super b-factory workshop, hawaii, april 20-22, 2005 lattice studies for low momentum compaction in...

Super B-Factory Workshop, Hawaii, April 20-22, 2005

Lattice studies for low momentum compaction in

LER

M.E. BiaginiLNF-INFN, Frascati


SBF Parameters (J. Seeman)

L (cm-2s-1) 7x1035 1036

E (GeV) 8 3.5 8 3.5x (cm) 15 15 15 15y (mm) 1.5 1.5 1.5 1.5

x / y (nm) 28/0.3 28/0.3 40/0.4 40/0.4l (mm) 1.8 1.8 1.8 1.8

Nb 6900 6900 6900 6900I 6.8 15.5 10 23

(mrad) ±15 ±15x .105 .105 .108 .108y .107 .107 .105 .105

HER LER HER LER


Present LER Lattice

• C = 2200 m• 6 sextants, 90° FODO lattice +

dispersion suppressor• One IR• Dipoles 0.45 m• Momentum compaction +0.0012


New SBF lattice

• Guideline: use the existing tunnel and magnets

• Different layout using present LER dipoles & quads

• Several lattices considered: – “KEKB-like” 2.5 (H. Koiso, K. Oide)– “KEKB-like” 3.5 – “KEKB-like” 7.5 – “SSC-booster-like” 3.5 (U. Wienands)


New SBF lattice

• For flexibility, easy chromaticity correction, and c tunability the “KEK-B like” 2.5 lattice was suitable to our needs

• Preliminary lattice with no IR insertion• Two lattices were studied:

– Low negative c (-1.6x10-4)

– Low positive c (+7x10-4)


“KEK-B like” 2.5

• Ring is 2200 m long and has 6 sextants and 6 long straight sections with no dispersion

• Each sextant houses two 2.5 cells and two dispersion suppressors

• One cell has 4 bending units, each one has 3 LER dipoles, and 6 independently powered LER quadrupoles (16 families in one sextant)

• Phase advance/cell is 2.5 • Each straight section has 4 FODO cells



• Lattice allows for chromaticity correction by non-interleaved sextupoles connected by a –I pseudo-transformerQF2 QF6QF4 QF4 QF2

QD1 QD3 QD5 QD3QD5 QD1



• Total number of independently powered quadupoles is 16 so far

• Preliminary design has 2 sextupoles families to correct arc chromaticity

• “Local” chromaticity correction to be implemented once IR is inserted


Negative c lattice (-1.6x10-4) Arc + Dispersion suppressor


Negative c lattice One sextant


Negative c lattice

Whole ring


Beam Dynamics with c < 0• Bunch is shorter with a more regular shape

• Longitudinal beam-beam effects are less dangerous

• Microwave instability threshold is higher

• Sextupoles can be relaxed since head-tail disappears


Bunch lengthening• The rms length l of the equilibrium charge

distribution in the bunch, neglecting the lengthening process coming from the short-range wakefields, is:

• However the bunch has to remain short up to the design current. The Boussard criterion can be used to estimate the µ-wave threshold:

• For current values beyond the threshold, assuming purely inductive ring impedance, l is:

RF

cE

ring

s

cE

l Vh

eE

EL

Ecc

2

)/(

ring

l

eff

E

cth LnZ

eE

EI

)/(

/2

2

23

3

25 )/(cos2

2eff

sRF

ringlnZ

hV

IL


DANE

• A negative momentum compaction lattice (-0.017 for e-, -0.019 for e+) was designed and

implemented last year to study bunch length• In both e+ and e- rings a bunch shortening was

observed and -wave threshold was increased (higher for e+ ring that has lower impedance)

• Not used in collision since a vertical blow-up observed in the e- beam (larger impedance due to clearing electrodes)


Measured DANE bunch length

1

1,2

1,4

1,6

1,8

2

2,2

2,4

0 5 10 15 20

I [mA]

[cm]

alfa < 0 alfa > 0

e+

Bunch length vs bunch current for

VRF = 110 kV and 120 kV

-wave

Bunch length vs bunch current forVRF = 165 kV

0,5

1

1,5

2

2,5

3

3,5

0 5 10 15 20 25

alfa < 0alfa > 0

I [mA]

[cm] e-

Potential well

-wavePotential well


SBF• Due to HOM losses and beam loading at high current

(from 15.5 to 23 A), very high RF voltage will be needed (from 33 to 55 MV)

• With such a high voltage to have desired bunch length the |c| value must be larger than that achieved (1.6x10-4)

• Microwave instability threshold is very low (less than 0.5 mA) for the case studied, a bunch shortening is however observed in preliminary simulations (A. Novokhatski)

• A positive low c = +7x10-4 lattice was then designed

l = 1.8 mm, Ith ~ 3 mA• A larger negative momentum compaction lattice is

under study


Preliminary bunch length simulations (A. Novokhatski)

c < 0l = 1.75 mmIth < 0.5 mA

c > 0l = 1.8 mmIth > 3 mA


Positive c lattice (c = +7x10-4, x = 40 nm )

Arc + Dispersion suppressor


Positive c lattice One sextant


Positive c lattice Whole ring


IR design

• IR design has to cope with:– low-*– crossing angle– local chromaticity correction– radiation background issues (M. Sullivan)– shared QD1 quadrupole in LER & HER– vertical chicane to bring LER beam to collide

with HER beam

• First QD at 0.35 m from IP, shared by both beams (constraint)


Preliminary LER IR

x* = 25 cmy* = 3 mm

x* = 15 cmy* = 1.5 mm

Possible improvements: adjust phase advance between last arc bend and QF to optimize Touscheck lifetime

QD-QF doublet


Preliminary HER IR

x* = 25 cmy* = 3 mm

x* = 15 cmy* = 1.5 mm

Decreased distance between QD Doublet, to decrease peak y

Second QD necessary to keep HER beam focused

super b-factory workshop, hawaii, april 20-22, 2005 lattice studies for low momentum compaction in...

Documents

super bfactory workshop

sextant slide

ring slide

negative c lattice

frascati slide

wienands slide

herlerherler slide

inserted slide