NewNewLHC quench calculations and the LHC quench ... forum 07-01-2009 - N… · 01/07/2009 ·...

NewNewNewNew LHC quench calculations and the LHC quench calculations and the LHC quench calculations and the LHC quench calculations and the luminosity limit for heavyluminosity limit for heavyluminosity limit for heavyluminosity limit for heavy----ion collisionsion collisionsion collisionsion collisions

R. Bruce*, S. Gilardoni, J.M. Jowett R. Bruce*, S. Gilardoni, J.M. Jowett

and

D. Bocian**, B. Dehning, A. Siemko

CERN

* also at MAX-lab, Lund University** also LARP Toohig Fellow

• Background and introduction: Bound Free Pair Production

• Tracking: Distribution at the IP and at impact

• FLUKA simulation of shower in magnet

• Thermal network simulation (Dariusz)

– Thermodynamics of magnet structure

– Network model

OutlineOutline

07/01/2009 R. Bruce, D. Bocian, AP Forum 1

– Network model– Validation of the model– Steady state beam loss heat load simulation

• Comparison between optics version

• Simulation uncertainties

• Summary

Motivation (1)Motivation (1)The LHC will run ~1 month/year with heavy ions. Nominal parameters:


• Although the stored energy in the Pb82+ beam is much lower than in the proton beam, beam loss mechanisms peculiar to ions may limit luminosity. Most serious are:

– Collimation inefficiency

– Bound free pair production (BFPP)

Motivation (2)

• Important to predict the quench limit as accurately as possible to estimate the impact of these beam losses. Same holds true for proton losses.

• Earlier estimates of quench limit make simplifying assumptions about the distribution of beam losses or the thermal behaviour of magnets


• To make more accurate estimates, these factors need to be accounted for

• Here we calculate the quench limit for a specific beam loss mechanism – BFPP – combining tracking, FLUKA shower simulations and a thermal network simulation of the heat flow in a magnet

Bound free pair production

• During Pb82+ operation in the LHC, electromagnetic interactions between colliding beams take place at IP:

– Bound Free Pair production (BFPP):

( )Cross section for (several authors)Bound-Free Pair Production (BFPP)


Compare: σhadr=8 barn

( )

[ ]

1/2

- +1 2 1 21s ,

PP5 2

1 2

1 27

e e

has very different dependence on ion charges (and energy)

log

for

0.2

log

CM

CM

Z Z

Z Z

Z Z

A B

Z ZA BZ

+ → + + +

σ ∝ γ +

∝ =

≈

γ +

K

b for Cu-Cu RHIC

114 b for Au-Au RHIC

281 b for Pb-Pb LHC

We use BFPP values from Meier et al, Phys. Rev. A, 63636363, 032713 (2001), includes detailed calculations for Pb-Pb at LHC energy

• BFPP creates 1-electron ions with altered magnetic rigidity:

• These ions follow locally generated dispersion function dxfrom IP

BFPP at IP2δ=0.012

Magnetic rigidity change

Secondary Pb81+ beam emerging from IP and impinging on beam screen


from IP

• Lost in localized spot where aperture Ax and δ satisfy

• Apart from significant luminosity decay, induced heating risks to quench superconducting magnets

S. Klein, NIM A 459459459459 (2001) 51

Beam screenBeam screen

Main Pb82+ beam

BFPP tracking

• Distribution leaving IP does not correspond to the bunch distribution, but to the distribution of collision points

• Spatial distribution in each plane is narrower by a factor

• As it propagates through the lattice, the distribution


• As it propagates through the lattice, the distribution changes, in the same way as an unmatched beam at injection

• Beam size at a later point

• Tracking with matrix formalism, off-momentum optics calculated by MAD-X, analytical algorithm finds impact in MB.B10R2

• LHC optics 6.500 as reference case, comparison with 6.503 later

• At IP2: losses at s=378.9 m downstream in end of dispersion suppressor dipole, spot size around 0.5 m

• IP1 and IP5: losses in connection cryostat in missing dipole, less critical. Will focus on IP2.

