Further Luminosity Upgrade

Y. Funakoshi (KEK)Contents
ŝ Three ways of considering luminosity

upgrade Comparison with the design
 History of luminosity improvements
 Luminosity limiting issues

ŝ SummaryComparison with design
LER

HER

Horizontal emittancenm

18 (18)24 (18)

Total beam current

mA

1454 (2600)

949 (1100)

Number of bunches

1184 (~5000)

Bunch current

mA

Bunch spacing

nsec

1.23 (0.52)

0.801 (0.22)
8 (2)Synchrotron tune

-0.0236-0.0207

Betatron tunes
νx/νy45.511/43.553(45.52/45.08)

44.513/41.582(47.52/43.08)

59/0.62 (33/1)

61/0.7 (33/1)

0.090/0.053(0.039/0.052)

0.078/0.045(0.039/0.052)

Beta’s at IP βx/βycm

Beam-beam parametersξx/ξyPeak luminosity/cm2/sec

8.26*1033 (1*1034)( ): design values

History of Luminosity improvements

Luminosity limiting issues
ŝ Luminosity formula

γ k RL Ikξ yL=
2ere Rξy β y*Three key parameters for higher luminosity
Beam currents
Vertical beta function at IP
Vertical Beam-Beam parameterBeam currents
ŝ Record values at KEKB HER 1006mA(record) cf. 1100mA(design) LER 1650mA(record) cf. 2600mA(design)ŝ Issues Effect on luminosity
 Hardware tolerance Bunch spacing problem
 RF powerEffect of higher beam currents on luminosity
ŝ Machine study on June 26 2002
 Methodś Decreased number of bunches by 33% (with the same bunchspacing)ś Increased bunch currents
ś Observed achieved luminosity (times 1.33)
 Resultsś Increase of LER beam current did not result in a higher peakluminosity. -> due to ECI ?
ś Increase of beam currents in both rings resulted in a higher
peak luminosity.
ŝ Increase LER beam current in actual physics run
 Resultś Higher LER current did not bring a higher peak luminosity.

Increase LER beam current
Machine Study (June 2002)
Machine Study:
ILER (max) = 1380mA-> 1835mA
IHER (max) = 720mA-> 958mA
Lpeak= 5.58 /nb/sec-> 7.42 /nb/sec
Physics Run:
ILER (max) = 1380mA
IHER (max) = 950mA
Lpeak= 7.3 /nb/sec

Increase beam currents of both rings

Machine Study (June 2002)
Machine Study:
ILER (max) = 1380mA-> 1835mA
IHER (max) = 815mA-> 1084mA
Lpeak= 6.16 /nb/sec-> 8.19 /nb/sec
Physics Run:
ILER (max) = 1380mA
IHER (max) = 950mA
Lpeak= 7.3 /nb/sec

Increase of LER beam current in
actual physics run (June 2002)

LER single beam blowup
From Fukuma
Beam blowup at LER (long train)
54/320/4(1train, regular 4 rf bucket spacing)

Beam size@I.P.(micron)

428 Jun 2002
330 Oct 2002
29 Jan 2003

202 Oct 2002
10600

800

1000

1200

1400

Beam current (mA)

16001800

Hardware tolerance to high beam currents
ŝ IP beam chamber Present restriction: 2200mA in the sum of the twobeams
 Investigation on the cause of the vacuum leak,Temperature rise of IP chamber -> more currents?
ŝ Other hardware components
 We experienced a lot of troubles due to high beamcurrents.
 No substantial difficulty at the present level of beamcurrents.
 HOM heating issues
ś It is desirable to decrease bunch currents by increasing thenumber of bunches. -> Bunch spacing problem
 Next target
ś 1.1A (HER), 2.0A (LER)

Bunch spacing problem
ŝ Observations
 The specific luminosity depends on bunch spacing.
 Longer bunch spacing gives a higher specificluminosity.
ŝ Cause of the problem
 This problem can not be attributed to the beam-beameffect.
 The LER single beam blowup does not explain thisproblem, since the problem occurs even below thethreshold of the blowup.
 No conclusive cause for this problem has not beenfound yet.
ś A synergistic effect of the beam-beam and the ECI might berelevant to this problem.

