R. Bruce, 2014.06.13

Today’s meeting

• ATS optics proposed for use in 2015 (see LBOC, Evian)

• Several differences compared to nominal optics

– Checks needed!

• From collimation side

– Check protection in case of fast failures (this talk)

– Check cleaning (next talk by Daniele)

Work in progess:

TCT impacts during fast failures in 2015 ATS optics

R. Bruce, S. Redaelli

Acknowledgement: C. Bracco, S. Fartoukh, B. Goddard, L. Lari

R. Bruce, 2014.06.13 2

R. Bruce, 2014.06.13

Outline

• Introduction

– Calculation of collimation hierarchy margins

– Fast beam failures

• Simulations of beam dump failures in ATS 2015 and nominal optics

– Comparison with TCT damage limit

– Variations in energy spread, emittance, tail population, phase advance

• Summary

R. Bruce, 2014.06.13

Collimation hierarchy

• Limit on β*

– IR6 dump protection must protect TCTs

– TCTs must protect triplet aperture

• Must ensure sufficient margins so that we are protected also in case of orbit or optics drifts

TCPTCS7 Protecte

d aperture

TCS6/TCDQ

TCTTCLA7

Secondaryhalo

Pri

mary

halo

Tertiaryhalo

Kicked beam

R. Bruce, 2014.06.13

What are we afraid of?

• Accident scenario: beam dump failure

– LHC filling scheme has “hole” – abort gap – to allow rise of the 15 dump kickers (MKD) from zero to full field - standard dump. Errors possible:

• Asynchronous dump: all dump kickers firing simultaneously at the wrong moment

• Single-module pre-fire: one module fires, followed by re-trigger of others

– These failures could give

Abort gap

intermediate kicks to some bunches and send beam directly onto sensitive equipment (TCTs / aperture)

R. Bruce, 2014.06.13

Received kicks

• Waveform of kick in urad from one MKD provided by B. Goddard

• Asynchronous dump: total kick is sum of 15 kickers

• Single module pre-fire: Re-triggering time (650 +50 N) ns, N integer distance from pre-firing kicker

• Below certain kick amplitude, nothing is hit

• Above certain kick amplitude, everything intercepted by TCDQ

• Single module pre-fire more dangerous: stay longer time at small kicks => more bunches potentially affected

Dangerous “window”

zoom

R. Bruce, 2014.06.13

Calculation of hierarchy margins

• Margins calculated using square sum of errors except van der Meer scans (see Evian 2011 and Chamonix 2012).

• Components of critical margins: orbit, optics errors, lumi scans, positioning errors and setup errors

• Philosophy: Margins should be respected more than 99% of time => risk of damage < 1 in ~300 years for TCTs, less than 30000 years for triplet (see Evian 2010-2011).

• Underlying assumption: 1 asynchronous dump per year expected

8

What Can Happen?

Error case:

1. We need an asynchronous dump ore one module pre-trigger while we are at low beta* (probability 10-7 per second).

2. We need to be out of orbit tolerance from IR6 to a TCT in one IR (probability 10-2).

3. We need to be at maximum beta beat error from IR6 to a TCT in one IR (probability 10-2).

4. Both errors must point in the same bad direction (probability 0.25).

Then one TCT is at risk for damage (benign damage). Still very unlikely, due to phase advance conditions that must be met.

5. The TCT is out of tolerance with respect to triplet aperture (probability 10-2).

6. We are fully squeezed, still separated (aperture assumption).

7. Beams have additional beam-beam offset reserved for van-der Meer scan (possible?).

Then the triplet aperture can be hit by fraction of a bunch, if conditions for TCT hit

(see above) are met.2012.03.09

LMC 2012

More likely 2015

R. Bruce, 2014.06.13

Underlying assumptions

• Neglecting out-scattering, assuming also TCSG IR6 “black absorber”. Optimistic!

• Assumed in the calculation of margins: we are protected if the effective settings in σ provides “shadowing”. Example:

• Phase advance from dump kicker to IR6 protection is 90 deg

• Phase advance between protection device and sensitive equipment: n*180 deg.

– I.e. TCTs also at 90 deg from dump kicker. Worst case!

– Pessimistic assumption – TCTs often at a more favorable phase advance, which gives additional margin

R. Bruce, 2014.06.13

Phase advances

• Fractional phase advances from MKD to TCTs in different optics:

• Preferable: phase advance is >40 deg away from 90 or 270 deg

• B2 IR5 most critical, since no cleaning insertion in between

(deg) 7TeV nominal, 55 cm ATS 2015, 55cm

Beam1

TCTH.4L1.B1 56 124

TCTH.4L2.B1 257 57

TCTH.4L5.B1 47 40

TCTH.4L8.B1 336 160

Beam2

TCTH.4R1.B2 198 106

TCTH.4R2.B2 170 137

TCTH.4R5.B2 176 77

TCTH.4R8.B2 19 170

R. Bruce, 2014.06.13

Simulations of ATS optics

• With ATS: No more additional margin from phase advance

• New SixTrack version available (L. Lari et al, IPAC2013,IPAC2014)

– SixTrack simulates dump failures with full collimation system in place, including scattering, and realistic bunch distribution

– Independent check of TCT impacts possible

• Could conclude on suitable margins based on damage onset

R. Bruce, 2014.06.13

Simulation setup

• Simulation setup

– 6.5 TeV post-LS1

– Single module pre-fire, MKD.A5R6 fires (most downstream kicker)

– Collimator settings: 2 σ retraction (more pessimistic than mm kept)

– Optics: Nominal 55cm and ATS 2015 55cm, B1 and B2

– Separate simulation for each bunch with 25 ns spacing, different kicks. Post-processing: sum all bunches

– Keeping perfect machine, but simulating scan over TCT settings to see what retractions are acceptable

– Gaussian beam with 3.5 um emittance, energy spread 1.1e-4

R. Bruce, 2014.06.13

Example result ATS 2015

• Losses around ring B2 with perfect machine, normalizing to 1.5e11 p/bunch

• Summed over bunches that give any contribution at TCTs.

