Beam scrubbing: specifications of beams and transverse stability considerations

G. Iadarola, H. Bartosik, N. Mounet, G. Rumolo

Many thanks to:T. Argyropoulos, T. Bohl, S. Cettour Cave, K. Cornelis, H. Damerau,

J. Esteban Muller, F. Follin, S. Hancock, W. Hofle, C. Lazaridis, L. Kopylov , H. Neupert, Y. Papaphilippou, B.Salvant, E. Shaposhnikova, M. Taborelli, C. Zannini

and the SPS operator crew

Outline

• Electron cloud and scrubbing at the SPS

• A “doublet” scrubbing beam for the SPSo Simulation studieso First tests at the SPS

• Stability considerationso First observationso e-cloud driven instabilitieso Impedance driven instabilities

G. Arduini, K. Cornelis et al.

400%

Electron cloud and scrubbing at the SPS

• In the past e-cloud has been strongly limiting the performances with LHC beams with 25 ns spacing (detrimental effects both on vacuum and beam quality)

• Scrubbing runs regularly performed over the years with evident beneficial effects on dynamic pressure rise and beam quality

• Status in 2012: No degradation due to e-cloud for nominal beam parameters. Emittance blow up observed on trailing bunches of the last batches for larger bunch population

2000 (48 b. - 0.8x1011 ppb @inj.)

G. Arduini, K. Cornelis et al.

400%

Electron cloud and scrubbing at the SPS

• In the past e-cloud has been strongly limiting the performances with LHC beams with 25 ns spacing (detrimental effects both on vacuum and beam quality)

• Scrubbing runs regularly performed over the years with evident beneficial effects on dynamic pressure rise and beam quality

• Status in 2012: No degradation due to e-cloud for nominal beam parameters. Emittance blow up observed on trailing bunches of the last batches for larger bunch population

2000 (48 b. - 0.8x1011 ppb @inj.)

G. Arduini, K. Cornelis et al.

400%

Electron cloud and scrubbing at the SPS

• In the past e-cloud has been strongly limiting the performances with LHC beams with 25 ns spacing (detrimental effects both on vacuum and beam quality)

• Scrubbing runs regularly performed over the years with evident beneficial effects on dynamic pressure rise and beam quality

• Status in 2012: No degradation due to e-cloud for nominal beam parameters.

• Emittance blow up observed on trailing bunches of the last batches for larger bunch population

2000 (48 b. - 0.8x1011 ppb @inj.)

2012 (288 b. - 1.35x1011 ppb @inj.)

G. Arduini, K. Cornelis et al.

400%

Electron cloud and scrubbing at the SPS

• In the past e-cloud has been strongly limiting the performances with LHC beams with 25 ns spacing (detrimental effects both on vacuum and beam quality)

• Scrubbing runs regularly performed over the years with evident beneficial effects on dynamic pressure rise and beam quality

• Status in 2012: No degradation due to e-cloud for nominal beam parameters. Emittance blow up observed on trailing bunches of the last batches for larger bunch population

2000 (48 b. - 0.8x1011 ppb @inj.)

2012 (288 b. - 1.45x1011 ppb @inj.)

2012 (288 b. - 1.35x1011 ppb @inj.)

Scrubbing runs at the SPS

• ~1-2 weeks periods (typically once per year) devoted to condition the beam chambers (i.e. lower the Secondary Electron Yield) by means of the electron cloud itself

• Scrubbing is performed at the injection energy for the LHC type beams (26 GeV) in cycling mode refilling the machine every ~40 s

• The achievable dose rate is typically limited by heating and/or outgassing on some sensitive machine elements (e.g. kickers, septa, beam dumps)

Scrubbing runs at the SPS

• ~1-2 weeks periods (typically once per year) devoted to condition the beam chambers (i.e. lower the Secondary Electron Yield) by means of the electron cloud itself

• Scrubbing is performed at the injection energy for the LHC type beams (26 GeV) in cycling mode refilling the machine every ~40 s

• The achievable dose rate is typically limited by heating and/or outgassing on some sensitive machine elements (e.g. kickers, septa, beam dumps)

43.2 s

Scrubbing runs at the SPS

• ~1-2 weeks periods (typically once per year) devoted to condition the beam chambers (i.e. lower the Secondary Electron Yield) by means of the electron cloud itself

