Outcome of Hierarchy Limit MDs:
Can we Tighten the IR7 Margins?
A. Mereghetti, on behalf of the LHC Collimation Team
07 Nov 2016
A.Mereghetti
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Tightening IR7 Margins
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In IR7, there are two stages of the LHC betatron collimation system
(out of three):
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TCP (primary collimators):
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TCSG (secondary collimators):
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first stage of cleaning (i.e. first to be impacted by beam particles)
smallest settings (Ns);
aimed at catching secondary halo, i.e. particles escaping from TCPs;
second smallest settings (Ns);
TCLA: shower absorbers;
During regular operation, the IR7 collimation system respect a specific
hierarchy and all collimators keep their role – e.g. no TCSG becomes
a primary collimator;
Reducing IR7 margins between TCPs and TCSGs may imply a loss of
the hierarchy;
In RunII, two MD activities were carried out, to spot possible hierarchy
breakages when tightening margins and identify sources/mitigations:
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MD314 (2015, MD2): qualify different TCP-TCSGs retractions (mainly as cleaning
inefficiency), identify breakages and possible mitigations;
MD1447 (2016, MD1): spot actual reasons for breakage;
07 Nov 2016
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MD314 (2015)
Main outcomes:
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Feasibility of 2s-retraction between IR7 TCPs and TCSGs 
operationally deployed in 2016!
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IR7 hierarchy broken on B1V only when 1s-retraction is deployed 
hint of TCSG.D4L7.B1 as responsible for hierarchy breakage, but no
time to verify this hypothesis and to find the actual origin of breakage;
•
Deploying measured beam sizes (instead of nominal ones) in setting
collimator gaps solves the breakage – but it does not help in spotting
its origin!
Broken hierarchy
on B1V!
07 Nov 2016
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MD1447 (2016)
Qualification with betatron LMs of the 2s / 1s retractions;
MD1447
Courtesy of D.Mirarchi
2016 OP
TS1
Comm.
1s
2s
B1H
2.83E-4
2.75E-4
2.2E-4
2.4E-4
B1V
1.95E-4
1.45E-4
1.3E-4
1.3E-4
B2H
2.04E-4
2.84E-4
2.8E-4
2.7E-4
B2V
1.03E-4
2.21E-4
1.7E-4
2.0E-4
2016: TCLAs@11s
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Cleaning inefficiencies at 2s-retraction a
bit worse (~10%) than what seen in
qualification for operation;
B1V: hierarchy breakage;
B2: cleaning inefficiency clearly
decreases when moving from 2s- to 1sretractions;
MD314 (2015)
Broken hierarchy
on B1V!
MD314 (2015)
MD314 (2015)
07 Nov 2016
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Hierarchy Breakage
[email protected]
Hierarchy breakage found on B1V when deploying 1s-retractions (as in 2015)
2015
2016
TCLAs@14s
TCLAs@11s
TCLAs@10s
[email protected]
TCLAs@11s
TSG.D4L7.B1
07 Nov 2016
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Source of Hierarchy Breakage
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Tilt angles lead to (fake) large measured beam sizes 
using measured beam sizes to set collimator gaps allow to
by-pass hierarchy breakage (as seen in 2015);
Identification of collimator(s) at the origin of breakage
(suspect: TCSG.D4L7.B1, from 2015) and possible cures most promising: angle to compensate a possible tank
misalignment;
Lack of time (CO): we inserted the 3 largest tilts and
removed them one by one until breakage is seen again;
PRSTAB 15-051002
A4L7.B1: 500mrad;
D4L7.B1: -350mrad;
A5L7.B1: -300mrad;
After CO correction
(residuals)
07 Nov 2016
A.Mereghetti
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Source of Hierarchy Breakage (II)
•
•
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Tilt angles lead to (fake) large measured beam sizes 
using measured beam sizes to set collimator gaps allow to
by-pass hierarchy breakage (as seen in 2015);
Identification of collimator(s) at the origin of breakage
(suspect: TCSG.D4L7.B1, from 2015) and possible cures most promising: angle to compensate a possible tank
misalignment;
Lack of time (CO): we inserted the 3 largest tilts and
removed them one by one until breakage is seen again;
PRSTAB 15-051002
A4L7.B1: 500mrad;
D4L7.B1: -350mrad;
A5L7.B1: -300mrad;
After CO correction
(residuals)
07 Nov 2016
A.Mereghetti
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Source of Hierarchy Breakage (III)
•
•
•
Tilt angles lead to (fake) large measured beam sizes 
using measured beam sizes to set collimator gaps allow to
by-pass hierarchy breakage (as seen in 2015);
Identification of collimator(s) at the origin of breakage
(suspect: TCSG.D4L7.B1, from 2015) and possible cures most promising: angle to compensate a possible tank
misalignment;
Lack of time (CO): we inserted the 3 largest tilts and
removed them one by one until breakage is seen again;
PRSTAB 15-051002
A4L7.B1: 500mrad;
D4L7.B1: -350mrad;
A5L7.B1: -300mrad;
After CO correction
(residuals)
07 Nov 2016
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Conclusions
Hierarchy limit:
• Hierarchy breakage found on B1V when 1s TCP-TCSG retractions are deployed (as
seen in 2015 )  tilt angle of tank translates in (fake) large beam sizes  if used to
set collimator gaps, these allow to by-pass the hierarchy breakage (as seen in 2015 );
• TCSG.D4L7.B1 at the origin of hierarchy breakage:
• Candidate collimator found in 2015;
• Smallest beam size  operational margins become relevant;
• One of the collimators with largest tilt angle;
• Re-alignment:
• very stable  centres change within 100mm;
• Performed in 1h:10m (i.e. ~20 more than 2015 record);
Tank tilt (i.e. overall tilt of collimator) at the origin of hierarchy breakage:
 1s retraction should be feasible, provided that we take care of tilt angles…
 1.5s retraction not explicitly studied, but should be feasible, without taking care of tilt
angles…
Implications on impedance: see next talk!
07 Nov 2016
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Spare Slides
07 Nov 2016
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Alignment - Centres
Time required: 1h:10m (i.e. ~20m more than 2015 record) – harder scraping conditions (i.e. cross-talk
between beams, B1 tails scraped down to <3s);
Esp: false spikes (possibly due to vibrations of the other jaw)  visible on 100Hz BLM data (not in 1Hz);
Reproducibility within 100mm
07 Nov 2016
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Alignment – Beam Size Ratio
Time required: 1h:10m (i.e. ~20m more than 2015 record) – harder scraping conditions (i.e. cross-talk
between beams, B1 tails scraped down to <3s);
Esp: false spikes (possibly due to vibrations of the other jaw)  visible on 100Hz BLM data (not in 1Hz);
07 Nov 2016
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