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Introduzione agli acceleratori e loro applicazioni:

High Luminosity LHC

Gabriele ChiodiniIstituto Nazionale di Fisica Nucleare

Sezione di Lecce

Lezioni per il Dottorato di Ricerca in Fisica dell’Università del Salento

Anno accademico 2015-2016 II Semestre

(20 ore, 4 CFD)

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/31Corso acceleratori e applicazioni - HL-LHC G. Chiodini - Luglio 2016

Sommario

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1.La luminosità di LHC2.Le sfide di HL-LHC3.Fasci ad alta brillanza


Layout di LHC■ Lungo 26.7 km ■ Simmetria height-fold. ■ 8 archi di 2.8 km ognumo ■ 16 dispersion suppressors

per fare il matching tra archi e sezioni diritte

■ 8 long straight sections di 700 m ognuna (dette anche zone di inserzione).

■ 2 camere a vuoto e 2-in-1 magneti sc di bending


LHC longitudinal

• frf=400 MHz

• fr=11.22 kHz

• h=frf/fr=35640

• Emittanza longitudinale = area del bunch = dE x dt = dEdPhi/2pifrf (eVs)

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Zona d’interazione (IR)

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194 mm

~ 260 m

Common vacuum chamber

D2

D1 D1

D2

Triplet Triplet

D1,D2 : separation/recombination dipoles

Machine geometry in H plane

IP

beam1

beam1

beam2

beam2~ 40 m

• Nelle IR i due fasci sono combinati in una singola camera a vuoto e quindi separati di nuovo sul piano orizzontale

• I fasci passano dalla camera a vuoto esterna a quella interna alternativamente • La focalizzazione nel IP e’ ottenuto con un tripletto di quadrupoli.


Luminosità di picco all’IP

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n

brrev NFfMLεβπ

γ 2

*4=

LHC-specific

Injectors-specific

(Round beams)


β*

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▪ Il fattore di riduzione F dovuto all’angolo di crossing è inferiore a 1:

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▪ β* entra in F con θc e σ*: ridurre β*

bsotto un certo valore non si guadagna più in luminosità perchè decresce anche F.

L vs. β*

•Tripletto di quadrupoli di inserzione ad alte performance

▪ Fasci piatti o crab-cavity mitigano questo effetto.


Limite β* dovuto alla apertura della macchina

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John Jowettβtriplet ~ 4 km

β* = 60 cm

γβ

εσ

*

*

∝triplet

Il tripletto di quadrupoli definisce il limite di apertura della macchina per beam squeezed. β* è limitato dall’inviluppo del fasci.


ε = 2.5 µm θ = 145 µrad

β* = 0.6 m ! F = 0.81

Limite β* dovuto al crossing angle

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2θ

•Minimizzare effetti beam-beam •Evitare una diminuizione drastica del fattore F


Da LHC a HL-LHC

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What to do to make this jump ?

M. Lamont

• Increase 5 x L• Increase 10 x LT

• Target (very ambitious) of the upgrade: 200 – 300 fb-1/y (×10 today)

• Radiation damage limit of IR quadrupoles (~400 fb-1)


Da LHC a HL-LHC

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Parameter Nominal 25ns – HL-LHC

Bunch population N 1.15 2.2

Number of bunches 2808 2748

Beam current [A] 0.58 1.12

Crossing angle [µrad] 300 590

Beam separation [σ] 9.9 12.5

β* [m] 0.55 0.15

Normalized emittance 3.75 2.5

εL [eVs] 2.51 2.51

Relative energy spread [10 1.20 1.20

r.m.s. bunch length [m] 0.075 0.075

Virtual Luminosity (w/o CC) [10 1.2 (1.2) 21.3 (7.2)

