(1)modified collimation layout & optics (2) performance limits for ion beams

(1)Modified Collimation Layout & Optics

(2) Performance Limits for Ion Beams

John Jowett (CERN, Beams Dept.)With thanks for contributions from:

Ralph Assmann, Giulia Bellodi, Roderik Bruce, Thomas Weiler

J.M. Jowett, Conceptual Design Review LHC Phase II Collimation, 2 April 2009 1

Plan of talk

(1)– Modified layout of IR7 for cryo-collimators– Rematch of IR7 optics– Collimation problem for ion beams

(2)– Performance limits with heavy ion beams– Reference to Executive Summary on ion

collimation– BFPP Luminosity limit for ion beams– Cryo-collimators in IR2 for ALICE experiment– Further possible installations

ConclusionsJ.M. Jowett, Conceptual Design Review LHC Phase II Collimation, 2 April 2009 2

(1)Modified Collimation Layout & Optics

(for cryogenic collimators)


LHC Collimation Insertions IR7: Betatron collimation insertion

– Treat changes for installation of cryogenic collimators– Effects in later talks (T. Weiler, G. Bellodi)

IR3: Momentum collimation insertion– Similar layout, different optics– Expect to install cryogenic collimators there too but

details not treated yet For further details:

– All layout and optics plots shown in this talk, plus more, are available at http://cern.ch/jowett/Talks/2009-04-02 in a form where you can mouse-over to see details of elements names etc.


http://cern.ch/jowett/Talks/2009-04-02

IR7 Optics overview, Beam 1


Beam 2 has F and D quads inverted, but imperfect (left-right, x-y) asymmetry, so has to be treated separately.IR7 optics is constant – no change with energy, β-squeeze, etc.

Primary collimators.Secondary collimators.

x

y

xD

Beam 1

Making space, IR7 right, Beam 1


Move outer group of elements 3 m away from IP into missing dipole space.Move inner group of elements 3 m towards IP to (roughly) compensate change in geometry.Similarly on right of IP7.

x y

xD Outer groupInner group

Zoom on displacements along reference orbit


This vacates enough space in the right places to install the cryogenic collimators.N.B. this is in Courant-Snyder coordinate s, so we do not see the change in geometry of the LHC.

Before

After

Global Cartesian Coordinate System

Global coordinates, in the straight part of the betatron collimation insertion section around IR7:– X is longitudinal– Y is vertical– Z is radial

w.r.t. Courant-Snyder coordinates.

Use (Z,X) as coordinates in the machine plane


Displacements of reference orbit, Beam 1


Zoom

Radial displacement of IP7 and straight section due to non-commutativity of rotations and translations is small enough (0.019 mm) to neglect.

Radial displacement of reference orbit between shifted sections by 30 mm.N.B. Not the displacement of elements!

Longitudinal displacement mainly reflects change in length of reference orbit – can be fixed.

Displacements of moved elements, Beam 1, left of IP7

In the global cartesian frame, the displacements of the outer and inner groups of elements include a component from the angle (“curvature”) of the initial reference orbit.

MAD - and the LHC Layout Database - use the “beads on a necklace” method of laying out the machine so everything downstream of IR7 moves and the ring does not close … this is not real of course but has to be corrected in our description.


Outer group

Inner group

Corrected layoutSmall negative displacements of all elements downstream of IR7 along the reference orbit restores them to their original position in the global cartesian system and closes the ring.

New sequence descriptions created for both rings.


LHC circumference is changed by -1.872 mm.

Optical perturbations


Change in layout perturbs the optical functions, giving about 20% β-beating which must be corrected.Rematch IR7 for each ring without using the common quadrupoles that affect both.

0/x x 0/y y

0x xD D

0/x x

β-beating in whole Ring 1 β-beating in IR7, Ring 1

0/y y

0x xD D

Rematch of IR7, Beam 1


Perfect match – same transfer matrix over IR7 - (also for Ring 2) so can be used in modular way with all existing LHC optics configurations.Adjusted β-function peaks so available aperture is not changed significantly.

Quadrupole strengths before/after rematch


Before and after matching the strengths used for Beam 1. Light blue bars on left hand side plots are the maximum strengths available at 7 TeV.

Before:

After:

Aperture of nominal IR7, Beam 1 at injection


(n1 is a quantity conventionally used to assess aperture available to beams in the LHC. It includes x and y planes and various “tolerances” in a single number according to a recipe coded in MAD. Normally require n1 > 7.)

