Observations during stable beams (follow up: sudden beam ... · 2.2e+11 2.25e+11 2.3e+11 2.35e+11 2.4e+11 2.45e+11 2.5e+11 2.55e+11 2.6e+11 01:31:00 01:40:00 01:48:00 01:56:00 02:05:00

Observations during stable beams

(follow up: sudden beam losses, very preliminary ...)

to trigger discussions and possible strategies

W. Herr (ABP-CLIC), E. Laface (ABP)

Conditions

Conditions:

Squeeze to 3.5m with separation bumps on

Crossing angles in all IPs

Basic filling scheme: 3 bunches per beam,

≈ 1011/bunch

slots: 1 - 895 - 1786 / 1 - 892 - 1786

buckets: 1(3) - 8941(2) - 17851(3) / 1(3) -

8911(2) - 17851(3)

6, 7, 12, 13, ... bunches are derived from this

usually a tune split applied

Side-effects of these conditions

Unequal collision scheme:

Bunches 1, 1786: 3 head on collisions

Bunches 895/892: 2 head on, 1 long range collisions

Long range separation not very large, depends on

emittance, β-beating

Rather different tune spread and shift

A few observations

Sudden beam losses observed during a fill (very fast and

not reproducible, makes analysis difficult)

Adding a witness bunch (7x7) indicates that loss is

beam-beam related.

Here some of the observations

Discuss and test some of the possible causes

Very first occasion: Sunday, 20.6.

1.4e+11

1.5e+11

1.6e+11

1.7e+11

1.8e+11

1.9e+11

2e+11

15:00:00 15:33:00 16:06:00 16:40:00 17:13:00 17:46:00 3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11

11.5

12

12.5

Beam Cur

rent

Normalize

d Emittan

ce [1e-6]

Time

Beam 1 BCTBeam 1 Horizontal Emittance SL

Beam 1 Vertical Emittance SLBeam 1 Horizontal Emittance WS

Beam 1 Vertical Emittance WS

1.4e+11

1.5e+11

1.6e+11

1.7e+11

1.8e+11

1.9e+11

2e+11

15:00:00 15:33:00 16:06:00 16:40:00 17:13:00 17:46:00 3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11

11.5

12

12.5

Beam Cur

rent

Normalize

d Emittan

ce [1e-6]

Time


Beam 2 Vertical Emittance SLBeam 2 Horizontal Emittance WS

Beam 2 Vertical Emittance WS

Small loss on beam 2 with emittance increase, followed

by strong loss on beam 1

Note: emittance increase seen by wire scanners too

large

Friday, 25.6.

1.2e+11

1.4e+11

1.6e+11

1.8e+11

2e+11

2.2e+11

2.4e+11

2.6e+11

01:06:00 01:40:00 02:13:00 02:46:00 03:20:00 03:53:00 3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11

11.5

12

12.5

Beam Cur

rent

Normalize

d Emittan

ce [1e-6]

Time


Beam 1 Vertical Emittance SL

1.2e+11

1.4e+11

1.6e+11

1.8e+11

2e+11

2.2e+11

2.4e+11

2.6e+11

01:06:00 01:40:00 02:13:00 02:46:00 03:20:00 03:53:00 3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11

11.5

12

12.5

Beam Cur

rent

Normalize

d Emittan

ce [1e-6]

Time


Beam 2 Vertical Emittance SL

Simultaneous loss in both beams together with

emittance increase (vertical)

Note: happened about 1 min after a lumi scan

Saturday, 26.6.

0.26

0.27

0.28

0.29

0.3

0.31

0.32

0.33

11:23:00 11:40:00 11:56:00 12:13:00 12:30:00 12:46:00

Tune

Time

Beam 1 Horizontal TuneBeam 1 Vertical Tune

0.285

0.29

0.295

0.3

0.305

0.31

0.315

0.32

0.325

11:23:00 11:40:00 11:56:00 12:13:00 12:30:00 12:46:00

Tune

Time


No intensity loss, but strong tune change during

collapse ? Locked on some mode ?

If real, tune difference very large for coherent

beam-beam modes (unless ..)

Sunday, 27.6.

2.2e+11

2.25e+11

2.3e+11

2.35e+11

2.4e+11

2.45e+11

2.5e+11

2.55e+11

2.6e+11

01:31:00 01:40:00 01:48:00 01:56:00 02:05:00 02:13:00 02:21:00 0.5

1

Beam Cur

rent

Beam Size

Time

Beam 1 BCTBeam 1 Horizontal Size

Beam 1 Vertical Size

2.2e+11

2.25e+11

2.3e+11

2.35e+11

2.4e+11

2.45e+11

2.5e+11

2.55e+11

2.6e+11

01:31:00 01:40:00 01:48:00 01:56:00 02:05:00 02:13:00 02:21:00 0.5

1

Beam Cur

rent

Beam Size

Time



Modest loss and emittance increase on beam 2

Something is happening on the beam before the second

loss

What happened to the tunes ?

Sunday, 27.6.

0.3

0.302

0.304

0.306

0.308

0.31

0.312

0.314

0.316

0.318

0.32

0.322

01:31:00 01:40:00 01:48:00 01:56:00 02:05:00 02:13:00 02:21:00

Tune

Time


0.3

0.302

0.304

0.306

0.308

0.31

0.312

0.314

0.316

0.318

0.32

0.322

01:31:00 01:40:00 01:48:00 01:56:00 02:05:00 02:13:00 02:21:00

Tune

Time


Strong fluctuation of peak signal on beam 2 before the

loss, quiet after .., additional modes, tune split, different

bunches ?

Any other observation ?

Sunday, 27.6.

