PEP-II NEW IR ISSUES

M.BiaginiLNF, INFN

PEP-II MAC Break-out Session SLAC, Oct. 10th 2003

Luminosity Upgrade & IR issues• To increase Luminosity in PEP-II there are few key

points (brute force):

– Decrease y*

– Decrease z

– Increase number of colliding bunches

– Increase currents

– Increase beams separation to decrease the effect of parasitic crossings

• All these... leaving the present IR as much unchanged as possible !

Luminosity Upgrade & IR issues (cont’d)

• These goals are problematic with the present IR:

– Q1 is not strong enough to lower y*

– Need to push Q1 closer to IP (gradient increases, balance between peak y in Q1 and

chromaticity increase)– Parasitic crossings can be an issue (as now in

by_2 pattern), degrading luminosity and tune shifts (separation is not enough)

– At higher currents beam backgrounds can be a big problem (Sullivan)

Luminosity Upgrade & IR issues (cont’d)

– At higher currents and shorter bunches HOM heating, beam pipe temperature and instabilities all grow

– Decreasing z the peak current increases and also increases

the probability of trapping modes (bunch spectrum is larger)

– Chromaticity correction can be a problem: with present optics the functions at the nearby sextupoles have been already increased in order to operate with present sextupoles

– Vacuum pipe apertures can also be an issue when decreasing * (peaks increase) and can limit beam lifetimes AND produce backgrounds

– A smaller bunch length could affect the Touschek lifetime in LER (not an issue now) if there is not a corresponding increase in dynamic aperture

First solution• Introduce a small horizontal crossing angle:

- increases beam separation- decreases effect of PC- allows for safe by_2 pattern collision

• All operating Factories, except for PEP-II, have a small crossing angle: CESR ±2.3 mrad, KEK-B ±11 mrad, DANE ±12 to ±14.5 mrad. KEK-B has a reduction in L for bb tune shift > 0.05, and it is going to add crab cavities to get to higher luminosity

• A preliminary design for PEP-II IR, with y* = 5 mm, has shown that ±3.5 mrad is “geometrically” easy to do. Need to remove 4 B1 slices (no need for further separation) and put 4 Q1 slices (need for more y-focusing gradient). The present correctors can match orbits to the present values.

• This solution has drawbacks:– Large angles can induce synchro-betatron resonances in the

beams. Piwinski angle: = z/x

– Unwanted beam-beam interactions at Parasitic Crossings

– Effect of off-axis trajectories in quadrupoles and solenoids on

the beam optics have to be carefully evaluated

– Luminosity and tune shifts are affected: L , (geometrical

reduction) + reduction for high at small angles (bb

simulations)

Crossing angle issues

CESR PEP-II

(z = y = 5

mm)

DANE KEK-B

(mrad) 2.3 3.5 12 – 14.5 11

(mrad) 0.1 0.12 0.22 – 0.29 0.57

Parasitic Crossings EffectThe unwanted beam interaction at the PCs has 2 effects: • x and y tune shifts are induced, similarly to the main IP,

depending on the beam separation at the PC • beam lifetime is affected, if the separation is lower than 10

x

222

22ye

y

222

22xe

x

yx

yx

2

rN

yx

yx

2

rN

J. Jowett, Handbook of Accelerator Physics and Engineering: Beam-beam tune shifts for gaussian beams

x, y = beam separation at PCsGaussian beam distribution

PEP-II PCs Tune shifts (June 2003)Y on log scale

0.0001

0.001

0.01

0.1

1

10

0 0.5 1 1.5 2 2.5 3 3.5

HER - PC tune shifts vs main IP

HER x

HER y2PC/I P

sPC

(m)

0.0001

0.001

0.01

0.1

1

10

0 0.5 1 1.5 2 2.5 3 3.5

LER - PC tune shifts vs main IP

LER x

LER y2PC/I P

sPC

(m)

by_1

by_2

by_3

by_4

The PC tune shifts are normalised to the main IP tune shifts

Tune shifts due to PCs: new IR, c. a.

