SLAC/PEP-II and INFN/LNF-AD Collaboration for Very High Luminosity Factories studies M. Biagini, LNF...

SLAC/PEP-II and INFN/LNF-AD

Collaboration for Very High Luminosity Factories studies

M. Biagini, LNFCommissione Nazionale Gruppo I, LNF 12/11/03

OUTLINE

• PEP-II mid and long term plans

• The Task Force

• PEP-II Luminosity upgrade

• Super B-Factory

• SLAC-LNF Collaboration on:

– IR upgrade & Backgrounds

– RF & Feedbacks

• The PEP-II B-Factory reached the design luminosity

(3x1033 cm-2 s-1 in October 2000) and a peak

luminosity of 6.6x1033 cm-2 s-1 (June 2003),

delivering a total of 146 fb-1 for the BaBar

experiment (June 2003).

• The “twin” collider KEK-B in Japan has obtained analogous performances. Their success has shown that complex accelerators can be designed and operated. However the particle physics items studied at these colliders demand still higher peak luminosities, and to collect, in a reasonable amount of time, a huge amount of data of good quality.

• In this frame, both SLAC and KEK laboratories are

studying upgrades and improvements to their

machines, with the aim to reach, in 10 years, peak

luminosities of the order of 1035 to 1036 cm-2 s-1.

• At present the PEP-II Staff is studying the

possibility to reach 3.x1034 cm-2 s-1 peak

luminosity with a minor, not invasive, modification

of the present machine layout. This design, having

a small impact on the accelerator, could be

operating in 2006-2007 already.

Why a Task Force?• PEP-II is at present still a factor 1.6 lower in peak luminosity

with respect to KEK-B.

• A Task Force called “PEP-II Mid-Project Evaluation” has been established in 2003 by the SLAC Director in order to focus on the main problems PEP-II is facing at present and find suitable solutions.

• The Task Force is divided in subgroups, each of them addressing a specific item.

• Help from other high energy laboratories experts have been asked. Two LNF physicists have been asked to be part of the Task Force, for the IR design and Feedback subgroups.

Task Force Structure

LNF/DAGallo

from web page

LNF/DA

LNF/DALNF/DA

SerioDragoMarcellini

Biagini

Biagini

Boscolo

PEP-II Mid-Project EvaluationFeedback Systems Subgroup

Primary ContactsEric R. Colby SLAC

Dmitry Teytelman SLAC

At LBNL:Walt BarryJohn CorlettJohn ByrdLarry Doolittle

At INFN/Frascati:Mario Serio

At KEK:Makoto Tobiyama

At SLAC:John FoxSam HeifetsRon AkreRay LarsenNavid HassanpourLiane BeckmanAndy YoungUli WienandsJohn Seeman

Contact List

From w

eb page

From w

eb page

PEP-II Luminosity Upgrade

PEP-II Upgrade ParametersThe road to 3x1034…

UnitsLuminosity6.6x1033 1.2x1034 1.8x1034 2.3x1034

3.3x1034 cm-2 s-1

I+ 1550 2700 3600 3600 4500 mAI- 1175 1600 1800 2000 2200 mAy* 12. 9 8.5 6.5 6. mmx* (+/-) 40/28 28 28 28 28 cml (+/-) 10.5/12 9/11 8.5/9 7.5/8 6.5/7.5 mmNb 1034 1450 1500 1700 1700Emitt. x (+/-) 30/49 40/40 44/40 40/40 40/44 nmEmitt. y (+/-) 1.8/1.8 1./1.5 1.4/1.5 0.9/1.1 0.9/1.1 nmCross. angle 0 0 0 0 ±4/7 mradx (+/-) .10/.04 .09/.05 .10/.06 .10/.06 .10/.076y (+/-) .08/.04 .08/.06 .09/.06 .09/.06 .09/.07

Date Jul 03 Jul 04 Jul 05 Jul 06 Jul 07

Modified IR for the PEP-II upgrade

• The main design issues for the PEP-II upgrade are:

– lower y* (from 12 mm to 6 mm)

– shorter bunches (from 11 mm to 7 mm)– small crossing angle to increase the number of

colliding bunches and to decrease the parasitic crossing tune-shift

– higher collision frequency (from 1100 to 1700 bunches)

