a novel low b * scheme s. fartoukh (cern, be/abp)

S. Fartoukh OMCM Workshop, CERN, 20-22 June, 2011 1

A novel low scheme S. Fartoukh (CERN, BE/ABP)

Performance goal of the HL-LHC

Layout & optics limitations of the existing LHC

An “Achromatic Telescopic Squeezing” (ATS) scheme to overcome the limits

An overview of the optics challenges and mitigation measures

Conclusions and outlookMain References:Optics challenges and solution for the Phase I Upgrade, Chamonix 2010 & SLHC-PR38Breaching the Phase I optics limitations for the HL-LHC, Chamonix 2011 & SLHC-PR53

S. Fartoukh OMCM Workshop, CERN, 20-22 June, 2011

2

Performance goal for the HL-LHC Integrated luminosity: ~250 fb-1 /year, i.e. about 1 fb-1 per fill.

Technique for leveling not yet decided:Crab-cavity, X-angle, *,…

0.E+00

2.E+34

4.E+34

6.E+34

8.E+34

1.E+35

0 5 10 15 20 25

Lum

inos

ity

(cm

-2 s

-1)

time (hours)

1035 - no level Level at 5 1035 34

Average no level

Average level

The “effective target” is a luminosity increased by a factor of 10 w.r.t. nominal.

Concept of “Virtual” luminosity:Need more than 5E34, typically ~1.E35 cm-2s-1 “stored”, even if not usable due to limitations on the machine side or on the detector side.

Running luminosity:Sustained to 5E34 cm-2s-1 with leveling during 3-5 h + decay of a few hours:

Illustration from E. Todesco


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Luminosity vs. * in the Xing plane (with hour-glass effect) for different values of * in the other plane: nominal emittance and bunch length, ultimate intensity, no crab-cavity

Case of round beam optics(saturation due to Xing angle)

0.16

cm

1.0c

m

0.40

cm

100cm

7.5c

m16cm

40cm

Example of flat optics:* =30 cm in the crossing-plane* = z =7.5 cm in the other planec = 10 in the plane of biggest *

Peak lumi ~5.6 1034cm -2s – 1

“Equivalent” round optics:* =15 cm in both planesc = 10 Peak lumi ~3.5 1034cm -2s – 1

1. The “virtual” performance of the two optics becomes equivalent with crab-cavity (~8-9E34), 2. In all cases the two options requires to push the beam parameters beyond ultimate and * a factor 4 to 8

below nominal (55 cm), which cannot be only given by Nb3Sn triplets (25% improvement w.r.t. NbTi).

Main layout and optics constraintsLayout constraints

• More or less fixed L* ~ 20 m (detector)

• Constant length of the FFS (matching section) LFFS ~250 m from the IP till the arcs (Q7)

• Beam fully separated, Dsep=194 mm, in the arcs


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Optics constraints

1) Strength of the IR quadrupoles (specifications for the min./max. gradient).

2) Aperture of the IR magnets, not only triplet (IT) but also D1, and matching section (D2, Q4, Q5).

3) Correction of the chromatic aberrations within the strength of the arc sextupoles (chroma Q’, Q’’,.., but also off-momentum -beating, spurious H and V dispersion induced by the crossing angle in IR1/5):

All these constraints can be quantified by the maximum allowed peak beta function max reached in the triplet (IT),

This max limit coming from the ring then gives the optimum IT aperture (max)½, then the IT gradient & layout for a given technology (NbTi or Nb3Sn) and then *

min 1/max/G½.

L*=23 mLFFS=268m

Triplet Separation dipoles D1/2 Matching section Q4/5/6


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1) IR quadrupole strength at high max (low *) Neglecting the aperture and chromatic related constraints, the existing insertions IR1/5 are squeezable

down to *=15 cm, i.e. max~16 km.