• Scrubbing is performed at the injection energy for the LHC type beams (26 GeV) in cycling mode refilling the machine every ~40 s

• The achievable dose rate is typically limited by heating and/or outgassing on some sensitive machine elements (e.g. kickers, septa, beam dumps)

43.2 s

Beam requirements for scrubbing

• The beam parameters need to be chosen in order to maintain a strong e-cloud flux on the chamber’s wall

• Integrated dose is more important than peak flux need for a reliable operation, reduce stress on sensitive machine elements

• Beam quality requirements less tight than for the LHC filling but only as long as it does not compromise the scrubbing efficiency

Beam requirements for scrubbing

• The beam parameters need to be chosen in order to maintain a strong e-cloud flux on the chamber’s wall

• Integrated dose is more important than peak flux need for a reliable operation, reduce stress on sensitive machine elements

• Beam quality requirements less tight than for the LHC filling but only as long as it does not compromise the scrubbing efficiency

Beam requirements for scrubbing

• The beam parameters need to be chosen in order to maintain a strong e-cloud flux on the chamber’s wall

• Integrated dose is more important than peak flux need for a reliable operation, reduce stress on sensitive machine elements

• Beam quality requirements less tight than for the LHC filling but only as long as it does not compromise the scrubbing efficiency

2006 Scrubbing Run

E. Benedetto et al.

Outline

• Electron cloud and scrubbing at the SPS

• A “doublet” scrubbing beam for the SPSo Simulation studieso First tests at the SPS

• Stability considerationso First observationso e-cloud driven instabilitieso Impedance driven instabilities

Why do we need a dedicated “scrubbing beam”?

1 1.2 1.4 1.6 1.810

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[C

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Why do we need a dedicated “scrubbing beam”?

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10-1

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101

SEY

EC

cu

rren

t (E

e>

50eV

) [m

A/m

]

MBB – 25ns beam

What do we need?

What do we have?

Why do we need a dedicated “scrubbing beam”?

Possible issue:

• The beam is still degraded due to EC

• The dose is not sufficient to continue scrubbing in a reasonable time

Possible solution:

• A “scrubbing beam” which exhibits a lower multipacting threshold

• The 25 ns beam is the ideal scrubbing beam for the 50 ns beam

• What could we used to scrub for the 25 ns beam?

1 1.2 1.4 1.6 1.810

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10-2

10-1

100

101

SEY

EC

cu

rren

t (E

e>

50eV

) [m

A/m

]

MBB – 25ns beam

What do we need?

What do we have?

Why do we need a dedicated “scrubbing beam”?

Possible issue:

• The beam is still degraded due to EC

• The dose is not sufficient to continue scrubbing in a reasonable time

Possible solution:

• A “scrubbing beam” which exhibits a lower multipacting threshold

• The 25 ns beam is the ideal scrubbing beam for the 50 ns beam

• What could we used to scrub for the 25 ns beam?

1 1.2 1.4 1.6 1.810

-6

10-5

10-4

10-3

10-2

10-1

100

SEY

Req

. scr

ub

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g d

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[C

/mm

2 ]

1 1.2 1.4 1.6 1.810

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10-3

10-2

10-1

100

101

SEY

EC

cu

rren

t (E

e>

50eV

) [m

A/m

]

MBB – 25ns beam

Scru

bbin

g be

am

What do we need?

What do we have?

Why do we need a dedicated “scrubbing beam”?

Possible issue:

• The beam is still degraded due to EC

• The dose is not sufficient to continue scrubbing in a reasonable time

Possible solution:

• A “scrubbing beam” which exhibits a lower multipacting threshold

• The 25 ns beam is the ideal scrubbing beam for the 50 ns beam

• What could we used to scrub for the 25 ns beam?

1 1.2 1.4 1.6 1.810

-6

10-5

10-4

10-3

10-2

10-1

100

SEY

Req

. scr

ub

bin

g d

ose

[C

/mm

2 ]

1 1.2 1.4 1.6 1.810

-4

10-3

10-2

10-1

100

101

SEY

EC

cu

rren

t (E

e>

50eV

) [m

A/m

]

MBB – 25ns beam

Scru

bbin

g be

am

What do we need?