Crossing angle

Recently Yunhai Cai has simulated a crossing angle in his beam-beam code and confirms Ohmi’s beam-beam result that an x crossing angle results in a significant luminosity reduction for very high tune shifts when one is near the ½ integer

Parasitic crossings

The introduction of a crossing angle increases the beam separation at the parasitic crossings and thereby decreases the beam-beam tune shifts from these near collisions. The effects we have already seen in by2 bunch patterns from parasitic crossings would be greatly reduced.For PEP-II, the parasitic crossings occur at:

0.63, 1.26, 1.89 and 2.52 m in the by2 bunch pattern and

0.945, 1.89 m in the by3 bunch pattern

Crossing angle and parasitic crossings

See M. Biagini’s talk on parasitic crossings

See Y. Cai’s and K. Ohmi’s talks on beam-beam simulations

M.Sullivan, IR Upgrade, ICFA e+e- Factories, Oct. 2003

The Super B-Factory

New techniques of the Next Generation B-Factory

• Beam lifetimes will be low continuous injection.• Very low y

* (6 to 12 mm1.5 to 3 mm).• Higher tune shift (trade beam-beam lifetimes for tune shifts).• Higher beam currents (x 10 or so) (Watch total power!).• Higher frequency RF (more bunches).• Bunch-by-bunch feedbacks at the 1 nsec scale. (StartedFox)• Very short bunch lengths (2-3 mm).• High power vacuum chambers with antechambers and improved or

no bellows. (Started)• Reduce energy asymmetry to save wall power.

J.Seeman, Future Very High Luminosity Options for PEP-IIICFA e+e- Factories, Oct. 2003

Advanced B Factory with 476 MHz RF Frequency

• E- = 8 GeV• E+ = 3.5 GeV• I- = 4.8 A• I+ = 11 A• y

* = 2.2 mm• x

* = 15 cm• Bunch length = 2.5 mm• Crossing angle = ~15. mrad• Beam-beam parameters = 0.15

– (Needs some testing but 0.12 now!)• N = 3450 bunches• L = 5 x 1035 cm-2s-1

• Site power with linac and campus = ~120 MW.


Advanced B Factory with 952 MHz RF Frequency

• E+ = 8 GeV• E- = 3.5 GeV• I+ = 6.8 A• I- = 15.5 A• y

* = 1.5 mm• x* = 15 cm• Bunch length = 1.8 mm• Crossing angle = ~15. mrad• Beam-beam parameters = 0.15• N = 6900 bunches• L = 1.0 x 1036 cm-2s-1

• Site power with linac and campus = ~120 MW.


E36 B-Factory +/- 12 mrad xing angle Q2 septum at 2.5 m

30

20

10

0

-10

-20

-30

cm

-7.5 -5 -2.5 0 2.5 5m

Q1

Q1Q1

Q1

Q2

Q4Q5

Q2

Q4Q5

3.1 GeV

9 GeV

Conclusions

• The parameters of a Super-PEP-II were studied with RF frequencies of 476 MHz and 952 MHz.

• At the present, for about 120 MW of total power, linac and campus included:

• 476 MHz provides a luminosity of about 5x1035 and • 952 MHz provides a luminosity of about 1x1036 with

beam-beam parameters of 0.15. • Coupled-bunch beam-dynamics effects, IR design, and

vacuum systems will be studied next.


SLAC-LNF Collaboration

• Design of a modified Interaction Region with crossing angle (similar to DANE and KEK-B design) to increase the number of colliding bunches, while keeping small the parasitic crossing tune-shift, and then increase luminosity. This work has started already.

• Study of RF and feedbacks upgrades for high currents operation

• Design of a brand new IR for the Super B-Factory, with SC quadrupoles (HERA or CESR type), ultra-

low y* and study of the crossing angle option.

SLAC-LNF Collaboration (detail)

• Design of a new IR for y* = 6 mm, using pm quadrupoles and lattice studies for chromaticity correction (2002);

• Backgrounds studies, in particular study of the Touschek effect for the LER ring (2003);

• Design of a longitudinal feedback kicker for high beam currents (original design developed at LNF and applied to LER) (2001);

• Bunch shortening options for lower y* design (2004);

• Study of the RF issues at high currents (2003);• Assessment of the longitudinal feedback (LFB) and

low-level RF (LLRF) able to cope with a higher number of bunches (1700) and higher currents (2003).

