Dynamic Effects in LHCMagnets

LHC-MTA, LHC-MMS, SL-POPresented by L. Bottura

Prepared for the VLHC Annual Meeting Port Jefferson, NY, October 16-18, 2000

Overview

What are dynamic effects ? Why are they important ? What do we know about them ?

physics phenomenology

How will we deal with them at LHC ? Summary and ideas for the VLHC

Dynamic Effects

What are dynamic effects ?

Static Field Quality in LHC Magnets

0.705

0.7075

0.71

0.7125

0.715

0 5000 10000Current (A)

Tran

sfer

func

tion

(T/k

A)

MBP2N1

-8-6-4-202468

0 5000 10000Current (A)

b2 (u

nits

@ 1

7 m

m)

aperture 1

aperture 2

MBP2N1

-30

-20

-10

0

10

20

30

0 5000 10000Current (A)

b3 (u

nits

@ 1

7 m

m)

aperture 1

aperture 2

MBP2N1

geometric (linear)contributionT = 0.713 T/kA

persistent currentsδT = -0.6 mT (0.1 %)

partial compensationof persistent currentsat injection

1 % ironsaturation

systematic b2from two-in-one geometry

ironsaturation

Ramp Rate Dependent Effects

strands coupling currents eddy (coupling) currents in cables eddy currents in metallic components

beam screen wedges ...

Cable Coupling Currents - 1

Systematic effectsexpected on allowedmultipoles

B1, b3, b5, … in MB B2, b6, b10, … in MQ

for the Rc value in MTP1N2 see:R. Wolf, et al., IEEE Trans. Appl. Sup., 7 (2), 797,

1997

Normal sextupole during ramps

-3

-2

-1

0

1

2

0 2 4 6 8 10field (T)

B3 -B

3 geo

met

ric (G

auss

@ 1

0 m

m)

35 A/s50 A/s

10 A/s20 A/s

MTP1N2

Rc ≈ 6.7 µΩ

Random, non-negligible effectsexpected on non-allowed multipoles

e.g. b2, a2 in MB

for the ∆Rc value in MTP1N2 see:R. Wolf, et al., IEEE Trans. Appl. Sup., 7 (2), 797,

1997

Normal quadrupole during ramps

-8

-6

-4

-2

0

2

4

0 2 4 6 8 10field (T)

B2-B

2 geo

met

ric (G

auss

@ 1

0 m

m)

35 A/s50 A/s

10 A/s

20 A/s

MTP1N2

Cable Coupling Currents - 2

∆Rc ≈ 5 µΩ

-5

0

5

10

15

20

25

30

B1 A1 B2 A2 B3 A3 B4 A4 B5 A5 B6 A6 B7 A7

harmonic (-)

harm

onic

s at

ram

p (u

nits

@ 1

7 m

m)

Expected Ramp Rate Effects

MB bending dipolesField Quality WG, MB-99-02

MQ quadrupolesField Quality WG, MQ-99-07

Rc=15 µΩ, 10 A/s at injection

systematic rampharmonics

uncertainty andstatistical spread

-4

-2

0

2

4

6

8

10

B1 A1 B2 A2 B3 A3 B4 A4 B5 A5 B6 A6 B7 A7

harmonic (-)

harm

onic

s at

ram

p (u

nits

@ 1

7 m

m)

expected for seriesMBP1A1-A1MBP1A1-A2MBP2N1-A1MBP2N1-A2MBP2N2-A1MBP2N2-A2

Decay and Snap-back at Injection - 1

-100

-80

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-40

-20

0

20

0 500 1000current (A)

b3 (u

nits

@ 1

7 m

m)

MBP2N1

-10

-8

-6

-4

-2

0

700 750 800current (A)

b3 (u

nits

@ 1

7 m

m)

MBP2N1

Snap-backat the startof theaccelerationramp

decayduringinjectionMeasured b3 in MBP2N1

prototype dipole duringramp to injection and

subsequent energy ramp

Decay and Snap-back at Injection - 2

-10

-8

-6

-4

-2

0

0 5000 10000 15000time from start of injection (s)

b3 (u

nits

@ 1

7 m

m)

MBP2N1

decay during simulated10,000 s injection

exponential fitτi = 900 s

Decay and Snap-back at Injection - 3

snap-back fit:∆b3 [1-(I-Iinj)/∆I]3

∆b3= 3.7units∆I = 27A → ∆B = 19 mT

snap-backdecay

-2

-1

0

1

2

3

4

5

B1 A1 B2 A2 B3 A3 B4 A4 B5 A5 B6 A6 B7 A7

harmonic (-)

deca

y (u

nits

@ 1

7 m

m)

expected for seriesMBP1A1-A1MBP1A1-A2MBP2N1-A1MBP2N1-A2MBP2N2-A1MBP2N2-A2

0

1

2

3

B1 A1 B2 A2 B3 A3 B4 A4 B5 A5 B6 A6 B7 A7

harmonic (-)

deca

y (u

nits

@ 1

7 m

m)

Expected Decay and Snap-back

MB bending dipolesField Quality WG, MB-99-02

MQ quadrupolesField Quality WG, MQ-99-07

Dynamic Effects

Why are they important ?