Tracking


IP2

Beam screen

Main Pb82+ beam

Secondary Pb81+ beam

Longitudinal Pb81+ ion distribution on screen

FLUKA shower simulation

• FLUKA simulation to estimate the heat load in the dispersion suppressor dipole at IP2

• impact coordinates of lost BFPP particles from tracking fed as starting conditions to FLUKA

• 3D model of LHC main dipole


FLUKA modelreal magnet

Simulated power deposition

15

20P HmWêcm3L

8 bins

4 bins

2 bins

1 bin

Power deposition from FLUKA in the inner coil layer, averaged

over width of coil,

normalized with BFPP cross section and luminosity:

Ptot = σBFPP L Eparticle

Energy deposition longitudinally in hottest bin, different radial binnings.

88 bins in φ (cable), 5 cm longitudinal cell size


in LHC design report: Quench limit=4.5 mW/cm3

Beam impact10-5 10-4 10-3 10-2 10-1 100 101 P HmWêcm3L

1300 1350 1400 1450z HcmL0

5

1032 bins

16 bins

Interpolation of power deposition

• However, now more accurate methods to estimate the quench limit exists – thermal network model (see later slides)

• Detailed map of power deposition in the coil needed

• Strand positions not compatible with R-φ mesh


compatible with R-φ mesh used in FLUKA

• Interpolating the “best possible” FLUKA mesh

• Applying global scaling factor to compensate for insulation, helium space in cables etc.

• Mathematica program automatically generates network input from FLUKA output

Input to network simulation

• Combining detailed simulated energy deposition from “real beam loss” with thermal network model of magnet

• input to network model:W/m







– Network model

OutlineOutline





• Summary

ModelModelllinging of quench levels of quench levels induced by steady state induced by steady state beambeam lossloss heat heat loadload

Thermodynamics of magnet structure

Network Model

Validation of the model

Steady state beam loss heat load simulation


Steady state beam loss heat load simulation

More details:D. Bocian, B. Dehning, A. Siemko, Modeling of Quench Limit for Steady State Heat Deposits in LHC Magnets, IEEE Transactions on Applied Superconductivity, vol. 18, Issue 2, June 2008 Page(s):112 – 115; CERN-AB-2008-006, 2008;

D. Bocian, B. Dehning, A. Siemko, Quench Limit Model and Measurements for Steady State Heat Deposits in LHC Magnets, accepted for publication in IEEE Transactions on Applied Superconductivity, 2009

Thermodynamics of magnet structureThermodynamics of magnet structure

Heat transport in the cableHeat transport in the cable

Courtesy C. Scheuerlein

MB magnet – inner layerRutheford type cable


NbTi + Cu

He(inside cable)

Cryogenic System

He(bath)

Collar

Yoke

Insulation

cable

1.9K / 4.5K

> 4.5K

Courtesy C. Scheuerlein

Thermodynamics of magnet structure Thermodynamics of magnet structure

Heat transport in the coil at 1.9KHeat transport in the coil at 1.9K

A heat transfer in the main dipole

Cold bore


inner layer outer layer

Electrical insulation is the largestElectrical insulation is the largest thermalthermal

barrier at 1.9 K against coolingbarrier at 1.9 K against cooling


Heat transfer in the Heat transfer in the magnet magnet coilcoil


A sketch of the heat transfer in the magnet

at nominal operation (a) and at quench limit (b).

MB – arc magnet,Tb=1.9 KHEAT FLOW

LIMITS

� heat flow barriers

- cable insulation

- interlayer insulation (MQM)

- ground insulation

- helium channel around cold bore (for temperatures above 2.16 K)

MQ – arc magnet,Tb=1.9 K


Heat flow limitsHeat flow limits


�bath temperature 1.9 K

- Transition HeII → HeI:helium channels are blocked = less effective heat evacuation due to the changing of heatevacuation path

�bath temperature 4.5K

- lower temperature margin (worst case: MQM0.45K)

- Helium channels does not play dominatingrole (heat conduction of He I and polyimide is the same order)

MQM – LSS magnet,Tb=1.9/4.5 K MQY – LSS magnet,Tb=4.5 K






– Network model

OutlineOutline


– Network model• Model construction• Model of the superconducting cable and coils

– Validation of the model– Steady state beam loss heat load simulation



• Summary

ROXIE

magnet field

distribution,

temperature

margin

TECHNICAL

DRAWINGS

detailed magnet

coil geometry

OTHER

non beam induced

heat sources

Hysteresis losses

Eddy currents, etc.

