Special e -cloud bunch spacing: injectors

Special e-cloud bunch spacing: injectorsH. Bartosik, G. Iadarola, G. Rumolo, E. Shaposhnikova

Acknowledgements: G. Arduini, T. Argyropoulos, T. Bohl, S. Cettour Cave, H. Damerau, J. Esteban Muller, F. Follin, B. Goddard, S. Hancock, W. Höfle, L. Kopylov, C. Lazaridis, Y. Papaphilippou, M. Taborelli, H. Timko

LBOC, 5. November 2013

LBOC, 5. November 2013 2

Outline

o Motivation

o Expectations from simulations

o Experience with the doublet beam in the SPS• Comparison with simulations

o Doublet beams for the LHC• Bunch splitting at SPS injection • Bunch splitting at SPS flat top• Bunch splitting at LHC injection

o Expected beam parameters


Motivation

1 1.5 2 2.510

-4

10-2

100

102

104

SEY

Hea

t loa

d [W

/hc/

beam

]

DipoleQuadrupoleDrift

3

25 ns scrubbing @450 GeV (2011 + 2012)

scrubbing beam, hopefully … (2015)

dipoles with scrubbing beam

Scrubbing beam:o Lower SEY threshold in

LHC dipoleso Should allow to further

scrub at 450 GeV with high efficiency as far as the scrubbing curve of copper allows


PyECLOUD simulations – 5 ns doubletso The 5 ns doublet beam shows a much lower multipacting threshold compared to

the standard 25 ns beam

1.1 1.2 1.3 1.4 1.510

-6

10-4

10-2

100

102

SEY

Scr

ubbi

ng d

ose

(50e

V) [

mA

/m]

0.50e11ppb0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppbStd. 25 ns

10 20 30 40 50 60 700

2

4x 10

11

Bea

m p

rof.

[p/m

]

10 20 30 40 50 60 701

1.5

2

2.5

3

3.5

Time [ns]

Ne /

Ne(0

)

LHC dipoles


PyECLOUD simulations – 5 ns doubletso The 5 ns doublet beam shows a much lower multipacting threshold compared

to the standard 25 ns beam

o Efficient scrubbing with the doublet beam expected from e- energy spectrum for a wide range of intensities

o Intensity larger than 0.8x1011 p/b preferable for covering similar horizontal region as the standard 25 ns beam with nominal intensity

-15 -10 -5 0 5 10 1510

-4

10-3

10-2

10-1

100

101

102 sey = 1.50

Position [mm]

Scr

ubbi

ng c

urre

nt (5

0eV

) [A

/m2 ]

0 200 400 600 800 100010

-4

10-3

10-2

10-1

100 sey = 1.50

Energy [eV]N

orm

aliz

ed e

nerg

y sp

ectru

m

0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppbnom. 25 ns

0 200 400 600 800 100010

-4

10-3

10-2

10-1

100 sey = 1.50

Energy [eV]

Nor

mal

ized

ene

rgy

spec

trum


LHC dipolesLHC dipoles


PyECLOUD simulations – 2.5 ns doubletso The 2.5 ns doublet beam shows a lower multipacting threshold compared to

the standard 25 ns beam, but higher threshold compared to 5 ns doublets

10 20 30 40 50 60 700

2

4x 10

11

Bea

m p

rof.

[p/m

]

10 20 30 40 50 60 701

1.5

2

2.5

3

Time [ns]

Ne /

Ne(0

)

1.1 1.2 1.3 1.4 1.510

-5

10-4

10-3

10-2

10-1

100

101

SEY

Scr

ubbi

ng d

ose

(50e

V) [

mA

/m]

1.1 1.2 1.3 1.4 1.510

-6

10-4

10-2

100

102

SEY

Scr

ubbi

ng d

ose

(50e

V) [

mA

/m]

0.50e11ppb0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppbStd. 25 ns

LHC dipoles


PyECLOUD simulations – 2.5 ns doubletso The 2.5 ns doublet beam shows a lower multipacting threshold compared to

the standard 25 ns beam, but higher threshold compared to 5 ns doublets

o Similar e- energy spectrum as with 5 ns doublets

o E-cloud build-up is concentrated in central part of the chamber less favorable compared to the 5 ns doublets