What do we have?

A “doublet” scrubbing beam for the SPS

• Due to RF limitations in the PS, impossible to inject bunch to bucket with spacing shorter than 25 ns

• A shorter “effective spacing” can be obtained injecting long bunches from the PS and capturing each bunch in two neighboring buckets train of doublets

0 10 20 30 40 50 60 70

Lo

ng

. be

am p

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0 10 20 30 40 50 60 70Time [ns]

E

A “doublet” scrubbing beam for the SPS

• Due to RF limitations in the PS, impossible to inject bunch to bucket with spacing shorter than 25 ns

• A shorter “effective spacing” can be obtained injecting long bunches from the PS and capturing each bunch in two neighboring buckets

0 10 20 30 40 50 60 70Lo

ng

. be

am p

rofi

le

0 10 20 30 40 50 60 70Time [ns]

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. be

am p

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0 10 20 30 40 50 60 70Time [ns]

E

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Lo

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. be

am p

rofi

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0 10 20 30 40 50 60 70Time [ns]

E

A “doublet” scrubbing beam for the SPS

0 10 20 30 40 50 60 70Lo

ng

. be

am p

rofi

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0 10 20 30 40 50 60 70Time [ns]

E

0 10 20 30 40 50 60 70Lo

ng

. be

am p

rofi

le

0 10 20 30 40 50 60 70Time [ns]

E

• Due to RF limitations in the PS, impossible to inject bunch to bucket with spacing shorter than 25 ns

• A shorter “effective spacing” can be obtained injecting long bunches from the PS and capturing each bunch in two neighboring buckets train of doublets

25 ns 25 ns

Why should it work?

• Mechanism of e-cloud enhancement

10 20 30 40 50 60 70

0.5

1

1.5

2x 10

11

Bea

m p

rof.

[p

/m]

10 20 30 40 50 60 701

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

Time [ns]

Ne /

Ne(0

)

Below the threshold all the electrons produced after a bunch passage are absorbed before the next one small accumulation over subsequent bunch passages

PyECLOUD simulation

Std 25 ns beam

Why should it work?

• Mechanism of e-cloud enhancement

10 20 30 40 50 60 70

0.5

1

1.5

2x 10

11

Bea

m p

rof.

[p

/m]

10 20 30 40 50 60 701

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

Time [ns]

Ne /

Ne(0

)

More e- production and shorter e- decay accumulation possible

PyECLOUD simulation

Std 25 ns beam

Doublet beam

Simulation study

• The doublet beam shows a lower multipacting threshold compared to the standard 25 ns beam if the intensity is larger than 0.8e11ppb (1.6e11ppb from the PS)

-0.02 -0.01 0 0.01 0.020

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0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppb1.20e11ppb6btc 25ns1.20e11ppb

Intensity per bunch of

the doublet (b.l. 4 ns)

(b.l. 3 ns)

1.15 1.2 1.25 1.3 1.35 1.4 1.4510-6

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10-2

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102

SEY

Sc

rub

bin

g d

ose

(5

0eV

) [m

A/m

] MBB - 26GeV

PyECLOUD simulation

-0.02 -0.01 0 0.01 0.020

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Position [m]

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Simulation study

• The scrubbed region is smaller to be used, with radial steering, as a last stage of the scrubbing

MBB - 26GeV PyECLOUD simulation

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Position [m]

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[A/m

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0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppb1.20e11ppb6btc 25ns1.20e11ppb

Intensity per bunch of

the doublet (b.l. 4 ns)

(b.l. 3 ns)

First tests at the SPS

First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement

• The production scheme has been successfully tested for a train of (2x)72 bunches with 1.7e11 p per doublet

42 660

1

2

3

Time [ms]

200

MH

z R

F V

olta

ge [

MV

]

4

-10

1st inj.

Thanks to T. Argyropoulos and J. Esteban Muller

First tests at the SPS

First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement

• The production scheme has been successfully tested for a train of (2x)72 bunches with 1.7e11 p per doublet

42 660

1

2

3

Time [ms]

200

MH

z R

F V

olta

ge [

MV

]

4

-10

1st inj.