Nominal collision optics with max=4.5 km *=55 cm (@ 205 T/m in the triplet)

Can be pushed up to max=16 km *=15 cm (@ 205 T/m in the triplet)

Flexibility given by the large cofocal length of the existing IT, P=()Q3-exit ~1km (independent of *)

Q4 ~ 1.5 km Q4~ 6 km

Below *=15 cm, gradient limitations are observed in the matching section quadrupoles (Q5/Q6 0 T/m) and in the dispersion suppressor (Q7 200 T/m).

Re-optimizing the matching section layout will not drastically improve the situation.


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2) Aperture constraints vs. triplet cofocal length P=()@Q3exit (example given for a 120 mm-135 T/m triplet, max~12 km *=25cm, and nominal matching section)

Case Nom. P Low P

Grad.[T/m] 132.74 136.41

Lq1=Lq3 [m] 8.70 8.50

Lq2 [m] 7.40 7.30

L* [m] 23.0 23.0

D(q1-q2a) [m] 2.50 2.70

D(q2a-q2b) [m] 1.00 1.00

D(q2b-q3) [m] 3.00 2.90

Beta_max [m] 11910 11810

P [m] 891 328

Beta_Q4 [m] 3750 2125

Beta_Q5 [m] 2220 1340

Q5 D2/Q4 TANTAS-IT-D1

DS and MS gradients well within limits but Aperture bottle-neck in the TAN-D2-Q4-Q5 (12km max is a bit too much for 120 mm coil_ID)

MS aperture restored (except at the TAN) but quad. gradient at the limit in the MS (Q4/5/60) and DS (Q7~200T/m)

TAN

A difficult optimization game!

The game is over for max > 13 km at nominal

matching section aperture


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3) Chromatic aberrations: a zoology of effects to sort out (illustration given for the 15 cm * optics of slide 5, with a full crossing angle of 580 rad, V in IR1 and H in IR5)

No specific IP1-IP5 phasing: off-momentum -beat, Q’’, spurious dispersion.

Phasing IP1 and IP5 by /2 only partly solves the problem: off-momentum -beat in half of the ring, huge Q’’’, still spurious dispersion.

IP1 IP5

IP1

IP1

IP1 IP5

IP5

IP5

Montague function

Montague function

Tune vs.

Tune vs. H & V dispersionwith X-angle

H & V dispersionwith X-angle


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A complicated strategy set up for the former upgrade project (Phase I): active correction anticipating the chromatic kicks induced by the IT by a series of small kicks given upstream by the sextupoles

Two implications:1. An overall new optics is required to fulfill specific betatron phasing conditions.

2. Only half the sextupole families participate effectively to the correction of the triplet

max 11 km (instead of max 17 km for the correction of Q’ only).

A by-product:The new phasing configuration allows the correction of the spurious dispersion by modest H or V orbit

bumps (a few mms) in the arcs surrounding the low- IR’s.

Sextupole gradients (beam1) vs. *

Transition @ *=1.5 m

550A

- 550A

*=30 cm


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Illustration for the 120 mm-120 T/m Phase I triplet : squeezed optics with *=30 cm (max~11 km)

Off-momentum -beating envelop after correction (W=100 =10% @ =10-3)

Vanishing in the collimation IR’s Vanishing in the new IT of IR1 & IR5 Two sectors of sextupoles needed per IT!

IP1 IP3 IP5 IP1IP7

Betatron tunes vs. energy Almost linear up to =1.5 10-3

(with Q’ matched to 2 units)

Bu

cket:

×

Min

. mom

entu

m w

ind

ow:

Mom

entu

m collim

ator: ×


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Most of the ingredients are already there to blow-up the ’s in the arcs at 7 TeV! 1) Huge aperture margin in the arcs at 7 TeV : ~ factor 16 margin to increase the beta’s in the arcs. 2) About 150 quadrupole knobs moderately used (IR2/IR8) or not used at all (IR4/IR6) in pp collision @7 TeV.