0 200 400 600 800 100010

-4

10-3

10-2

10-1

100 sey = 1.50

Energy [eV]

Nor

mal

ized

ene

rgy

spec

trum


-15 -10 -5 0 5 10 1510

-4

10-3

10-2

10-1

100

101

102 sey = 1.50

Position [mm]

Scr

ubbi

ng c

urre

nt (5

0eV

) [A

/m2 ]

0 200 400 600 800 100010

-4

10-3

10-2

10-1

100 sey = 1.50

Energy [eV]N

orm

aliz

ed e

nerg

y sp

ectru

m


LHC dipolesLHC dipoles


Production of 5 ns doublet beam at SPS injectiono Injection of long (~10 ns) bunches into the SPS with low RF voltage

0 10 20 30 40 50 60 70

Long

. bea

m p

rofil

e

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

E

0 10 20 30 40 50 60 70Long

. bea

m p

rofil

e

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

E

0 10 20 30 40 50 60 70Long

. bea

m p

rofil

e

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

E


Production of 5 ns doublet beam at SPS injectiono Injection of long (~10 ns) bunches into the SPS with low RF voltage

o Fast voltage ramp in order to capture each bunch in two neighboring 200 MHz buckets

0 10 20 30 40 50 60 70

Long

. bea

m p

rofil

e

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

E

0 10 20 30 40 50 60 70Long

. bea

m p

rofil

e

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

E

0 10 20 30 40 50 60 70Long

. bea

m p

rofil

e

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

E

25 ns 25 ns5 ns


Tests of the 5 ns doublet beam in the SPSo First machine tests in the SPS at the end of 2012-13 run in order to

• validate the doublet production scheme at SPS injection• obtain first indications about the e-cloud enhancement

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

42 860

1

2

3

Time [ms]

200

MH

z R

F Vo

ltage

[MV

]

4

-20

1st inj.

0.92 0.94 0.96 0.98 1 1.02-0.02

0

0.02

0.04

0.06

Time [s]

Bea

m p

rofil

e [a

.u.]

Turn

0.92 0.94 0.96 0.98 1 1.02

100200300400500


Tests of the 5 ns doublet beam in the SPSo First machine tests in the SPS at the end of 2012-13 run in order to

• validate the doublet production scheme at SPS injection• obtain first indications about the e-cloud enhancement

o The production scheme has been successfully tested • for a train of up to (2x)72 bunches with 1.7e11 p/doublet• injecting a second batch without degrading the circulating beam has been shown• Cycle included the start of acceleration to estimate capture losses (around 10%)

36043600 360235983596359435923590

1

2

3

Time [ms]

200

MH

z R

F Vo

ltage

[MV

]

4

0

1st inj. 2nd inj.

42 860-20

0 5 10 15 200

0.5

1

1.5

2

2.5

Time [ns]

Long

itudi

nal b

eam

pro

file

[a.u

.]

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

After 1st inj.After 2nd inj.


Experience with the 5 ns doublet beam in the SPSo Stronger pressure rise with doublet beam indicates enhanced e-cloud build-up in the SPS arcs

• Direct comparison of standard and doublet beam within the same supercycle

25ns std. (1.6e11p/bunch)

(1.7e11p/doublet)25ns “doublet”

the curves represent pressure gauges in the center of the SPS arcs


Experience with the 5 ns doublet beam in the SPSo Stronger pressure rise with doublet beam indicates enhanced e-cloud build-up in the

SPS arcs• Direct comparison of standard and doublet beam within the same supercycle

o Clear enhancement observed also in the dedicated e-cloud monitors• Shown here for the MBB type chamber• Good agreement with PyECLOUD simulations• Build-up with doublet beam is concentrated in central region (SPS MBB chamber)

PyECLOUD simulationMeasurements


Ways of producing doublets for the LHC (I)o “Long bunch splitting” at SPS injection (5 ns doublets)

• Demonstrated in MDs (see previous slides)• Possible issues in the SPS

− Transverse beam stability due to enhanced e-cloud losses and emittance growth

− Transverse damper (after LS1 can damp the common oscillation mode of doublets but not the pi mode)

− Acceleration: RF power, longitudinal stability, LLRF (doublets treated as single bunch)

− Beam quality at extraction (however not critical for scrubbing)

• Possible issues in the LHC (to be treated in separate talks)− Transverse damper− Beam instrumentation in general, beam control and machine protection− Anything else?


