Orbit effect from main dipoles in SPS at high energy

K. Cornelis

Stephane Cettour Cave

Orbit after alignment at 6000ms (FT optics)

• Plan H RMS= 1.608• Plan V RMS= 1.226

The B-field in SPS dipoles

Magnetising field from windings (depends on geometry of windings)

Field due to polarisation in the yoke (depends on geometry of the yoke, composition of the yoke, and field)

Yoke and windings geometry such that best field quality at 300 GeV.

SPS dipoles

-1000 0 1000 2000 3000 4000 5000 6000 70000

0.5

1

1.5

2

2.5

Bfield

current (A)

Fiel

d

450 GeV

300 GeV

Static fields of dipoles• Yoke composition as homogenous as can be

(laminations of special melt).

• Magnets measured and shimmed to give the correct at 1T (~220GeV).

• Different places of the yoke will come to saturation at different magnetising fields, resulting in a change of geometry of the fields. In identical magnets this change should be the same but, e.g. shims have a different effect then the yoke when in saturation.

• From about 2004 until about 2010, uncorrected dipoles have been installed. Recent beam based measurements with uncorrected C- coated magnets showed errors from 10 to 25 mrad

-1000 0 1000 2000 3000 4000 5000 6000 70000

0.5

1

1.5

2

2.5

Bfield

current (A)

Fiel

d

Dynamic effects from induced currents in vacuum chamber

dB/dt

B ~ -dB/dt

B

x

Chromaticity effect from Eddy Currents

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000-0.2

00.20.40.60.8

11.21.41.6

Horizontal chroma error from eddy currents in a cngs cycle

Cycle time

Chro

mati

cyty

corr

ectio

n

0 1000 2000 3000 4000 5000 60000

0.10.20.30.40.50.60.7

horizontal chroma error from eddy currents in pilot cycle.

Cycle time

chro

mati

city

com

pens

ation

Dipole error from decaying eddy currents

0 200 400 600 800 1000 1200 1400

-2

-1.5

-1

-0.5

0

0.5

1

Radi

al p

ositi

on

Time in cycle

2000 4000 6000 8000 10000 12000 14000

-3

-2

-1

0

1

2

3

4

5

0 2000 4000 6000 8000 10000 12000 140000

1000

2000

3000

4000

5000

6000

7000

LHC fast

Amplitude and decay time depend on shape of vacuum chamber, thickness and specific resistance of material.

Effect : 6 mrad/magnet at 26 GeV0.35 mrad/magnet at 450 GeV

If magnet differences are at the % level, the orbit kick is less than .05mrad/magnet at 450 GeV

Possible sources of “local” errors

Weird vacuum chambers

Possible sources of “local” errors: Earth loops

Earth loops around magnets can enhance the field suppression

Short between spires in the coil gives a strong field supression proportional to dB/dt

Normally the short circuit gets worse rapidly and the orbit becomes so bad that the beam is destroyed and the magnet has to be changed

In position 3.26 a temperature dependent dB/dt kick could be observed 10

/8/2

012

20:2

4

10/8

/201

2 21

:36

10/8

/201

2 22

:48

10/9

/201

2 0:

00

10/9

/201

2 1:

12

10/9

/201

2 2:

24

10/9

/201

2 3:

36

10/9

/201

2 4:

48

10/9

/201

2 6:

00

10/9

/201

2 7:

12

-160

-140

-120

-100

-80

-60

-40

-20

0

Kick

at m

ax d

B/dt

/B

Time after restart

The problem is there since October but it does not get worse.

Summary and Conclusions• In the horizontal plane, the orbit at high energy is not only due to

misalignment, but results also from dipole errors.• During several years the shimming of dipoles was neglected resulting in

horizontal orbit kicks of a few tens or micro radians. • The errors due to eddy currents in the vacuum chamber are negligible

at 450 GeV.• Earth loops around magnets can make a bigger effect but it is difficult to

know how much. We have no total picture of the state of the machine. The bad earth connections will be removed during LS1.

• These errors are normally stable in time.• Short circuit between windings can give huge kicks, but it normally kills

the magnet in a short time. However, it can not be excluded that very weak shorts stay stable and vary slowly in time.