Effect of an uncorrected ramp

During uncorrected ramps (Rc=15 µΩ, 10 A/s)

∆b1(MB)= 5.4 → ∆Q = 0.054 vs. 0.003

∆b2(MQ)= 17 → ∆Q = 54 ∆b210-4= 0.09 vs. 0.003

∆b3(MB)= 1.0 → ∆ξ = 52 ∆b3 = 52 vs. 1

(source: O. Bruening, SL-AP)

During uncorrected snap-back

∆b1(MB)= 2.6 → ∆Q = 0.026 vs. 0.003

∆b2(MQ)= 1.7 → ∆Q = 54 ∆b210-4= 0.009 vs. 0.003

∆b3(MB)= 3.3 → ∆ξ = 52 ∆b3 = 172 vs. 1

(source: O. Bruening, SL-AP)

Effect of an uncorrected snap-back

Dynamic Effects

What do we know about them ?

Physics of Coupling Currents

Physical model for strand/cable couplingavailable

A. Devred, T.Ogitsu, CERN 96-03, 1996

Can be controlled at the strand/cable productionlevel: LHC target interstrand resistance Rc > 20 µΩ ± 5 µΩ obtained through controlled coating and accelerated

oxidation

Reproducible → measured on 100 % ofmagnets

Basic understanding of physics principleavailable: flux-creep (accounts for 10 % … 30 % of effect) interaction between cable transport current re-

distribution and filaments magnetizationL. Bottura, et al., Field Errors Decay and "Snap-Back" in LHC Model Dipoles, IEEE Trans. Appl Sup., 7(2), 602,

1997R. Wolf, The Decay of the Field Integral in SC Accelerator Magnets Wound with Rutherford Cables, Proc. of

15th Mag. Techn. Conf., Beijing, Oct. 20-24, 1997

Cannot be controlled at production

Not reproducible → measured on 110 % ofmagnets

Physics of Decay and SB - 1

Physics of Decay and SB - 2

The current distribution isnot uniform in the cables Supercurrents, BICC’s joints …

The current distributionchanges in time , causinga variable rotating field

Bself

Bext

non-uniform dB/dt

transport + loop currentloop

current

Physics of Decay and SB - 3

Magnetization of a typical LHC strand

-0.03-0.02-0.01

00.010.020.030.040.050.060.07

0 0.2 0.4 0.6 0.8 1field (T)

M (T

)

+∆B−∆B

The field changeaffects themagnetization Mof the SC strand

Average M drops decay

Net decrease ofmagnetization

The magnetization state is re-established as soon as thebackground field is increased snap-back

Physics of Decay and SB - 4

The background field change necessary is of thesame order of the internal field change in thecable ≈ 100 A change in current imbalance ≈ 10 mT average internal field change (vs. 5…20 mT

measured)

B

Physics of Decay and SB - 5

-0.008

-0.006

-0.004

-0.002

0

0.48 0.5 0.52 0.54B (T)

M (T

)

measured

computed

Copper strands

NbTi strand Demonstration experimentat Twente University.Courtesy of M. Haverkamp

ICurrent & Duration

Snapback ?

Qt t

IPre Injection Duration & Current

Snapback ?

Measurements of Decay and SB - 1

and (many) others:Number of pre-cyclesQuenchRamping speed…

Parameters affecting decay and SB

Measurements of Decay and SB - 2

0

20

40

60

80

100

deca

y (%

)

nomina

l4 k

A flat-to

p

1 minu

te fla

t-top

30 m

in pre

-injec

tion

15 m

in inj

ectio

nqu

ench

operation variant

Measured on a smallseries (≈10) of 1-mLHC model dipoles

Large spread (1 orderof magnitude)depending on thepowering history andconditions

1000 3700 64009100 11750

3600

900

300

60

-3.00E-01

-2.50E-01

-2.00E-01

-1.50E-01

-1.00E-01

-5.00E-02

0.00E+00

b3 S

napb

ack

(uni

ts @

Rre

f = 1

0 m

m)

Flat Top Current (A)

