super b-factory workshop, hawaii, april 20-22, 2005 lattice studies for low momentum compaction in...
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Super B-Factory Workshop, Hawaii, April 20-22, 2005
Lattice studies for low momentum compaction in
LER
M.E. BiaginiLNF-INFN, Frascati
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Present LER Lattice
• C = 2200 m• 6 sextants, 90° FODO lattice +
dispersion suppressor• One IR• Dipoles 0.45 m• Momentum compaction +0.0012
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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)
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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)
Super B-Factory Workshop, Hawaii, April 20-22, 2005
“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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
“KEK-B like” 2.5
• Lattice allows for chromaticity correction by non-interleaved sextupoles connected by a –I pseudo-transformerQF2 QF6QF4 QF4 QF2
QD1 QD3 QD5 QD3QD5 QD1
Super B-Factory Workshop, Hawaii, April 20-22, 2005
“KEK-B like” 2.5
• 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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Negative c lattice (-1.6x10-4) Arc + Dispersion suppressor
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Negative c lattice One sextant
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Negative c lattice
Whole ring
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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)
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Preliminary bunch length simulations (A. Novokhatski)
c < 0l = 1.75 mmIth < 0.5 mA
c > 0l = 1.8 mmIth > 3 mA
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Positive c lattice (c = +7x10-4, x = 40 nm )
Arc + Dispersion suppressor
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Positive c lattice One sextant
Super B-Factory Workshop, Hawaii, April 20-22, 2005
Positive c lattice Whole ring
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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)
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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
Super B-Factory Workshop, Hawaii, April 20-22, 2005
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
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