2. Possible causes for SPS ZS2 misalignment
B. Balhan
12/09/2017

3. Outline Introduction ZS Alignment ZS misalignment Possible causes
Design flaws
Beam loss response
Spark rate
Proposed improvement
08/03/2017
Slawg#15

4. Introduction 5 ZS (Electrostatic septa ) installed in SPS LSS2
3 m long, wire 60 µm for ZS1 and ZS2, 100 µm for ZS3 4 and 5
Nominal gap 20 mm, nominal voltage 220 kV
Remote adjustment of upstream and downstream anode position +/- 2mm.
12/09/2017
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5. ZS individual alignment in laboratory
In order to avoid alignment process in the SPS, all ZS are aligned in the lab using a “ ZS alignment jig” to copy all references, all ZS are therefore identical.
Feet position and spacing
Up/Down Tilts
Up/Down alignment targets
Anode positioning with Electrical and Mechanical references
Cathode gap with Electrical and Mechanical references
ZS are placed on 3 spherical Feet, keeping the positioning.
12/09/2017
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6. SPS alignment LSS2
Since all ZS are identical, once the girder feet are correctly aligned, ZS can be removed and replaced without need for an alignment campaign in the ring.
Tunnel alignment sequence:
The girder is placed at nominal position
Quadrupoles ( ) targets are used as references.
Tilts are checked and adjusted if required (radial & longitudinal).
Vertical and Radial positions are checked and adjusted if required.
12/09/2017
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7. ZS2 installation during YETS 2016
ZS2 has been changed using an ZS#17 previously installed in LSS6 till 2004.
ZS#17 history:
Assembled, aligned and tested in 1995,
Installed in LSS6 from 1998 to 2004
Replaced HV components (cathode, deflectors) in 2009, no alignment check done.
12/09/2017
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8. ZS 2 misalignment
During run 2016 setting up (19/04/2016), beam alignment sequence found misalignment of the ZS 2 Anode, after investigation we decided to adjust tank positioning (feet installed on the girder) to retrieve correct anode position.
~2 mm
The cathode move accordingly and therefore reduced the gap to 18 mm, belatedly discovered after 1 year of operation, explaining poor loss profile.
More info; Studies on slow extraction losses in LSS2 L.Stoel MSWG meeting 2017 #11
12/09/2017
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9. Possible causes of misalignment
The laboratory alignment is done at atmospheric pressure,
→ no vacuum force to compensate for mechanical play.
Mitigation for future: difference should be measured using a view port.
Possible causes:
Initial misalignment in1995,
Mechanical play in the motorisation system,
Isolated shaft/anode connection damaged by initial 300°C bake-out.
12/09/2017
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10. Known design flaws Al2O3 ring damaged by 300 °C bake out,
→ replaced on LIU version, not baked anymore.
To much mechanical play ( ~ 0 .1 mm),
→ improved on LIU version
12/09/2017
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11. Beam loss response Beam loss profile came back to expected levels.
Actual wire positioning and displacement response seems to be normal.
BLM during ZS2 Upstream anode scan
BLM during ZS2 downstream anode scan
BLM during ZS2 Parallel anode scan
BLM during ZS2 Angular anode scan
12/09/2017
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12. Spark rate
2015
2016
2017
Cathode at 18 mm
The normal Z2 spark rate is normally very low,
ZS2 sparked a lot during 2015 run and early 2016, but improved since cathode realignment.
Spark rate reduced since gap opening, but not as low as before: the cathode oxidation layer may have been damaged by losses during ‘misaligned period’.
ZS2 now also has a higher spark rate with high intensity LHC beam (observed during “Hiradmat” run) → Observation indicate the ZS2 is not in perfect shape.
12/09/2017
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13. Proposed mitigation to prevent misalignment in future
Mitigation for future: difference should be measured using a view port.
No high temperature bake out of ZS anymore.
Tentatively: add external measurement of anode position.
12/09/2017
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14. Possible displacement precision improvement
For 2016 scans, the anode accuracy has been improved from 100 µm to 50 µm (software setting adjusted). Bear in mind the wires thickness is 60 µm & 100 µm.
An estimation has been done by Ch. Boucly to improve acquisition and control of these displacements ~ 100 kCHF. It comprises of:
New cabling required
New control (software + hardware DAC/ADC 16 bit)
Use motorisation in servo mode instead positioning?
Test to be scheduled in 867 to validate the estimated accuracy.
12/09/2017
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15. What to do next?
ZS#17 misalignment origin can only be confirmed after ZS removal and alignment check in 867 (cool down period!).
Mechanical modifications implemented to reduce problems with LIU type ZSs.
Position accuracy improved to 50 μm, not an issue for 2017 and 2018 operation, but should we improve further?
ZS2 position satisfactory for Diluter MD in 2018 (see slide 10).
Spark rate acceptable, although higher than usual.
12/09/2017
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