Aperture of nominal IR7, Beam 2 at injection


Somewhat different from reflected Beam 1

Cryo-collimator optics IR7, Beam 1 at injection


n1 of the cryo-collimator optics is different

Cryo-collimator optics IR7, Beam 2 at injection


n1 of the cryo-collimator optics is different

n1 before and after, Ring 1, IR7


n1 before and after, Ring 2, IR7


1

Optics V6.503 6C

min in IR7 Ring

ryo-collimator.84 6.9

optic

1 Ri

s 6.8

n

3 7.191

g 2n

(2) Performance of LHC with Heavy Ion Beams


Design parameters with 208Pb82+ nuclear beams

The LHC will run ~1 month/year with ion beams, initially Pb

Although the stored energy in the Pb beam is much lower than in the proton beam, beam loss mechanisms peculiar to ions may limit luminosity. Most serious are:– Collimation inefficiency (different physics from protons)– Bound free pair production (BFPP)


Parameter Units Nominal Energy per nucleon TeV 2.76Initial ion-ion Luminosity L0 cm-2 s-1 1 ×1027

No. bunches, kb 592Minimum bunch spacing ns 99.8* m 0.5 /0.55Number of Pb ions/bunch 7 ×107

Transv. norm. RMS emittance mm 1.5Luminosity half-life (1,2,3 expts.)

h 8, 4.5, 3

Ultraperipheral reactions in nuclear collisions


82 82 208 81

[ 0.012]

2082 80 208 28

Bound-free pair production(BFPP), 280 b:

Pb Pb Pb b eP

20882 82 8GDR 207 82

[ 0.04

208 20 28

8]

Electromagnetic dissociation (EMD), 100 b:does not form spot on beam pipe

Pb Pb Pb n

2-neutron EMD process, 29 bforms spot on other side of pipe from B

Pb

206 82

[ 0.009

82 208GDR208 2 82 8208

6]

FPP

Pb Pb Pb 2P nb

0 10 0 1 1

0 1

Magnetic rigidity change in nuclear reaction

( , ) ( , ) = 1Z AZ A Z AA Z

C.f. hadronic interactions, 8 b

Luminosity Limit from BFPP in collisions


0

100

200

300

400

sm

0.020

0.02

xm 0.03

0.02

0.01

0

ym

0

100

200

300

400

sm

0.03

0.02

0.01

0

ym

IP2

Main Pb82+ beam

Longitudinal Pb81+ ion distribution on screen

9.5 10 10.5 11 11.5 12

0.2

0.4

0.6

0.8

Secondary Pb81+ beam emerging from IP and impinging on beam screen, ~ 25 W of power at design luminosity may quench dipole magnet in dispersion suppressor at fraction of design luminosity (see other papers and talks).Very similar to isotopes emerging from primary collimator (see G. Bellodi talk).

82 82 82 20208208 208 8 81

[ 0.012]

Pb Pb b ePbP

Main and secondary beams from IP2


208 81Pb (BFPP)

206 82Pb (EMD-2n)208 82Pb (main)

Optimal position for one cryo-collimator?

Cryo-collimators as cure for BFPP Not considered up to now because of inviolability

of cold sections of LHC Location of cryo-collimators may need to be

different from IR7 (one seems enough).– Smaller movements of more dipoles? – Requires further detailed study

Layout adjustments and optics rematch in IR2 should be acceptable– More work to do because of multiple optics in

ramp and squeeze Comparison with FLUKA studies for IR7 (talk by F.

Cerruti) suggests that 25 W at design L should be OK


Further possible installations Momentum collimation insertion IR3

– Expected to be similar to IR7, details to be worked out

Other experimental IRs?– ALICE (IR2) is dedicated heavy-ion detector but

ATLAS (IR1) and CMS (IR5) also want heavy-ion collisions

– Consider cryo-collimators in those IRs also ? – Possible interference with FP420 ?– Need for same luminosity? With design

luminosity in 3 experiments, short lifetime from burn-off would impose time-sharing or luminosity levelling with β* (A. Morsch).


Conclusions Installation of cryogenic collimators in IR7

– New layout, geometry and optics satisfying all requirements

– Solution for collimation in both p-p and ion modes (talks by T. Weiler and G. Bellodi)

IR3 still to be treated but should be similar Cryo-collimators in IR2 can raise luminosity limit

for Pb-Pb collisions– Needed soon! Pb-Pb is earliest phase and design

luminosity to be approached in 2-3 years Possible installations in IR1 and IR5

– Requires decisions, guidance on luminosity sharing in heavy-ion operation, and further study

– Possibly useful in p-p running


Backup slides


Reminder: Ion beam energies in LHC


2

Pb p

8208

82 7 TeV

For our fully-stripped Pb

7 Te

nuclei ("ions

V 574 TeV

208 2.76 TeV 2.76 TeV 574 TeV82 (kinetic energy of mosquito at 1m/s)(

") in the same magnetic field as 7 TeV protons:

n

E Z E Z

AE A

kinetic energy of 1 mm diameter grain of sand at 40 km/h)