New IR configuration, ±3.5 mrad crossing angle

x=28 cm, y=5 mm, xLER=.098, y

LER=.076, xHER=.076, y

HER=.062

Nb =1700, Npart/bunch=1.2x1011/6x1010 (+/-)

(modified John’s numbers)

Y on log scale

10-6

10-5

0.0001

0.001

0.01

0.1

1

10

0 0.5 1 1.5 2 2.5 3 3.5

HER - PC tune shifts vs main IP

x

y

PC/I P

sPC

(m)

0.0001

0.001

0.01

0.1

1

10

0 0.5 1 1.5 2 2.5 3 3.5

LER - PC tune shifts vs main IP

x

y

PC/I P

sPC

(m)

Luminosity & tune shifts with crossing angle

L N2

4 y z2tg2 / 2 x

2

For >> tg (/2). z = bunch length, = crossing angle

P. Raimondi, M. Zobov, “Tune shift in beam-beam collisions with a crossing angle”, DAFNE Tech. Note G-58, 2003

x p

reN

2x

z2tg2 / 2 x

2 z2tg2 / 2 x

2 y

y p

reN2

y

y z2 tg2 / 2 x

2 y

Luminosity & tune shifts with crossing angle

• These formulae are similar to the head-on collision ones if we simply substitute the horizontal beam size with:

• A large crossing angle introduces strong coupling between the horizontal and longitudinal planes, provided that z > x (this is almost always true).

• Luminosity and tune shifts are reduced with the crossing angle. However, since:

the horizontal tune shift drops faster than the luminosity.

x z2tg2 / 2 x

2

L ~ x2 z

2tg2 / 2 1/ 2 ; x ~ x2 z

2 tg2 / 2 1 ; y ~ x2 z

2tg2 / 2 1/ 2

Luminosity with crossing angleLuminosity geometric reduction due to the crossing angle, vs c.a.Y scale: Luminosity with c.a. normalized to the head-on one. Lhead on = 3.3x1034

0.9

0.92

0.94

0.96

0.98

1

1.02

0 0.002 0.004 0.006 0.008 0.01 0.012

Luminosity: crossing angle vs head-on

LCA/LHO

half crossing angle (rad)

Tune shifts with crossing angle

Tune shifts geometric reduction due to crossing angle, vs. crossing angle. Y scale: tune shift with c.a. normalized to the head-on one.Horizontal drops faster. Beam footprint is smaller.

0.8

0.85

0.9

0.95

1

1.05

0 0.002 0.004 0.006 0.008 0.01 0.012

HER Tune shifts: crossing angle vs head-on

x

y

CA/HO

half crossing angle (rad)

0.8

0.85

0.9

0.95

1

1.05

0 0.002 0.004 0.006 0.008 0.01 0.012

LER Tune shifts: crossing angle vs head-on

x

y

CA/HO

half crossing angle (rad)

Cai’s beam-beam simulation

Looks like small crossing angles are worst than large crossing angle!!

2nd solution ?• Beam-beam simulations for PEP-II (Omhi, Cai)

show that even a small crossing angle can degrade luminosity, so...

Back to head-on collision... ?

• To decrease y*, Q1 needs to be more powerful add more slices. Fill the gap between B1 and Q1? (Sullivan, Ecklund)

• To increase beam separation B1 field could be increased (SR increases too...) ?

• The values of present correctors may not be large enough to match orbits to the present values at IR ends ( no perturbation is wanted outside IR)

• The PC effect starts to be important

HER PC tune shifts in by_2 patternvs. y* and

Comparison of PC tune shifts for different y* and crossing angles.The head-on solution is the red curve

0.0001

0.001

0.01

0.1

5.5 6 6.5 7 7.5 8 8.5 9 9.5

HER - PC X tune shift vs main IP for by_2 pattern

HER 0mradHER 1mradHER 2 mardHER 3.5 mradHER 4 mradHER 5 mrad

2PC/I P

y* (mm)

0.0001

0.001

0.01

0.1

1

5.5 6 6.5 7 7.5 8 8.5 9 9.5

HER - PC Y tune shift vs main IP for by_2 pattern

HER 0mradHER 1mradHER 2 mardHER 3.5 mradHER 4 mradHER 5 mrad

2PC/I P

y* (mm)

LER PC tune shifts in by_2 patternvs. y* and

Comparison of PC tune shifts for different y* and crossing angles.The head-on solution is the red curve

0.0001

0.001

0.01

0.1

1

5.5 6 6.5 7 7.5 8 8.5 9 9.5

LER - PC Y tune shift vs main IP for by_2 pattern

LER 0mradLER 1 mardLER 2 mardLER 3.5 mradLER 4 mradLER 5 mrad

2PC/I P

y* (mm)

0.0001

0.001

0.01

0.1

5.5 6 6.5 7 7.5 8 8.5 9 9.5

LER - PC X tune shift vs main IP for by_2 pattern

LER 0mradLER 1 mradLER 2 mardLER 3.5 mradLER 4 mradLER 5 mrad

2PC/I P

y* (mm)

Conclusions

• The present IR is very crowded, it is not simple to modify it without perturbations. But 2 solutions exist

• Head-on solution with low- shows huge effect of PCs

• Crossing angle can still be a good solution if we can afford L degradation by pushing other parameters

Need to study in more detail

with bb simulations of PC

the head-on vs crossing angle options !