Flat Top Duration (s)

b3 Snapback as function of Flat Top Current & DurationMBSMS 12V1

Measurements of Decay and SB - 3

Space of parameters: pre-injection duration injection duration flat-top current flat-top time magnet temperature ramp-rates …too large for series

measurements

Modelling of Decay and SB

Flat Top Duration InfluenceMBSMS5V1

-0.182704

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0 500 1000 1500 2000 2500

Flat Top Duration (s)

b3 -

Snap

back

(uni

ts)

MeasurementNeuronAnalytic

Flat Top Duration t @ 11750 A

Injection

Error Plot

-30%

-20%

-10%

0%

10%

0 500 1000 1500 2000

Pre Cycle Duration (s)

Rel

ativ

e Er

ror (

%)

AnalyticNeuron

Analytical modelaccurate to 30 %

Neural networkaccurate to 5 %

Dynamic Effects

How will we deal with them ?

Optimized ramp to minimize effects Cycling policy to guarantee reproducibility Feed-forward from the LHC magnetic reference Feed-forward from previous operating cycles Feed-back from on-line (BI) measurements

Control of Dynamic Effects

Optimized Ramp

0

2000

4000

6000

8000

10000

12000

-4000 -2000 0 2000 4000

time from start of injection (s)

dipo

le c

urre

nt (A

) energyramp

preparationand access

beamdump

injectionphase

injection

pre-injection

I ≈ t2

I ≈ et

I ≈ t

A. Faus-Golfe, LHC Project Note 9, 1995.

L. Bottura, P. Burla, R. Wolf, LHC ProjectReport 172, 1998.

coast coast

The LHC Magnetic Reference

Machine Operating

Conditions: I, dI/dt, T

Machine OperatingHistory:

I(-t), dI/dt(-t), T(-t)

B1, B2,angle,

multipoles

MultipolesFactory

Courtesy of Q. King

Inside the Multipoles Factory

dataBase tablesfrom seriesmeasurements on100 % of magnets

dataBase tablesfrom seriesmeasurements on10 % of magnets

machineoperatingconditions:Ι, dΙ/dt, T

machine poweringhistory:Ι(-t), dΙ/dt(-t), T(-t)

multipoles fromreferencemagnets

multipoles from BI:tune (b2),chromaticity (b3)

linear physicalmodel of

reproducibleeffects

non linearmodel of decayand snap back

non linearadjustment for

actualpoweringconditions

B1, B2, angle, multipoles

Reference Magnets Control Interface

C

Gateway MultipolesFactory

DB

ISM18 MagnetTest Benches

WorldFIPfieldbus

Real-time LHC controls network

FBPowerConverter

Real-TimeLHC Control

System

Instrumented Magnet

3-10Hz

Courtesy of Q. King

Dynamic Effects

Summary and ideas for a VLHC

WGs, Workshops, Seminars !

Working and study groups Dynamic Effects Working Group (active 3 yrs, now dormant) Interdivisional LHC Controls Project (active since early 2000) Machine Commissioning Committee (planned)

International Workshops and seminars Seminars on Dynamic Effects in Super-Conducting Magnets and

their Impact on Machine Operation, October 6th, 1995. LHC Workshop on Dynamic Effects and their Control, February 5th

to 7th, 1997. LHC Controls-Operation Forum, December 1st-2nd, 1999.

vital to understanding, involvement and planning

Open Issues

Reproducibility cycle-to-cycle ? Spread among octants ?

5 cable and 3 magnet manufacturers

Accuracy of predictive scalings ? assume 80 % for the moment, 20 % residual error

A deterministic model of decay and snap-backseems to be out of reach...

Perspective for LHC

Treasured TeV and HERA experience Physics principle of decay and SB assessed, a

working empirical scaling available 100 % cold measurements Involvement of machine control and operation

teams for early integration Sector test (early 2004) can verify conceptual

design of machine control

5 years to go before the first p is injected !

Ideas for VLHC - 1-1

.2-1

-0.8

-0.6

-0.4

-0.2

0

0 500 1000 1500 2000time (s)

b3 (u

nits

@ 1

0 m

m)

0.05 A/s

0.5 A/s 0.25 A/s0.1 A/s

A slow ramp out of injection can help...

dξ/dt ≈ 2 units/sdξ/dt ≈ 0.2 units/s

magnetization loss due to on-going current diffusion in SCcable…

… vs. magnetization recoverydue to field sweep…

… causes a decrease in SBamplitude !

Ideas for VLHC - 2-2

-10

12

0 0.5 1field (T)

b3 (u

nits

@ 1

0 m

m)

-1.5

-0.5

0.5

0.55 0.6 0.65field (T)

b3 (u

nits

@ 1

0 m

m)

A snapback-free injection and acceleration start continuous B1 ramp, injection on-the-fly ∆B1 ≈ 15 mT

B1 ramp

“standard”