Simulation of a Large Beam Spot in a 15cm Cryotarget Cell

D. Mack1/20/15

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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?

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?”

(This is the best-case scenario. Unconstrained halo models will make the interception worse and can be tailored to give you any answer you want.)

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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 number 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)

(+-2mm,+-2mm)

Position of Beam rel. TgtXoffset, Yoffset

(0.mm,0.mm) up to (2mm,2mm)

Pitch of Tgt rel. Beam Pitch: 0 degrees to 3 degrees (a barely noticeable 5mm over the 10cm long

entrance tube)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 units on all the following Excel plots are mm.

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No Misalignments – No Worries

-10 -8 -6 -4 -2 0 2 4 6 8 10

-10

-8

-6

-4

-2

0

2

4

6

8

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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 slideright hand plot - spot size turned off)

-10 -8 -6 -4 -2 0 2 4 6 8 10

-10

-8

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-4

-2

0

2

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8

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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

-8

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-2

0

2

4

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8

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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

-8

-6

-4

-2

0

2

4

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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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-8

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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 3 degrees

Whether this pitch is credible may be questioned.

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.

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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

-8

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-4

-2

0

2

4

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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

-10

-8

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-2

0

2

4

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Ytgt vs XtgtStd FR, spot 0.2mm; offset 2,-2mmtube wall UStube wall DS

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One Way to Fix – Reduce RMS Spot Size to 0.2mm,0.2mm

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

20

40

60

80

100

120

Scraping vs Spot SizeX,Y = (2mm,-2mm)

RMS spot size (mm)

Scra

ping

(ppm

)

The scraping dropped by 5-10 for every symmetric 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

-10

-8

-6

-4

-2

0

2

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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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-8

-6

-4

-2

0

2

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Ytgt vs XtgtFR 4x4mm, spot 0.2mm; offset 2,-2mmtube UStube DS

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Another Way to Fix – Move Target or Beam (if you can figure out which way)

-10 -8 -6 -4 -2 0 2 4 6 8 10

-10

-8

-6

-4

-2

0

2

4

6

8

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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

-10

-8

-6

-4

-2

0

2

4

6

8

10

Ytgt vs XtgtStd FR, spot; offset 2,2mm tube wall UStube wall DS

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Another Way to Fix – Move Target or Beam (if you can figure out which direction)

-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)Sc

rapi

ng (p

pm)

-2.5 -2 -1.5 -1 -0.5 0 0.50

20

40

60

80

100

120

Scraping vs Y Positionrms spot size 0.8mm,0.8mm

Y Position (mm)

Scra

ping

(ppm

)

?

The scraping dropped by 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 “good” direction. Note that the extra cryo heat lead may become noticeable (100+Watts).

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Conclusions:Big spots are bad. Combining the observed large spot with significant misalignments can lead to scraping (100 ppm level).

A small spot leads to a bkg situation which is much less sensitive to misalignments or FR setting errors.

The spot size seems to be more critical than the FR size. (A gaussian has long tails.)

Halo may have played a role in Fall 2014 scraping, but a big spot and misalignments were sufficient.

Recommendations 1. We’d like spot sizes down to 0.2mm rms. A large spot size is potential trouble. Even reducing the spot size from 0.8mm0.5mm could lead to a huge reduction in scraping.

2. Revisit the specs for cryotarget alignment: i. what is the spec, does it allow for conspiracy in position and angle, and are we

achieving the specified tolerances? (check the burn scar on the DS window?)ii. If 0.2mm spots are no longer robustly achievable day to day, tighten tolerances

3. If the spot size cannot be reduced, try steering out of it. IF the problem is due to misalignments, in this example it took 1.5mm of beam steering (or

target elevation adjustment) in the right direction 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

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