1.32e-09

1.34e-09

1.36e-09

1.38e-09

1.4e-09

1.42e-09

1.44e-09

1.46e-09

1.48e-09

01:31:00 01:40:00 01:48:00 01:56:00 02:05:00 02:13:00 02:21:00

Bunch L

ength [

s]

Time

Beam 1 Mean Bunch LengthBeam 2 Mean Bunch Length

Bunch length of beam 2 slightly smaller, observed again

later

Tuesday, 29.6.

2.9e+11

2.95e+11

3e+11

3.05e+11

3.1e+11

3.15e+11

3.2e+11

13:20:00 13:28:00 13:36:00 13:45:00 13:53:00 14:01:00 14:10:00 14:18:00 0.5

1

Beam Cur

rent

Beam Size

Time



2.9e+11

2.95e+11

3e+11

3.05e+11

3.1e+11

3.15e+11

3.2e+11

13:20:00 13:28:00 13:36:00 13:45:00 13:53:00 14:01:00 14:10:00 14:18:00 0.5

1

Beam Cur

rent

Beam Size

Time



Similar observation, happened again a few minutes after

a lumi scan

Tuesday, 29.6.

0.3

0.302

0.304

0.306

0.308

0.31

0.312

0.314

0.316

0.318

0.32

0.322

13:20:00 13:28:00 13:36:00 13:45:00 13:53:00 14:01:00 14:10:00 14:18:00

Tune

Time


0.285

0.29

0.295

0.3

0.305

0.31

0.315

0.32

0.325

13:20:00 13:28:00 13:36:00 13:45:00 13:53:00 14:01:00 14:10:00 14:18:00

Tune

Time


Tunes: not very clear signature

Tuesday, 29.6.

1.36e-09

1.37e-09

1.38e-09

1.39e-09

1.4e-09

1.41e-09

1.42e-09

1.43e-09

1.44e-09

1.45e-09

13:20:00 13:28:00 13:36:00 13:45:00 13:53:00 14:01:00 14:10:00 14:18:00

Bunch L

ength [

s]

Time


Small change of bunch length on one beam

Thursday, 1.7.

2.25e+11

2.3e+11

2.35e+11

2.4e+11

2.45e+11

2.5e+11

2.55e+11

2.6e+11

2.65e+11

06:40:00 06:48:00 06:56:00 07:05:00 07:13:00 07:21:00 07:30:00 0

0.5

1

1.5

Beam C

urrent

Beam S

ize

Time



Beam 1 only, well known signature

Observation: large luminosity drop in ATLAS and CMS

(much less in ALICE, LHCb) about 5 min before loss,

(reconstructed beam size increase in both planes)

1.7.

0.27

0.28

0.29

0.3

0.31

0.32

0.33

0.34

0.35

0.36

0.37

06:40:00 06:48:00 06:56:00 07:05:00 07:13:00 07:21:00 07:30:00

Tune

Time


0.295

0.3

0.305

0.31

0.315

0.32

06:40:00 06:48:00 06:56:00 07:05:00 07:13:00 07:21:00 07:30:00

Tune

Time


Tunes: very clear signal during time of the loss

1.7.

1.3e-09

1.32e-09

1.34e-09

1.36e-09

1.38e-09

1.4e-09

1.42e-09

1.44e-09

1.46e-09

06:40:00 06:48:00 06:56:00 07:05:00 07:13:00 07:21:00 07:30:00

Bunch L

ength [

s]

Time


Small change of bunch length on one beam

What could be the reason ?

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Is it related to beam-beam ?

Yes, but how ... (cause versus symptom)

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Unequal collision pattern and emittances ?

Test with collision scheme equal number of collisions for

all bunches (10x10, but should have nominal intensities)

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Contribution of the (single) long range encounter ?

Test with equal number of collisions and no long range

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Reduction of dynamic aperture due to beam-beam ?

No ! Should cause bad life time and be worse after

emittance increase


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Unequal beam sizes (remember SPS) ?

Very unlikely, should lead to bad lifetime, but not to a

sudden loss

β-beating: as above, except for long range interaction

What if emittances are smaller that we think: strong

coherent dipole kicks when beams are moving, check !

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Coherent beam-beam, self exciting ?

Very unlikely with unequal bunches, but cannot

exclude, should never happen with tune split

(Note bene: 3rd order can drive coherent beam-beam,

for a while)


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(Small) excitation from outside source (hump ? not

always present) ?

Any excitation of one beam can (will) cause troubles

when beams are in collision (are all gadets off ?)

Remember: we have basically nominal intensity, i.e.

head-on beam-beam

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Any RF noise (e.g. phase) ?

Possible, needs to be studied


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Loss of Landau damping ?

Always possible, but requires collective motion (open a

big hole) and change of damping

Behaviour seems to be far too ”reproducible” for that

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Effect of crossing angle ?

Possible, if something moves the beam longitudinally

(RF noise)

Test without crossing angle (filling scheme) and maybe

with RF

Summary I

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Behaviour not yet understood

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Some candidates, need to be tested, ideally under

simplified conditions

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Beam-beam yes, but unlikely to be the cause, rather the

executioner ..

Summary II

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Recommend to test some of the possible culprits (first

attempts with 10 bunches)

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Transient dipole kick difficult to avoid, should try with

FB, possibly at end of fill first

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Must try to understand, retreat to ”working” scenario

not a long term solution

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For equal number of collisions per bunch and per IP, we

pay a big price

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Follow up continues ..

Documents

Observations during stable beams (follow up: sudden beam ... · 2.2e+11 2.25e+11 2.3e+11 2.35e+11 2.4e+11 2.45e+11 2.5e+11 2.55e+11 2.6e+11 01:31:00 01:40:00 01:48:00 01:56:00 02:05:00