Max. Luminosity [10 1 5.1

Levelled Pile-up/Pile-up density [evt. / evt./mm] 26/0.2 140/1.25

ΔQbb ~ -0.01Aim for ~250 fb-1/y


Strategia per upgrade

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!➽Maximize bunch brightness (beam-beam limit) ➽Minimize beam size (aperture in triplets) ➽Maximize number of bunches (beam power, e-cloud) ➽Compensate for F !▪ LHC Upgrade (HL-LHC): ▪ Smaller β*; new triplets quadrupoles; possibly new

technology (Nb3Sn instead of Ni-Ti). ▪ Mitigation measures for higher currents (e.g., collimator

system upgrade, cooling, beam-beam compensation wires) ▪ Flat beams or crab cavities

▪ Injectors’ Upgrade (LIU): ▪ Increase beam brightness ▪ Mitigation measures for higher currents (e.g., coating SPS

vacuum chambers).


ATS=Achromatic Telescopic Squeeze

13• …of course it requires larger aperture triplets

IR4 IR5 IR6

S. Fartoukh

/31Corso acceleratori e applicazioni - HL-LHC G. Chiodini - Luglio 201614• …of course it requires larger aperture triplets

Novel optics concept enabling the matching of ultra-lowβ∗ while correcting the chromatic aberrations induced bythe inner triplet (IT). !Two-stage telescopic squeeze: • Pre-squeeze performed by using exclusively, as usual, the

matching quadrupoles of the high luminosity insertions IR1 and IR5.

• Second stage, β∗ can be further reduced by some factor (typically 4 to 8) by acting only on the insertions on either side of IR1 and IR5 (i.e. IR8/2 for IR1 and IR4/6 for IR5).

ATS=Achromatic Telescopic Squeeze


Crab Cavities

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➔Crab Cavities as tool to maximize overlap among colliding bunches (i.e. virtual luminosity) and minimize pile-up density

➔Design molto difficile a seguito del limitato spazio trasverso ad LHC

At IP without crab cavities At IP with crab cavities


Virtual luminosityHL-LHC relies on a levelled luminosity not exceeding 5 × 10E34 cm−2s−1

This is achieved presently via a gradual reduction of β∗ in order to compensate for the proton burn-off during the physics store.

In order to sustain such a high luminosity over a typical period of 8-10 hours, the beam parameters, in particular the total beam current, shall correspond to a so-called virtual luminosity, which would be of the order of 2×10E35 cm−2s−1 should all the other parameters, for instance β∗, be pushed to the limit at the beginning of the levelling process.

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Luminosity levelling

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!L [1034 cm-2s-1]

!t [h]

Virtual peak luminosity (F=1)

leveling at 5x1034cm-2s-1

τeff=15 h, Tta=5 hleveling at 2.5x1034cm-2s-1

τeff=30 h, Tta=5 h

F. Zimmermann

➔Luminosity levelling mechanisms for more uniform pile-up during run ➔dynamic β* squeeze ➔Long-range and beam-beam wire compensators to reduce crossing angle


Pile-up density control

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z [m]

Baseline: CC in X-plane “only”Crab-kissing:

CC also in ||-plane

S. Fartoukh


Nuovi tripletti in ATLAS/CMS

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LHC

HL-LHC

Bore aperture : 70 ! 150 mm. Gain 3-4 in beam area (β* = 15 cm).

+ radiation shield for B and cryogenics


Dipoli ad alto campo e crio-collimatori

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The goal is to develop a 10 m long 11.2 T Nb3Sn dipole to replace a standard LHC dipole and provide space for collimators downstream of the straight sections.

FNAL prototype MBHSP02 reached 11 T in May 2013

NbTi 8.3 T

Nb3Sn 11 T

Collimator

15 m

Same deflection !


R&D magneti per HL-LHC

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• LHC dipoles feature 8.3 T in 56 mm (designed for 9.3 peak field)

• LHC IT Quads features 205 T/m in 70 mm with 8 T peak field

• HL-LHC: • 11 T dipole (designed for 12.3 T peak field, 60 mm)

• New IT Quads features 140 T/m in 150 mm > 12 T operational field, designed for 13.5 T).

