive to misalignments or FR size.
• The id of the existing 15 cm cell entrance tube looks fine. (But if all 12 GeV beam spots are > 0.5mm, revisit this.)
• The spot size seems to be more critical than minor errors in the FR size. (A gaussian has long tails.)
• Halo could have played a role in Fall 2014 scraping, but a big spot and misalignments were sufficient.
Recommendations 1. Continue to specify small beam spot sizes, say 0.2mm rms. A much larger spot size is
potential trouble. Scraping is extremely sensitive to beam spot size though, so a 0.5mm spot may be orders of magnitude better than a 0.8mm spot.
2. Check whether cryotarget alignment specs allow for conspiracy in position and angle, and if we are achieving the specified tolerances (look at the burn scar on the DS window?).
3. When the spot size cannot be reduced, try steering out of the scraping problem. In one example, we saw that it took 1.5mm of beam steering (or target elevation adjustment) to cure the problem.
Not explored: flow diverters, 4cm cell (has a longer entrance tube), Silviu’s cell (smaller radius).These will probably lead to tighter alignment specifications.
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Acknowledgements
Kallan for discussions and slides
Greg and Silviu for dimensions and drawings
Jay for discusssions
Dave Meekins and JP Chen for feedback on alignment
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The id for the standard entrance tube comes from Greg Smith starting with this drawing and tracking down the sub-drawings.
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The id for Silviu’s entrance tube comes from drawing 67153-56024
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Subsequent slides are FR 3mmx3mm
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Left – no misalignmentsRight – spot size off
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Ytgt vs XtgtFR 3mm, Std spot; no offsetstube walltube wall Alt.
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Ytgt vs XtgtFR 3mm, no spot or offsetstube walltube wall Alt.
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Left – misaligned by +2,-2mmRight – and pitched by 2.5mm
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Ytgt vs XtgtStd spot, FR 3mm; offset 2,-2mmtube walltube wall Alt.
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Ytgt vs Xtgtstd spot, FR3mm; -2,-2mm offset tube wall UStube wall DS fouls
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Left – offset and pitched as beforeupper Right – 0.2mm spot
lower right – move offset to 2,+2mm
-10 -8 -6 -4 -2 0 2 4 6 8 10
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Ytgt vs Xtgtstd spot, FR3mm; -2,-2mm offset tube wall UStube wall DS fouls
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Ytgt vs XtgtFR 3mm, 0.2mm spot; offset 2,-2mmtube wall US
-10 -8 -6 -4 -2 0 2 4 6 8 10
-1.00E+01
-8.00E+00
-6.00E+00
-4.00E+00
-2.00E+00
0.00E+00
2.00E+00
4.00E+00
6.00E+00
8.00E+00
1.00E+01
Ytgt vs XtgtFR 3mm, Std spot; offset 2,2mmtube wall US
Interception was 746 ppm.Probably enough to see O(100) Watts on tgt heat load.
As before, there are no fouls when the spot is reduced to
0.2mm, or the beam is steered 4mm.