First tests at the SPS

First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement

• The production scheme has been successfully tested for a train of (2x)72 bunches with 1.7e11 p per doublet

42 660

1

2

3

Time [ms]

200

MH

z R

F V

olta

ge [

MV

]

4

-10

1st inj.

0.92 0.94 0.96 0.98 1 1.02-0.02

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0.02

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0.06

Time [s]

Bea

m p

rofi

le [

a.u

.]T

urn

0.92 0.94 0.96 0.98 1 1.02

100

200

300

400

500

Thanks to T. Argyropoulos and J. Esteban Muller

42 36043600 360235983596359466 359235900

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z R

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ge [

MV

]

4

-10

1st inj. 2nd inj.

First tests at the SPS

First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement

• The possibility of injecting a second batch without degrading the circulating been has also been shown

42 36043600 360235983596359466 359235900

1

2

3

Time [ms]

200

MH

z R

F V

olta

ge [

MV

]

4

-10

1st inj. 2nd inj.

First tests at the SPS

First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement

• The possibility of injecting a second batch without degrading the circulating been has also been shown

0 5 10 15 200

0.5

1

1.5

2

2.5

Time [ns]

Lo

ng

itu

din

al b

ea

m p

rofi

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a.u

.]

First bunch (of 2 single) after the second inj.

After 1st inj.

After 2nd inj.

Profile of the first doublet

First tests at the SPS

First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement

• Clear enhancement observed both on e-cloud detectors and pressure in the arcs

MBA

MBB

First tests at the SPS

First machine tests have been conducted at the SPS at the end of 2012-13 run in order to validate the production scheme and obtain first indications about the e-cloud enhancement

• Clear enhancement observed both on e-cloud detectors and pressure in the arcs

MBA

MBB

25ns std. (1.6e11p/bunch)

(1.7e11p/doublet)25ns “doublet”

Outline

• Electron cloud and scrubbing at the SPS

• A “doublet” scrubbing beam for the SPSo Simulation studieso First tests at the SPS

• Stability considerationso First observationso e-cloud driven instabilitieso Impedance driven instabilities

Stability considerations

Beam quality requirements:

• For SPS scrubbing purposes the goal is to inject up to 4 batches (4 x 72 doublets) and store the beam at injection energy for ~30 s with limited losses and emittance blow-up

• Studies are ongoing in order to assess if this beam can be accelerated to 450 GeV and delivered to the LHC for scrubbing purposes tighter beam quality requirements

The present SPS damper:

• After the LS1 consolidation it will be able to detect and damp the motion of the centroid of each doublet Intra-bunch motion and “pi-mode” of the two bunchlets are not detected

Stability considerations

Beam quality requirements:

• For SPS scrubbing purposes the goal is to inject up to 4 batches (4 x 72 doublets) and store the beam at injection energy for ~30 s with limited losses and emittance blow-up

• Studies are ongoing in order to assess if this beam can be accelerated to 450 GeV and delivered to the LHC for scrubbing purposes tighter beam quality requirements

The present SPS damper:

• After the LS1 consolidation it will be able to detect and damp the motion of the centroid of each doublet Intra-bunch motion and “pi-mode” of the two bunchlets are not detected

Stability considerations

Beam quality requirements:

• For SPS scrubbing purposes the goal is to inject up to 4 batches (4 x 72 doublets) and store the beam at injection energy for ~30 s with limited losses and emittance blow-up

• Studies are ongoing in order to assess if this beam can be accelerated to 450 GeV and delivered to the LHC for scrubbing purposes tighter beam quality requirements

The present SPS damper:

• After the LS1 consolidation it will be able to detect and damp the motion of the centroid of each doublet Intra-bunch motion and “pi-mode” of the two bunchlets are not detected

Tests at the SPS: first instability observations

• The tests were carried out without the transverse damper and with high chromaticity (ξx,y~0.4) Under these conditions, instabilities could be observed even at low bunch intensities

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Tests at the SPS: first instability observations

• The tests were carried out without the transverse damper and with high chromaticity (ξx,y~0.4) Under these conditions, instabilities could be observed even at low bunch intensities

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[a

.u.]

25 ns slot

At inj.3 sec. after inj.