IR upgrade• Evaluate if the introduction of a small crossing angle,

together with a lower y*, could be done with the smallest perturbation of the present design.

• Replacement of 4 B1 slices with defocusing quadrupole slices, to increase Q1 quadrupole gradient to decrease y*. Present orbit correctors are be able to cope with a small crossing angle (±3.5 mrad).

• Effect of the crossing angle on the luminosity needs to be studied. Two different phenomena: long range beam-beam interactions at the Parasitic Crossings (PC) become as important as the main one and luminosity, as well as main IP tune shifts, are degraded.

• Choice to collide with or without a crossing angle is a trade-off between these two effects. It is important to determine the minimum beam separation required in order to have acceptable beam-beam tune shifts at the PC: this sets the choice on the angle value.

IR upgrade (cont’d)

• A study of the new IR issues has been presented at the PEP-II Machine Advisory Committee held at SLAC in October this year. An evaluation of the PC effect for both the present PEP-II and the upgraded version has been presented to the ICFA Workshop on e+e- High Luminosity Factories also held in October.

• The choice of the more suitable IR geometry and y* value is the most urgent item, and this study will continue next year to finalize the IR geometry. Once the IR has been designed the lattice of the two rings has to be also modified in order to correct for the increased chromaticity arising from the lower y*. This study will start next year, and it is a preliminary step towards the design of a new IR for y* = 1.5 mm using superconducting quadrupoles (HERA type) for the Super B-Factory project.

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 !

M.Biagini, PEP-II Machine Advisory Committee, Break-out Session, SLAC, Oct. 10th 2003

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


HER PC tune shifts in by_2 patternvs. y* and

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)

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

M.Biagini, Long range beam-beam interactions in PEP-II, ICFA e+e- Factories, Oct. 2003

LER PC tune shifts in by_2 patternvs. y* and

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)

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

M.Biagini, Long range beam-beam interactions in PEP-II, ICFA e+e- Factories, Oct. 2003

•The initial upgrade proposal replaced the last 4 slices of the B1 magnets with quadrupole field. However, this replacement introduces a ± 3.3 mrad crossing angle at the IP. Recent beam-beam simulations indicate a luminosity reduction for beams with even a small crossing angle.

•An alternative proposal is to strengthen the IP end of QD1 and remove some the outboard slices. This moves the center of the magnet closer to the IP while maintaining the head-on collision and hence the peak luminosity. However, this design has parasitic crossing effects that become quite large at the low y* values.

•There are several alternatives between these two proposals and a compromise solution may be the better design

•We might try to test the crossing angle lumi loss prediction if there is a measurable loss at small enough angles (+/- 0.5 mrad)

Summary

M.Sullivan, IR Upgrade, ICFA e+e- Factories, Oct. 2003

Touschek backgrounds

• Backgrounds studies have started this summer to evaluate the impact of the Touschek lifetime on LER.

• The Touschek lifetime, which was measured in LER but is not limiting the beam lifetime at the present beam current, could be strongly affected by the change in beam parameters foreseen by the luminosity upgrade.

• Moreover the Touschek scattered particles could represent a further source of background in the detector.

• It is foreseen to use the same simulation tools already successfully applied to DANE to evaluate the Touschek lifetime and to produce energy spectra of the Touschek scattered particles to be given as an input to GEANT.

Longitudinal kicker cavity • One of the problems of the high

currents option is the beam induced heating of structures as the kickers used in the longitudinal feedback. In particular, this problem has already arisen for the LER.

• An over damped cavity longitudinal kicker has been first conceived at LNF for DANE and it has been adopted by KEK-B and Bessy II. The same design has been applied to LER and the kicker is presently being built and it will be installed next December.

• This kind of device is easily cooled from the outside and it is expected to more than double the current capability of LER kickers.

Beam induced power

Existing drift-tube kicker New cavity kicker

Impedance

Beam spectrum

Beam induced voltage

Deposited power

x1.5

Feedbacks

• The PEP-II longitudinal feedback has been originally designed by a joint collaboration SLAC-LBNL-LNF, and it has proved to work very well to suppress multibunch instability at PEP-II, ALS and DANE.

• An evolutionary board (G-board) is presently under development, its capability to cope with the higher currents involved in the upgrade has however to be checked.