Specific Luminosity / bunch [10 30/cm2/sec/mA2]

Comparison of specific luminositywith 3 and 4 bucket spacing
4-bucket spacing
2001_7_10_2_44_50

4-bucket spacing
2001_11_10_16_24_54

3-bucket spacing
2001_7_12_1_56_43

3-bucket spacing
2001_11_9_17_56_23

Before summer shutdown in 2001

After summer shutdown in 2001

Ibunch(LER) * Ibunch(HER) [mA2]

Ibunch(LER) * Ibunch(HER) [mA2]

Specific Luminosity / bunch = L/Nbunch/Ibunch(LER)/Ibunch(HER)

Specific Luminosity / bunch [10 30/cm2/sec/mA2]

Comparison of specific luminositywith 4 and 24 bucket spacing24-bucket spacing
2002_6_14_18_26_3

4-bucket spacing
2002_6_13_7_40_43

Ibunch(LER) * Ibunch(HER) [mA2]

RF Power
ŝ LER
 We already have a full set of RF (ARES) cavities.
 We are ready to store a design beam current of 2.6A

from the viewpoint of RF power.
ŝ HER
 2 ARES cavities are lacking for a full set.
ś The design beam current of 1.1 A is maybe possible to stored.

 In summer 2003, 2 ARES cavities will be installed.
ś We will be able to store 1.2 A in autumn.

βy* (vertical beta function at IP)
ŝ Record values at KEKB HER 7.0mm (present) cf. 10mm (design) LER 6.2mm (present) cf. 10mm (design)ŝ Issues Dynamic and physical apertureś There may be small room for squeezing the vertical beta

function further.
 Bunch lengthś The bunch length in the usual physics run is alreadycomparable with the vertical beta function.

Luminosity with shorter bunch length
Machine Study June 14 2002
ŝ Vc (RF voltage)

HER: 12MV -> 14MV
 LER: 6.5MV -> 7.5MVŝ Bunch length (measured by bunch spectrum monitor)

HER: ~6.9mm -> ~6.3mm
 LER: ~7.7mm -> ~7.3mm
ŝ Luminosity
No significant improvement in the luminosity was observed withthe higher Vc operation.
ś Tuning time was insufficient?

A beam-beam simulation predicts a higher luminosity with themeasured bunch length by 6.5%.

Bunch Length with higher Vc
Vc=6.5MV
Vc=12MV
Vc=7.5MV

Vc=14MV204 bunches: 24 bucket spacingξy (Vertical Beam-Beam parameter)ŝ Record values at KEKB HER 0.045 (record) cf. 0.052 (design) LER 0.053 (record) cf. 0.052 (design)ŝ Issues Tune survey
 ECI
 Bunch spacing problem
 Crab cavity system

ECI (Electron Cloud Instability)
ŝ Solenoid coils
 If we switched off all the solenoids, the luminosity

would be less than half or we would not be able to store
the present beam current.
ŝ Does ECI limit the KEKB performance still now?
 The single beam blowup does not appear up to around

1800mA in the usual fill pattern.
 The bunch spacing problem has something to do withECI?
 A higher LER beam current brings a higher luminosity?
 We need more data…

ECI [cont’d]ŝ Effect of more solenoid There remains very small room for installationof solenoids.
 However, there is some indications that a small
amount of solenoids has effects than wasexpected.

Tune Survey
ŝ An enormous amount of efforts have been devoted

to the tune survey since the beginning of theKEKB commissioning.
ŝ At present there is almost no more promising tune
region than the present position.
ŝ In Factories 2001 workshop, I found that thefractional part of tune at CESR (.53,.58) is similarto ours.

Tune Survey(Simulation by Tawada)

Crab cavity
ŝ Beam-Beam simulation by using Ohmi’s code(Tawada) Luminosity will be doubled with the present machineparameters, if the crossing angle becomes zero.
-> ξy* ~0.1
ŝ Beam test of crab cavity system
 Present Plan: 1 cavity at Nikko section in HER (2005 or2006)
 My opinion: 1 cavity at Nikko section in both rings(2005 or 2006)

Effect of crab cavity system(simulation by Tawada)

Present KEKB

Summary [1/3]ŝ An introduction of the crab cavity system is the mostpromising way of raising the luminosity, if the simulationis correct.
 We should introduce it in both rings.
ŝ Increasing the HER beam current is effective for a higherluminosity.
ŝ Increasing the LER beam current might be effective for ahigher luminosity. -> We need more study.
ŝ To mitigate HOM related problems, it is desirable toincrease the number of bunches by reducing bunch
spacing. -> bunch spacing problem?

Summary [2/3]ŝ There is an indication that ECI has some effects

on the machine performance even below thethreshold of the single beam blowup .
 We maybe need more solenoids.

ŝ The next target of beam currents are:
 HER: 1.1A, LER:2A.

ŝ The present beam current limitation comes fromthe issue on the IP beam pipe.
 Study on this problem is one of the most important

issues at the present KEKB.

Summary [3/3]ŝ There is some (maybe small) room to get ahigher luminosity by shortening bunch
length and/or squeezing βy* further.