The Achromatic Telescopic Squeezing (ATS) scheme

Optics limitations with corresponding min. * for NbTi triplet

Cure

Aperture max ~13 km *min ~ 25 cm (for NbTi)

No miracle: New IT/D1 but also new 2-in-1 D2/Q4/Q5 needed with larger aperture

Optics “matchability”max ~16 km *min~ 20 cm (for NbTi)

Change the matching conditions at the IR boundariesBlow-up the functions in the arcs 81/12/45/56

Sextupole strength max ~11 km *min~ 30 cm (for NbTi)

Increase the sextupole efficiency at constant fieldBlow-up the functions in the arcs 81/12/45/56


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Nominal arc (180m) in sectors 81/12/45/56

Injection optics: * = 14 m in IR1 and IR5

- /2 FODO cells in sectors 81/12 and 45/56.- 4 sectors (23/34/67/78) “free” of any constraints. New integer tunes 62/60 (instead of 64/59).


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Nominal arc (180m) in sectors 81/12/45/56

Pre-squeezed optics: * = 60 cm in IR1 and IR5: “1111”

- “Standard” squeeze acting on IR1 and IR5 with /2 left/right phase advances for the low- IRs - “Up and down” sextupole powering scheme … till reaching the max current in 50% of the sextupoles of s45/12/45/56.


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arc increased by a factor of 4 in s45/56/81/12

Squeezed optics (round): * = 15 cm in IR1 and IR5: “4444”

Continuation of the squeeze acting only on IR2/8 for squeezing IR1, and IR4/6 for squeezing IR5.


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arc increased by a factor of 2 or 8 in s45/56/81/12depending on the * aspect ratio in IP1 and IP5

Squeezed optics (flat): *x/y = 7.5/30 cm alternated in IR1 and IR5: “8228”

S. FartoukhOMCM Workshop, CERN, 20-22

June, 201115

Montague functions (W=1000 =100% at =0.001)

…Then a series of fundamental chromatic properties (examples for “8228” optics)

1) Chromatic correction using only one sector of sextupoles per IT

Dispersion of only 50cm in the IT thanks to ±2.5 mm orbit bumps

induced in sectors 81/12/45/56

Closed orbit with X-scheme in IR8/IR1/IR2 and IR5 H and V dispersion

Tune vs. p (+/- 0.0015 window)

IP3IP7 IP7IP5IP1

2) Correction of the spurious dispersion induced by the X-angles in IR1 and IR5


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Optics challenges and mitigation measures

Optics “1111”(60cm/60cm)

“2222”(30cm/30cm)

“4444”(15cm/15cm)

“8228” or “2882”(7.5cm/30cm)

Peak ’s in the s81/12/45/56

180m (nominal) ~350 m

(in both planes)

~700 m

(in both planes)

~1.4km (for one plane)

~350 m (for the other)

105 turns DA[] > 50 (OK) 28 (OK) 15 (worrying) 11 (~)

… Assuming no field imperfection in the new IT/D1/D2/Q4 (courtesy of R.D. Maria)

… But a lot of room for improvement (with further developments needed): - Pushing the pre-squeezed * to reduce the ’s in the arcs by up to 50% at constant * in physics (with more sextupole current 550A 600A, additional sextupole at Q10, stronger Nb3Sn triplet). - Smaller emittance (3.75 2.5 m as worked out by the LIU project).

• Linear optics distortion due to the arc magnets during the squeeze - Good local closed orbit correction needed in the arcs and reliable CO feed-back. - Arc by arc accurate measurement and correction of b2 and a2 (dedicated corrector knobs available in the LHC, and optimized random b2/a2 seen by the beam thanks to magnet sorting).

• Dynamic aperture reduction (big ’s in the arcs combined with strong sextupoles and MB, MQ, MQM field imperfections).


