# Presentation: 138852

## Summary of OTR measurements taken at the exit of the acceleration tube

- Recycler Weekly Departmental Meeting
- October 3rd, 2007
- L. Prost, A. Shemyakin
- A. Warner, M. Sutherland,  K. Carlson, A. Burov

## GOALS

- Getting ready for YAG measurements
- Fix/improve data acquisition software (*T. Bolshakov*)
- Verify mechanics of the correction procedure
- Understand timing (gate w.r.t. beam pulse)
- Try to understand (better) what we see on the images
- Shape of the beam
- Optics
- Drop of intensity when pepper-pot is inserted
- Beam temperature
- Eventually, we have mostly worked on trying to focus the camera properly and calibrate the image for dimensions (i.e. pixels/mm)

## Prelude

- The camera used for the data presented is different from the one used during commissioning
- CID (now) vs CDD
- Gated (now) vs non-gated
- Slightly different controls
- All the images, extracted profiles,... shown in this presentation are for fixed camera biases
- Screen = 6 kV
- MCP = -700 to -770 V
- Gate/Photo cathode = -170 V
- THE TIMING SETUP WAS NOT PROPER FOR PART OF THE MEASUREMENTS REPORTED HERE
- Beam conditions: 5 kV (C.E. = -5 kV), 2 s pulse, nominal focusing settings

## Gated camera

- The camera we are using now makes it possible to acquire gated images
- Requires more attention to synchronization with beam pulse,...

- Acquisition window

- Gate pulse

- Control electrode pulse

- ‘Real’ gate position w.r.t. beam pulse depends on cable length...

**Beam image believed to be at flat-top of the beam pulse**

## Timing

## ‘2 s – 1 s’ image vs gated image

- ‘2 s – 1 s’ image vs gated image

- ‘Old’ analysis taking the difference of two images with different pulse length is likely inadequate
- Caveat: It is possible that the MCP was saturated

- 2 s – 1s pulse

- 1s Gated

*Good timing data*

- _Note_: this profile is quite different from previously published analyses (i.e. DIPAC’07)

- _Note_: this profile is in somewhat better agreement with previously published analyses (i.e. DIPAC’07)

## Gate scan

- Changed delay of the gate (300 ns width) to scan the whole beam pulse and find ‘real’ flat-top
- The actual delay between the real gate pulse and the one reported on the scope appears to be ~800 ns

- First visible image

- First visible image + 300 ns

- First visible image + 600 ns

- First visible image + 900 ns

- First visible image + 1.2 s

- First visible image + 1.5 s

- There is no real flat-top !!

*Good timing**data*

## Dependence on the gate width

- The total light collected is not proportional to the gate width
- Even though the *image* is not saturated it is possible that the MCP was in a non-linear regime
- Timing is also an issue, especially since the ‘flat-top’ is very small

- ~×2.5

- Gate ratio: 5

**?**

- 100 ns

- 20 ns

*Wrong timing data*

## Comments on timing

- From the gate scan and the very obvious difference between subtracting two images of different beam pulse lengths and the gated image of the equivalent width, we conclude:
- A gated camera is a requirement for any analysis of the beam optics in a pulse mode
- Because of the very short effective flat top, time jitter plays an important role in the quality of the data
- The importance of time jitter should decrease as we go to shorter gate width

## Calibration (dimensions)

## Calibration of the dimension

- We have relied on known displacements of moving parts in the set up (OTR screen, 2-mm hole) and their corresponding images in order to try to calibrate distances
- No fiducial available at this time
-  Different issues for X (horizontal) and Y (vertical) dimensions
- No moving parts horizontally
- Vertical expansion because of the angle between the camera and the OTR screen not quite 45
- Off-line scaling (rough): 14-15 pixels/mm
- Distortion also appears to be close to nominal at ~1.2
- By recording the displacement of the 2-mm hole while moving the OTR screen (vertical direction) we find a calibration of 12 pixels/mm
- This resolution is quite poor and significantly less than with the previous camera (20 or even 40 pixels/mm, depending on the lens)

## Estimation of distortion

- If one assumes no overall convergence/divergence angle in the beam, one can use the pepper-pot image to estimate dimensions at the OTR screen
- The pitch is 0.8 mm

- 1

- 2

- 3

- 0.8 mm

**11 pixels/mm in X-direction**
**13.5 pixels/mm in Y-direction**
_**Ratio: 1.2**_

- Note the poor resolution of the pepper-pot image

## Estimation of distortion (cont’)

- Based on the theoretical geometry
- Angle between OTR screen and camera 38.9
**Distortion ( i.e. expansion/compression ratio) is ****1.2**
**Consistent with result from pepper-pot estimation ****but...**
**... ****looking at the angle (*****over 4 hole images in this case*****) that the pepper-pot holes make as a function of the upstream lens current, we find a non-constant ratio !?**