Electron cloud driven instabilities

• By definition a scrubbing beam works in a severe e-cloud environment and is therefore prone to e-cloud instabilities

7.35 7.36 7.37 7.38 7.39 7.4

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am

pro

file

[p

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]

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tral

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nsi

ty [

m-3

]

-0.02 -0.01 0 0.01 0.020

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Position [m]

Sc

rub

bin

g c

urr

en

t (5

0e

V)

[A/m

2]

0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppb1.20e11ppb6btc 25ns1.20e11ppb

-0.02 -0.01 0 0.01 0.020

0.2

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0.8

1sey = 1.30

Position [m]

Sc

rub

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t (5

0e

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[A/m

2]

0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppb1.20e11ppb6btc 25ns1.20e11ppb

PyECLOUD simulation

Electron cloud driven instabilities

• By definition a scrubbing beam works in a severe e-cloud environment and is therefore prone to e-cloud instabilities

7.35 7.36 7.37 7.38 7.39 7.4

x 10-6

0

1

2

3x 1011

Time [s]

Be

am

pro

file

[p

+/m

]

7.35 7.36 7.37 7.38 7.39 7.4

x 10-6

0

5

10

15x 1012 sey = 1.35

e- c

en

tral

de

nsi

ty [

m-3

]

-0.02 -0.01 0 0.01 0.020

0.2

0.4

0.6

0.8

1sey = 1.30

Position [m]

Sc

rub

bin

g c

urr

en

t (5

0e

V)

[A/m

2]

0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppb1.20e11ppb6btc 25ns1.20e11ppb

-0.02 -0.01 0 0.01 0.020

0.2

0.4

0.6

0.8

1sey = 1.30

Position [m]

Sc

rub

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g c

urr

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t (5

0e

V)

[A/m

2]

0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppb1.20e11ppb6btc 25ns1.20e11ppb

PyECLOUD simulation

HEADTAIL simulation

Impedance driven instabilities

HEADTAIL simulation studies (considering a single doublet) have been recently started

• Impedance model includes: wall impedance (6 different vacuum chambers, including the magnets iron) + low frequency trapped mode due to MKE kickers with serigraphy

Thanks to C. Zannini

Impedance driven instabilities

HEADTAIL simulation studies (considering a single doublet) have been recently started

• First simulations with zero chromaticity and no transverse damper show an instability with a “p” mode (bunches oscillating rigidly out-of-phase)

1.3 1011 ppb

Impedance driven instabilities

HEADTAIL simulation studies (considering a single doublet) have been recently started

• Dependence of the growth rate on chromaticity has also been investigated

These instabilities have a rise time of at least several thousands of turns so might well be damped by Landau damping from (natural) non-linearities

Summary

• Build up simulation studies have shown that that the “doublet” beam features and

enhanced scrubbing efficiency with respect to the standard 25 ns beam

• The production scheme has been successfully tested at the SPS at the end of

2012-13 run

• First indications from the e-cloud detectors and dynamic pressure rise look very

promising

• During the tests (carried out without transverse damper) instabilities could be

observed

• After LS1 the present SPS damper will be able to detect (and damp) the center of

mass motion of each doublet but will not be able to detect:

o Intra-bunch motion driven by e-cloud

o “pi-mode” driven by machine impedance

• By fighting this kind of instabilities, the high bandwidth feedback would help

preserving the beam quality and therefore increasing the scrubbing efficiency

Thank you for your attention!

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First tests at the SPS

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First bunch (of 2 single) after the second inj.

After 1st inj.

After 2nd inj.

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First bunch (of 2 single) after the second inj.

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First bunch (of 2 single) after the first inj.

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First bunch, 2nd batch

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al b

ea

m p

rofi

le [

a.u

.]

70th bunch, 2nd batch

0 5 10 15 200

0.5

1

1.5

2

2.5

Time [ns]

Lo

ng

itu

din

al b

ea

m p

rofi

le [

a.u

.]

1st bunch, 1st batch

First tests at the SPS

agdsgfsh

MBA-like Stainless Steel liner

25ns standard (1.6e11p/bunch)

25ns “doublet” (1.7e11p/doublet)

First tests at the SPS

agdsgfsh

MBB-like Stainless Steel liner

25ns “doublet” (1.7e11p/doublet)

25ns standard (1.6e11p/bunch)

SPS tests????

72 “doublets”

72 bunches Higher press. rise

Arcs

Arcs