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• Usual concerns related to the triplet at low * (not ATS specific)

1. Inner triplet (IT) stability

Tolerance to mechanical vibrations 1/√*: x*<*/10 requires micron to sub-micron stability level (peak to peak) for *=157.5 cm.

Tolerance to PC jitter 1/*: Q ~ requires 1 ppm to 0.5 ppm level (r.m.s.) for *=157.5 cm.

2. IT field quality (w/o forgetting new D1) … not yet studied in details but certainly not more than a fraction of units for the low order multipoles at 2/3rd of the coil aperture

Certainly manageable for round optics at least for large aperture NbTi quadrupoles rescaled from the existing MQX’s… (see E.Todesco et al. LHCPR1010).

Will be more tricky for equivalent flat optics (max doubled in one plane, halved in the other plane, at constant IT aperture and therefore constant field quality).

A battery of non-linear correctors may be needed (a fortiori for Nb3Sn triplet).

• The ATS concept offers a powerful and flexible machinery to reach very low * while correcting the chromatic aberrations induced.

• A series of LHC MDs have already started this year to validate it.

• Conceptually and operationally speaking, the actual insertions IR1 and IR5 are replaced by two 7 km long low-insertions containing the low- IRs proper (optically passive below a certain *), two LHC sectors (chromatic correction sections) , two “auxiliary” insertions (powerful matching section).

• The existing LHC seems ready to generate, swallow and take advantage of the big -beating bumps induced in the arcs during the squeeze of IR1 and IR5 thanks to its conception (aperture available in the arcs and “optics margins” of IR8/2/4/6 at 7 TeV, existing

sextupole scheme, sectorization of the a2/b2 corrector knobs, …). the optimized MB & MQ field quality as seen by the beam (very small systematic even

multipoles but b6 of the MQs at 7TeV, and random low order multipoles minimized by magnet sorting at installation),


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Summary

Reserve


19


20

)*,for effect glasshour (no

1~

plane -crossingnonin the * y toSensitivit

1

plane crossingin the * y toSensitivit

21

1*2

**

2*

zyx

AH

HN

A

y

x

zcx

b

)L(

• Round beam optics (*x= *

y)

The luminosity saturates without crab-cavity:

Performance vs. w/o crab-cavity

zc

yx

1

0

*

**

)L(

• Flat beam optics (*x≠ *

y)

The lumi is optimal fixing * in the crossing plane to

The lumi still increase when decreasing * in the other plane(and then saturates due to the hour-glass effect at very small *).

cm 352

* zx

c


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New TAS60 mm ID

New IT125 mm b.s. ID

≥150 mm coil_ID

NewD1~135-140 mm b.s ID ≥160 mm coil_ID

New TAN33.5-37.5 mm

elliptical chamber

NewD277 mm b.s ID

≥92 mm coil_ID

New Q4 (“MQYY” type)70 mm b.s ID

≥85 mm coil_ID

New Q5 (“MQYL” type) 70 mm coil_ID

A priori enough apertureat Q6 and beyond

(n1 ≥ 10)

Aperture requirement assuming Nb-Ti IT (nominal emittance, 7.5/30 cm or 30/7.5 cm flat optics, 13 full X-angle, spurious H&V dispersion corrected via orbit bumps in the arcs)

The above requirements are also compatible with a 15/15 cm *round optics and could be relaxed by ~10% (but for the TAS) with Nb3Sn triplet .


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Why does it work?.. Zoom in from IP4 to IP5 for the flat optics (beam1)

Equipping Q10 (MQML) with an MS becomes highly desirable for high arc.

y between the

12strong SD sextupoles

y (Q11 IP ) = 1.25×y

with y ~ 1/2 tan-1 (min/max)arc× *)V cst

x between the

9 strong SF’s one missing at Q10 to complete 5 -pairs

x(Q14 IP ) = 1.25×x

with x ~ - 1/2 tan-1 (min/max)arc× *)H cst

a novel low b * scheme s. fartoukh (cern, be/abp)

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