• Moreover the interconnection between the LFB and the RF system performances is very tight, so the LFB and LLRF sub-group of the Task Force will collaborate with the RF sub-group and concentrate on a reliable modeling of PEP-II longitudinal dynamics (both beam and RF), a key to evaluating the upgrade proposals, as well as on the analysis and redesign of the LLRF feedback.

J.Fox, SLAC, Alghero Workshop

(was 500 MHz)

(just one board instead of 22)

RF Issues• The RF cavities are the most critical component of the rings

at present. Their performances are a limit the achievable integrated luminosity at the moment. The upgrade in number of cavities will require high reliability and the study of suitable feedbacks.

• PEP-II is now operating routinely with about 2 A in the LER and 1.1 A in the HER. The longitudinal coupled bunch motion is stable with a reasonable margin at the present current rates. However grow/damp measurements made through the LFB system showed that the coupled bunch modes closest to instability are those excited by the fundamental mode of the RF cavities that are detuned toward lower frequencies by the beam loading effect.

• The klystrons saturation reduces the effective gain of the loops used to reduce the interaction between the beam and the cavity accelerating modes. As a consequence, the loop performances are less effective than expected, and the beams will become unstable at the current values required by the short and medium term machine upgrade.

RF Issues (cont’d)• Different ideas are under study to overcome this limitation.

The most direct cures (klystron and cavity rebuilding) are probably too invasive and expensive.

• A less expensive solution consists in installing a second, more linear power amplifier feeding the cavities in parallel to the klystron and only devoted to excite the loop correction signals. The evaluation of the RF power required and the study of the high power combiner are presently in progress. However, at the moment the most promising idea is the implementation of a new loop around the klystron to linearize and stabilize its small signal amplitude response (LNF idea).

• From preliminary analysis the loop seems feasible, with a sufficiently broad frequency response. A “simulink” Matlab model of this system is under elaboration; a hardware prototype will be probably built and tested during year 2004.

Bunch shortening studies• The bunch shortening is one of the major issues of the

luminosity upgrade, since the hourglass effect limits the

luminosity for y* < l, a lower y* means that the bunch

length has to be decreased too. However this cannot be done easily. Increasing the RF voltage is an expensive option. Other options foreseen are the addition of higher harmonic cavities, and/or to change the machine lattice to lower the momentum compaction (HER only).

• A brand new idea for bunch shortening (RF focusing) is presently being studied at LNF (Gallo,Raimondi,Zobov) for DANE2. The same design could be modified to suit the LER ring needs.

Hourglass effect

0

0,02

0,04

0,06

0,08

0,1

-0,02 -0,01 0 0,01 0,02

s (m)

1 mm

5 mm

2 cm

Bunch length

y*

Squeezing the vertical beam size by reducing the vertical -function is effective only if the bunch length is also reduced to about the * value.

A. GalloA. Gallo, The Strong RF Focusing: a possible approach to get short bunches at the IP, The Strong RF Focusing: a possible approach to get short bunches at the IP

Strong RF Focusing (SRFF)*

0 20 40 60 80 100 120

Bunch Length RF

IPs (m)

RF

RF

IP

Modulation of bunch lengthalong the ring with a minimum at the IP

* A. Gallo, P. Raimondi and M. Zobov :”Strong RF Focusing for Luminosity Increase”

DAFNE Technical Note G-60, 18/8/2003

Longitudinal phase space

RF input

RF center

RF output

IP

Bunch length

Energyspread

High RF voltage +

Magnetic lattice which correlates longitudinal position with energy

deviation (high momentum compaction)

From RF to IP

Personnel and funding requests

• For the 2004 the request is to have 6 month/person to participate to Machine Developments shifts, meetings and design studies :

– 3 month/person on IR design and backgrounds studies– 1 month/person on bunch shortening studies– 2 month/person on longitudinal feedback e RF issues

• The following LNF/AD physicists are at present part of this collaboration :

– M. E. Biagini (coordinator) for the IR design, beam-beam and lattice studies

– M. Boscolo (½ time) for the backgrounds studies – A. Drago for the longitudinal feedback (RF and broadband) – A. Gallo for the bunch shortening and RF studies – F. Marcellini for the longitudinal kicker operation – M. Serio for the longitudinal feedback (RF and broadband)

SLAC/PEP-II and INFN/LNF-AD Collaboration for Very High Luminosity Factories studies M. Biagini, LNF...

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