A. Verweij

R. Wolf

Contribution to the quench level

is order of 1-2%

Network Model Network Model

Model ConstructionModel Construction


HEAT FLOW

MODEL

MAGNET

QUENCH

LEVELS

FLUKA

beam loss profiles

Material properties

at low temperature

CRYODATA

MEASUREMENTS

model validation

Network Model Network Model



GROUND INSULATION

Courtesy G. Kirby

Network ModelNetwork Model



Network ModelNetwork Model

Helium in the Network Model


The volumes occupied by helium in the magnet are considered as:

-the narrow channels,

-semi-closed volumes = inefficient inlet of fresh helium.

The steady heat load, heat up the helium in the semi- closed volumes:

-Helium temperature well above critical temperature at Tb=4.5K

- Critical helium temperature reached already below the calculated quench limit

Network ModelCable modelling

µ-channel


Network ModelNetwork ModelCable modellingCable modelling


Network ModelNetwork ModelCoil modelCoil modelllinging







– Network model

OutlineOutline


– Network model– Validation of the model

• Measurements at the CERN test facility– Steady state beam loss heat load simulation



• Summary

heat

VALIDATION

measured

quench current

END

MAGNET

EXPERIMENT

Heat source

- quench heaters

- inner heating apparatus

ValidationValidation of of thethe Network MNetwork Modelodel


heat

predicted

quench currentHEAT SOURCE

MODEL

MAGNET

MODEL

MQY inner quench heater

3900

4000Ultimate current 3900 A

MQM magnets at 4.5 K

3700

3800

3900

4000

4100

4200

4300

4400

4500

4600

4700

25.00 30.00 35.00 40.00 45.00 50.00 55.00

P [mW/cm2]

I m

agne

t [A

]

MQM 627

MQM 677

MQMC 677

Ultimate current 4650 A

Nominal current 4310 A

ValidationValidation of of thethe modelmodel

MB magnet at 1.9K - Inner Heating Apparatus

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

I mag

net [

A]

measurements

simulation


2900

3000

3100

3200

3300

3400

3500

3600

3700

3800

9.00 11.00 13.00 15.00 17.00 19.00 21.00 23.00

P [mW/cm2]

I m

agne

t [A

]

MQY 609

MQM 659


MQY - outer quench heater

280029003000310032003300340035003600370038003900400041004200

25.00 30.00 35.00 40.00 45.00 50.00

P [mW/cm2]

I m

agne

t [A

]

MQY 609

MQM 659

Ultimate current 3900 A


2000

100.00 150.00 200.00 250.00 300.00 350.00

P [mW/cm2]

MQ magnet at 1.9 K - Inner Heating Apparatus

2000

4000

6000

8000

10000

12000

14000

30 35 40 45 50 55 60 65 70 75 80

P [mW/cm2]

Imag

net

[A]

Measurements - middle

Measurements - head






– Network model

OutlineOutline





• Summary

Quench limit simulationsQuench limit simulations

Heat deposition map in the MB dipole magnet coil� heat deposition map for nominal LHC ion beam intensity was created by interpolation of FLUKA data to the cable strand coordinates from ROXIE(Roderik)

�heat deposition map

W/m


Energy peak in the coil = 24.3 mW/mEnergy peak in the cold bore = 80.3 mW/m

ENERGY PEAK corresponds to the nominal LHC ion beam conditions (optics ver. 6.500)

�heat deposition map was implemented to Network Model

�the magnet current range from injection to ultimate values (761 A to 12840 A, nominal is 11850 A) was scanned by linear scaling of heat deposition map


Temperature map in the MB dipole magnet coil after heat load � temperature distribution for nominal LHC ion beam conditions, corresponding to 95% of loss energy peak in the coil (23.1 mW/m) and 95% loss energy peak in the coldbore (76.3 mW/m)

Ground insulationin the midplane

=


Peak temperature rise in the coil ∆T= 2.0 KPeak temperature rise in the cold bore ∆T=1.4K

For nominal LHC ion beam conditions (beam optics ver. 6.500)

� quenching cable is located at the coil mid-plane

�this temperature map corresponds to nominalmagnet current (11850 A)

= heat flow barrier


Energy peak in the coil = 24.3 mW/m and in the cold bore = 80.3 mW/mENERGY PEAK corresponds to the nominal LHC ion beam conditions







– Network model

OutlineOutline





• Summary

Extrapolation to other cases

• Redone tracking in v6.503, new FLUKA simulation

• Profile of power deposition in coil similar to v6.500 except global scaling factor. Scaling by integrated power:

0.020

0.025P HWêmL

0.020

0.025P HWêmL


• Gives approximate margin of 15% to quench limit in v6.503

• Question: what has changed ?