• Other bunches (going only on TCDQ or in dump line) not included

• TCT retraction important…

TCTs @ 10.3 σ

TCTs @ 6.9 σ

Beam

Beam

R. Bruce, 2014.06.13

Losses vs bunch

• As expected, only about 13 bunches (kick angles) out of 142 induce TCT losses. Simulating only these bunches to save CPU time

TCTs @ 10.3 σ

TCTs @ 10.3 σ

TCTs @ 6.9 σ

TCTs @ 6.9 σ

R. Bruce, 2014.06.13

Losses at TCTs vs retraction

• Summing all bunches, TCTs in IR1/5, both beams, ATS + nominal optics

• Compare with damage limits (A. Bertarelli, MPP workshop 2013)

– Plastic deformation: 5e9, Fragment ejection 2e10, 5th axis unusable 1e11. Under study by MME

R. Bruce, 2014.06.13

Remarks on results

• Constant “background” on IR5 TCT also at larger openings

• Losses with ATS start rising when passing the TCDQ level, and rise more rapidly after passing TCSG level

– Consistent with direct shadowing assumption and phase close to 90 deg

– See effect of out-scattering from TCSG

• Significantly higher losses on TCTs with ATS optics than nominal

– E.g. with TCT at the level of IR6 collimators, factor 20-30 more losses

• Reaching plastic deformation at 7.5σ for worst TCT in nominal optics and at 8.5σ with ATS. Fragment ejection reached at 8.0σ and 7.0 σ respectively

=> 1 σ difference

• Next step: how likely is each TCT setting, including errors?

R. Bruce, 2014.06.13

Errors

• As when calculating margins: look at drifts from orbit and b-beat

• Expect about 2 σ total error for a 99% confidence level => TCT could go as far in as 8.7 σ with these settings (about TCDQ level)

• With ATS optics, still just below plastic deformation, no ejection

Orbit in μm Survival functionof orbit in σ

R. Bruce, 2014.06.13

Variations in simulation

• The simulation result could be sensitive to several parameters – investigating through simulations of worst case (ATS B2)

– Decreased transverse emittance 3.5 um -> 1.7 um

– Increased energy spread 1e-4 -> 3e-4

– Error on TCSG-TCDQ retraction (increasing 0.5 σ -> 1.5 σ)

– Non-Gaussian tails from measured profile (see S. Redaelli, IPAC 13 or F. Burkart’s master thesis)

R. Bruce, 2014.06.13

Variations in simulation

• Assuming that β-beat and orbit error is absorbed into margin (can calculate a maximum TCT error as for hierarchy margins)

• Remaining optics error on phase need to be checked

• Sampling 1000 random machines with MQ errors to mimic realistic residual β-beat

• Resulting in up to 10 degrees phase difference MKD->TCT5

• Simulating optics with best (smallest) and worst (largest) phase advance

β-beatCompare 2012: RMS 0.02Peak 0.07±0.04

Phase MKD->TCT5

R. Bruce, 2014.06.13

Simulated variations

• ATS 2015 optics, B2 with above variations

R. Bruce, 2014.06.13

Simulated variations

• With linear scale…

R. Bruce, 2014.06.13

Observations

• Significant impact of phase advance in both directions

– ~0.5 σ difference between worst and best optics

– Possibilities to decrease phase advance MKD->TCT5?

• Over-populated tails -> worse, smaller emittance -> better

• Energy spread : not important

• TCDQ retraction: important for the a priori “safe” case, within margin

– Watch out with TCDQ tilt error – could have similar effect!

R. Bruce, 2014.06.13

Summary and outlook (1)

• ATS optics more challenging in terms of load on TCTs

– About 1 σ difference in where damage limit is crossed

– Would mean about 4-9 cm in β* in the range between 40cm and 80cm

– Similar issues for HL-LHC, but then we might have more robust TCTs

• With 99% margin, simulations suggest we are still safe, but very close to limit of plastic deformation.

– Only fraction of σ margin to fragment ejection

– Consistent with previous simple model of shadowing

– Very sensitive to phase advance. Can we change it?

– Uncertainties from emittance, tails etc

– Error margin on simulation result and on damage levels???

R. Bruce, 2014.06.13

Summary and outlook (2)

• Remember: 5th axis might not be usable (effort ongoing)

• Extremely important to verify early on in 2015 assumptions on orbit and β-beat. Underestimation of errors could be critical.

• What if we have more than 1 asynchronous dump per year?

– If we have 10 dumps, probability serious event goes from 1 in 300 years to 1 in 30 years

– Increase confidence interval from 99% to 99.9% and increase margin by ~0.3 σ ? But how do we extrapolate the tail of the distribution to 2015?

• Should probably calculate margins TCDQ->TCT, not TCSG->TCT

• Extra margins needed? To be discussed…

R. Bruce, 2014.06.13

Summary and outlook (3)

• Possible future studies:

– Update SixTrack with latest additions to scattering routine and repeat

– Scan over imperfect machines including both orbit and optics errors could provide additional cross-check.

– More systematic check of bunch timing wrt dump kicker

– Combined failures: What if magnets or RF etc fail and significantly perturb the machine before dump failure?

– Similar scans of triplet losses to assess margin TCT-triplet

• Note: Ongoing work! The results are very fresh and have not yet been scrutinized in detail

– Some simulations finished yesterday

– Don’t take them as final results yet…