Ways of producing doublets for the LHC (II)o “Bunch splitting” at high energy in the SPS (5 ns doublets)

• By sudden phase jump by 180° and recapturing each bunch in 2 neighboring buckets− A controlled phase jump will be possible with new module presently under

development for operation with ions (to be tested in 2014)− Preferably done at intermediate energy for cleaning-up uncaptured beam

before extraction

• Not tested yet• Possible issues in the SPS

− Transverse beam stability due to e-cloud − Transverse damper (during and after the splitting)− Acceleration of the needed high beam intensity: RF power, longitudinal stability− Splitting at high energy: LLRF, losses at high energy due to uncaptured beam,

longitudinal stability after the splitting, e-cloud effects after the splitting− Beam quality at extraction

• Possible issues in the LHC like in (I)


Ways of producing doublets for the LHC (III)o “Long bunch splitting” at LHC injection (2.5 ns doublets)

• Extracting long bunches (~5ns) from the SPS and capturing them in two neighboring LHC buckets 2.5 ns doublet spacing

• Not tested yet• Possible issues in the SPS

− Acceleration of the needed high beam intensity in the SPS: RF power, longitudinal stability

− Transverse beam stability due to e-cloud

• Possible issues in the LHC− Like in (I) and (II)− Several injections with RF voltage dips (feasible?)− Transverse damper (but should be easier than 5 ns doublets)− Losses due to uncaptured beam in the LHC during further injections


SPS RF power during acceleration (I)o Possible ways to alleviate RF power limitations

• Reduce ramp rate (example below for 3 times longer acceleration time Tacc)• Slightly less power needed in Q26, but other problems anticipated for high intensity (e.g.

TMCI)

o 1.6x1011 p/doublet seems within reach 0.8x1011 p/b • however controlled long. emittance blow-up will be needed to be checked in measurements

Q26 Q20


SPS RF power during acceleration (II)o Possible ways to alleviate RF power limitations

• Reduce ramp rate (example below for 3 times longer acceleration time Tacc)• Slightly less power needed in Q26, but other problems anticipated for high intensity (e.g.

TMCI)

o 2x1011 p/doublet out of reach with present 200 MHz RF system

Q26 Q20


Estimated beam parameters

o 1.6x1011 p/doublet within ~3 μm• Due to RF power limitation in the SPS• Assuming all benefits from larger

longitudinal parameters at PS injection

o Longitudinal emittance (first guess!)• ~0.45 eVs at LHC injection in case of

injecting doublets from the SPS• >0.6 eVs in case of injecting long

bunches for splitting in the LHC• Refined estimations from simulations and

instability considerations …


Summary and Conclusionso 5 ns doublet beam is the most preferable option as scrubbing beam

• Lowest SEY threshold in LHC dipoles• E-cloud covers the largest horizontal region

o Production of 5 ns doublet beam at SPS injection demonstrated in MDs• Enhanced pressure rise• Higher e-cloud activity in strip monitors as predicted by simulations

o Options for scrubbing beams for the LHC• 5 ns doublets at SPS injection main complications expected in the SPS: RF

power during acceleration and e-cloud along the cycle• 5 ns doublets at high energy in SPS main complications expected in the

SPS: RF power, losses at high energy (uncaptured beam), e-cloud effects• 2.5 ns doublets at LHC injection main complications expected in the SPS:

RF power, e-cloud effects• Other exotic ideas? (slip-stacking in the SPS or LHC, …)

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Special e -cloud bunch spacing: injectors