100 200 300 400 500strand no.

0.005

0.010

0.015

6.503

6.500

100 200 300 400 500strand no.

0.005

0.010

0.015

6.503 scaled by 1.2

6.500

9.5

10.0

mx

6.503

6.500

2000

3000

4000

bx HmL

6.500

Difference in optics• Phase advance after IP2 changed from v6.500 to v6.503

• Spot size larger - dependent on off-momentum β (calculated from starting conditions at IP2)

off-momentum β off-momentum µ

07/01/2009 R. Bruce, D. Bocian, AP Forum 35100 200 300 400 500 600 700s HmL

0.005

0.010

0.015

0.020

0.025

x HmL

6.503

6.500

300 320 340 360 380 400 420 440s HmL0

200

400

600

800

1000bx HmL

6.503

6.500

100 200 300 400s HmL8.5

9.0

6.503

100 200 300 400 500 600 700s HmL

10006.503

off-momentum β

central BFPP orbit

IP2 IP2impact

Squeeze, 5 TeV: preliminary result

• Tracking + FLUKA simulation for different β* and 5 TeV

=0.

5m

=0.

5m

=1.

1m

=5

m

=10

m

=0.

5m

,orb

.bu

mp

0.8

1.0

PêPquench


6.50

07

Te

Ve

q.b* =

0.5

6.50

37

Te

Ve

q.b* =

0.5

6.50

37

Te

Ve

q.b* =

1.1

6.50

37

Te

Veq.b

*=

6.50

37

Te

Ve

q.b*=

6.50

35

Te

Ve

q.b* =

0.5

6.50

07

Te

Veqb* =

0.5

m,

0.0

0.2

0.4

0.6

Possible alleviation methods• with orbit bump we could gain >factor 5:

– possible to introduce orbit bump around BFPP impact

– particles lost at second dispersion max, with larger off-momentum β

– nominal orbit shifted by 2-3.8 mm depending on optics

• cold collimators (R.W. Assmann et al):

– could be installed at a later stage

6σ envelope at IP2, no kicks


– could be installed at a later stage around IPs taking ion collisions

• at 5 TeV we gain a factor 3.5:

– lower field gives higher quench limit

– lower energy per BFPP particle

– larger geometric emittance gives larger spot size

– cross section only weakly energy dependent

• Increase of β*: not desired

6σ envelope at IP2, 4 kicks

Simulation uncertainties

• BFPP cross section: ~20%

• Changes in the optics (e.g. beta beating) could change the spot size: ~10%

• Network model: ~ 30%.


• On top of this, uncertainty on the energy deposition from the FLUKA simulation, could in worst case be a factor 2. Dominating uncertainty for this specific beam loss but could be less in other cases.






– Network model

OutlineOutline





• Summary

Summary

• To make a detailed calculation of the quench limit for a main dipole due to a specific beam loss mechanism (BFPP during LHC Pb82+ ion operation), we have combined

– particle tracking,

– a FLUKA shower simulation of the heat load in a single magnet and

– a thermal network simulation of the heat flow in the magnet


• BFPP creates one-electron Pb81+ ions at the IP, which follow an off-momentum orbit and are lost in the dispersion suppressor in the case of IP2.

• At nominal performance, the estimated heat load is expected to be very close to, and possibly above, the quench limit.

• For this loss distribution, the quench limit is a factor ~2 higher than calculated in LHC report 44 and LHC design report

• Possible alleviation methods include orbit bumps and cold collimators

Acknowledgements

We would like to thank people who have helped during the course of this work:

• A. Ferrari, M. Magistris and the rest of the FLUKA team

• E. Todesco, M. Lamm, G. Ambrosio


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NewNewLHC quench calculations and the LHC quench ... forum 07-01-2009 - N… · 01/07/2009 ·...

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