## Focusing limitations

## Focusing depth limitation

- Properly focusing the camera is essential to resolve temperature but also to resolve sharp edges (i.e. beam boundaries)

- If we consider that what limits the resolution is when the ‘blur’ is larger than the pixel size (~ 0.07 mm), we should not be out of focus by more than  ~ 0.7 mm

- D

- L

- 

- 

- With D = 35 mm, L = 350 mm    0.1 ‘blur’

*Object*

*Image*

## Focusing depth limitation (cont’)

- With current settings, we cannot focus the whole beam properly
- Issue for determining the edge of the beam
- Issue for pepper-pot images

****** ****= 5 mm ******** ******** = 0.5 mm**

- 45

- 10 mm

- 10 mm

- Focus

- 

- At the edge of the beam the blur is larger than the diameter of the pepper-pot hole images (~0.35 mm)

## Illustration of focusing flaws with pepper-pot images

- Moving the beam with upstream corrector results in much sharper pepper-pot images (than for default values)
- i.e. better focusing

- Nominal corrector settings

- Some ‘*better*’ settings

- Between the two images, the beam moved by several mm onto the OTR screen

- Out of focus area

*Good timing data*

## Attempts to focus the camera have been quite unsuccessful

- Tried to move the OTR screen while keeping the image at the same position with the whole beam
- Quite inconclusive as for which profile is sharper

*Wrong timing data*

## Attempts to focus the camera have been quite unsuccessful (II)

- Using pepper-pot images is not very conclusive either

- OTR = 67 mm

- OTR = 63 mm

- OTR = 61 mm

- Best focusing ?

*Wrong timing data*

## Comments on focusing

- We also tried
- To use the edge of the pepper-pot screen as a reference of a sharp object
- The boundary looks very odd and not sharp at all in all cases
- To use an external light source and used the camera in the un-gated mode
- We failed to record an image
- Focus on wires which are in front of the OTR screen
- We could only see phantoms

## Other effects/anomalies

## Collected intensity depends on the location of the beam on the OTR screen (vertically)

- Because the light generated at the OTR is directional, the total light captured by the camera varies as a function of the vertical position of the beam

- G. Kazakevich

- Should not be a problem for YAG measurements

- 2005 data with old camera system (un-gated)

## September 2007 data – Same observations

- Pulse to pulse integrated light varies by ~ < 1% (20 pulses sample)

- When the beam is moved with upstream correctors the integrated light varies by several percent (*decrease* by 6% here)

*Good timing data*

## Time and/or position jitter

- Consecutive images show signs of beam motion and/or time jitter

**‘****Differential’ images**

- The crescents indicate beam motion (or jitter) between one image and the reference

**Profiles across the crescents (positive and negative sides) **

- Zero

- Motion: ~12-18 pixels with ~12 pixels/mm (*i.e. 1-1.2 mm*)

- 12 pixels

*Good timing data*

## Intensity drops when inserting pepper-pot and 2-mm hole (sometimes)

- Difference image between the whole beam and the 2-mm hole scraped beam

- Nominal corrector settings

- ‘Better’ settings

- Focusing (or lack thereof) does not seem enough to explain the pepper-pot image

*Good timing data*

## Other anomaly

*Aliasing*
- Probably due to wrong timing setup
- Only affects Sep 27 & 28 data

*Wrong timing data*

## Look out

- Some of the measurements (Sep 27 & 28) were made with the wrong timing configuration
- Increased time jitter
- Makes focusing that much harder
- May explain problems with un-gated acquisition
- Un-gated acquisition plus external light source could be a better way to focus the camera
- Probable cause for aliasing

## Prospective for YAG measurements

- Goal of YAG measurements:
- Resolve and eliminate ellipticity of the electron beam at the exit of the cooling section
- We want to resolve e = 1 – (minor radius/major radius) < 0.1
- OTR measurements provide
- Tests of the various procedures to acquire data
- Resolution information
- Any blur will decrease the achievable resolution for e
- Timing, focusing, calibration pix/mm for X/Y

## Conclusion

- So far OTR measurements are unsatisfactory but are serving their purpose
- Did not firm up the method(s) by which to ensure that focusing and camera alignment are adequate for quality images
-  Our struggles have been beneficial for the general understanding of the way measurements should be taken and issues to be aware of
- With proper timing settings and the gained experience, we should be able to improve upon some of our issues
- Lack of focusing depth is not one of them and would require a smaller camera aperture
- Enough light ?
- Java data acquisition program is working (*Timofei*)
- Just need some tweaks for the image analysis part

## Conclusion (II)

- Even with the timing issue being resolved...
- We do not see a good explanation of why the pepper-pot image intensity is so low
-  It looks like this is not the case for the 2-mm hole, with proper focusing
- It is not clear that we can distinguish between camera misalignment, bad focusing and *actual* beam temperature
- In addition, with the current setup, the camera resolution is too poor anyway
- We will revisit once more all the issues presented and hopefully come to a point where we are confident that YAG measurements will be successful