11/0

3/20

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Bottleneck tecnico: criogenia

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IT IT

ITIT

IT

ITIT

IT

RF

RF

Cryo power limitation in Pt 4, interdependency

of different systems with different cool-down time, reduced

flexibility and no/little redundancy


Hardware upgrade

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•New high field/larger aperture interaction region magnets

•Cryo-collimators and high field 11 T dipoles in dispersion suppressors

•Crab Cavities to take advantage of the small β*

•New collimators (lower impedance)

•Additional cryo plants (P1, P4, P5) •SuperConductor cables to allow power converters to be moved to surface

11/0

3/20

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Fasci ad alta brillanza

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Fasci ad alta brillanza Nb/ε*

(alta densità di particelle nello spazio fasi)

• Catena di iniezione ad alte performance

• LHC deve mantenere le performance (ottica e configurazione lungo tutto l’anello)


LHC injector chain

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LINAC2 (50 MeV)-PSbooster(1.4GeV)-PS(25 GeV)-SPS(450GeV)


LHC Injector Upgrade (LIU)

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LINAC2 (H+ a 50 MeV) rimpiazzato dal LINAC4 (H- a 160 MeV)

SPB (may-be fully replaced by Rapid Cycling Synchrotron): -new injection (from multi-turn betatron a charge exchange injection)-Top energy up to 2 GeV

SP need 2 GeV injection energy to reach the HL-LHC emittance

SPS to reach the HL-LHC emittance needs: -coating with amorphous carbon the wall vacuum chamber agains e- cloud -reduce beam impedance (particularly in extraction and dump inserction-new optic with tune Q=20 against several instabilities.


HL-LHC ad alta brillanza

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- Impedenza al fascio ridotta- Mitigazione del fenomeno dell’e- cloud- Correzioni ad effetti beam-beam nel tune Q


Wake field and beam impedance

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• Intense bunches generate electromagnetic fields when passing inside a structure (in particular Carbon collimators – opening of ~1 mm!!!)

• → results in an EM force, called wake field in time domain coupling with the beam !!!!

!!!• Avoid the abrupt transition for the beam fields at the location of the

beam passage (taper) • Reduce the resistivity of the material

Abrupt transition With taper

B. Salvant


Wake fields and instabilities

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• Wake fields can couple the head and tail of a bunch or consecutive bunches leading to instabilities

s

v~36 cm ~15 m

~1.6×1011 p


Collimatori a bassa impedenza

30 G. Arduini - LHC Machine Upgrade

New material: • Molibdeno-Grafite • Rame-Diamante


e- cloud

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Possible consequences: – instabilities, emittance growth, desorption, vacuum

Electron bombardment of a surface has been proven to reduce secondary electron yield (SEY) of a material as a function of the delivered electron dose. This technique, known as scrubbing, provides a mean to suppress electron cloud build-up.

Secondary emission yield [SEY] SEY>SEYth ➔ avalanche effect (multipacting)

SEYth depends on bunch spacing and population


e- cloud mitigation

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1. Strategy for the arcs (to be validated in 2015): • enhance electron cloud at 450 GeV to remove e-cloud in the dipoles “completely” with dedicated scrubbing

• Use of “doublet beam” to enhance scrubbing !!!• Need to leave with e-cloud in the quadrupoles (sufficient cryo-margin for that)

2. Strategy for the Insertion regions: • Coat the beam screen of all new elements (triplets in particular) with materials with low SEY (amorphous Carbon) and install clearing electrodes

Back-up scenario:Detrimental e-cloud effects can be mitigated by using specially conceived filling patterns: bunch trains with long enough gaps interspersed


beam-beam effects (1)

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• Strong non linear fields when counter-rotating beams share vacuum chamber.

• ➔ spread in betatronic frequencies ➔ risk of overlapping resonances driven by magnetic errors

S. Fartoukh

Head-on

Long-range

T. Pieloni


beam-beam effects (2)

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S. Fartoukh

• Minimize magnetic errors ➔ Paid off for the LHC

• Devise correction schemes and sorting ➔ Paid off for the LHC

• Initially expected to have limit at ΔQBB ~ 0.003/IP

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