Luminosity vs. single bunch current

with Pb ions at 2.76 A TeV

0.01 0.05 0.1 0.5 1 5 10I b m

1. ´1022

1. ´1023

1. ´1024

1. ´1025

1. ´1026

1. ´1027L cm2s-1

Visibility threshold on FBCT

Nominal *=0.5 m

Visible on BCTDC

Early *

=1 m

-1-2scm/L

A/ mbI

Visibility threshold on arc BPM

BFPP Quench limit, Collimation limit?N

ominal single bunch current

Visible on BCTDC

Thresholds for visibility on BPMs have improved (Sep 2008 data) giving greater flexibility for commissioning, possibility of longer fills.J.M. Jowett, Conceptual Design Review LHC Phase II Collimation, 2 April 2009 32

*=2 m

Machine Protection BLM thresholds to avoid quenches

– Most ion performance limitations are related to quenching magnets (discussed extensively elsewhere, not within scope of initial run) – see next slide

Beam dump– Possible damage to window etc. checked – Revolution frequency lock OK– Need to re-validate the XPOC checks of the

dump quality for the BI, also define the new references etc.

“Safe beam” intensity can be defined as same beam charge as protons


Beam loss monitor thresholds


Fragmentation of nucleus

Nucleons shower

BLM signal

Initial high (Bethe-Bloch) ionization from nuclear charge

~ Z2

FLUKA simulations of BLM signals for LHC MB, Pb nuclei and protons impinging on beam screen (R. Bruce).Implies that BLM thresholds to avoid quenching can be identical for Pb and p.

35

Robustness of collimator against mishaps

J.M. Jowett, Conceptual Design Review LHC Phase II Collimation, 2 April 2009

FLUKA calculations from Vasilis Vlachoudis for dump kicker single module prefire

Compares full nominal proton bunch train and nominal ion train.The higher ionisation loss makes the energy deposition at the impact side comparable to proton case, despite 100 times less beam power.Energy deposition nowhere exceeds p case.

LHC Pb-Pb is a new accelerator regime

Effects limiting future performance of LHC with Pb-Pb collisions are new and uncertain:– See other reports on bound-free pair

production, collimation, etc.– Loss patterns, quench limits, … – Data from RHIC and SPS has been exploited

and published.– Experience of first low intensity runs will help

test and calibrate simulations and assess needs for future improvements

– (May also be able to learn about performance limits in phases beyond Pb-Pb.)



Pair Production in Heavy Ion Collisions

- +1 2 1 2

42 23

P

2 21 2

P 4

Racah formula (1937) for in heavy-ion collisions

e e

1.7 10 b for Au-Au RHIC224 log 2 2

free pair pr

7 2. 10 b for Pb-Pb

oduction

LHCe

CMZ Z

Z Z Z Z

r

1/2

- +1 2 1 21s ,

PP5 2

1 2

1 27

Cross section for (several authors)

e e

has very different dependence on ion charges (and energy

Bound-Free Pair Production (

)

log

for

0.

BFPP)

2

logCM

CM

Z Z

Z Z

Z Z

A B

Z ZA BZ

b for Cu-Cu RHIC114 b for Au-Au RHIC281 b for Pb-Pb LHC

We use BFPP values from Meier et al, Phys. Rev. A, 63, 032713 (2001), includes detailed calculations for Pb-Pb at LHC energy

BFPP can limit luminosity in heavy-ion colliders, S. Klein, NIM A 459 (2001) 5137

Ion Collimation in LHC Collimation system essential to protect machine from

particles that would be lost, causing magnet quenches or damage

Principle of two-stage collimation for protons:– Particles at large amplitudes undergo multiple Coulomb

scattering in sufficiently long primary collimator (carbon), deviating their trajectories onto properly placed secondary collimators which absorb them in hadronic showers

Ions undergo nuclear fragmentation or electromagnetic dissociation before scattering enough– Machine acts as spectrometer: isotopes lost in other

locations, including SC magnets– Secondary collimators ineffective, two-stage principle

does not work


Example of 206Pb created by 2-neutron EMD

Green rays are ions that almost reach collimator Blue rays are 206Pb rays with rigidity change


Losses in physics at 177 A GeV Luminosity losses are

negligible– Because L is low

Collimation losses have been simulated– Different

distribution from high energy

– Well below quench limit (~50 W/m)


For one experiment colliding:100 h (hadronic+BFPP+EMD)L

From G. Bellodi

Beam 1, dispersion suppressor right of IP7

41J.M. Jowett, Conceptual Design Review LHC Phase II Collimation, 2 April 2009

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(1)modified collimation layout & optics (2) performance limits for ion beams

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