Transcript SLAC/PEP-II and INFN/LNF-AD Collaboration for Very High Luminosity Factories studies

SLAC/PEP-II and INFN/LNF-AD
Collaboration for Very High
Luminosity Factories studies
M. Biagini, LNF
Commissione 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/DA
Gallo
Biagini
Serio
Drago
Marcellini
LNF/DA
Biagini
Boscolo
LNF/DA
LNF/DA
PEP-II Mid-Project Evaluation
Feedback Systems Subgroup
Primary Contacts
Eric R. Colby
SLAC
Dmitry Teytelman SLAC
At LBNL:
Walt Barry
John Corlett
John Byrd
Larry Doolittle
At INFN/Frascati:
Mario Serio
At KEK:
Makoto Tobiyama
Contact List
At SLAC:
John Fox
Sam Heifets
Ron Akre
Ray Larsen
Navid Hassanpour
Liane Beckman
Andy Young
Uli Wienands
John Seeman
PEP-II Luminosity Upgrade
PEP-II Upgrade Parameters
The road to 3x1034…
Luminosity
I+
I y*
x* (+/-)
l (+/-)
Nb
Emitt. x (+/-)
Emitt. y (+/-)
Cross. angle
xx (+/-)
xy (+/-)
6.6x1033 1.2x1034
1550
2700
1175
1600
12.
9
40/28 28
10.5/12 9/11
1034
1450
30/49 40/40
1.8/1.8 1./1.5
0
0
.10/.04 .09/.05
.08/.04 .08/.06
1.8x1034
3600
1800
8.5
28
8.5/9
1500
44/40
1.4/1.5
0
.10/.06
.09/.06
2.3x1034 3.3x1034
3600
4500
2000
2200
6.5
6.
28
28
7.5/8
6.5/7.5
1700
1700
40/40 40/44
0.9/1.1 0.9/1.1
0
±4/7
.10/.06 .10/.076
.09/.06 .09/.07
Date
Jul 03
Jul 05
Jul 06
Jul 04
Jul 07
Units
cm-2 s-1
mA
mA
mm
cm
mm
nm
nm
mrad
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)
M.Sullivan, IR Upgrade,
ICFA e+e- Factories, Oct. 2003
Crossing angle and parasitic crossings
Crossing angle
See Y. Cai’s and
K. Ohmi’s talks
on beam-beam
simulations
Recently Yunhai Cai has simulated a crossing angle in his beambeam 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
See M. Biagini’s
talk on 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
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 mm1.5 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.
J.Seeman, Future Very High Luminosity Options for PEP-II
(StartedFox)
ICFA 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.
J.Seeman, Future Very High Luminosity Options for PEP-II
ICFA e+e- Factories, Oct. 2003
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.
J.Seeman, Future Very High Luminosity Options for PEP-II
ICFA e+e- Factories, Oct. 2003
E36 B-Factory +/- 12 mrad xing angle Q2 septum at 2.5 m
30
Q2
Q5
Q4
20
eV
G
3.1
Q1
10
Q1
9 GeV
cm 0
Q1
-10
Q1
-20
Q4
Q5
Q2
-30
-7.5
-5
-2.5
0
m
2.5
5
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.
J.Seeman, Future Very High Luminosity Options for PEP-II
ICFA e+e- Factories, Oct. 2003
SLAC-LNF Collaboration
• Design of a modified Interaction Region with crossing
angle (similar to DAFNE 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 lowlevel 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 BFactory 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)
M.Biagini, PEP-II Machine Advisory Committee, Break-out Session, SLAC, Oct. 10th 2003
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
M.Biagini, PEP-II Machine Advisory Committee, Break-out Session, SLAC, Oct. 10th 2003
HER PC tune shifts in by_2 pattern
vs. y* and f
HER - PC X tune shift vs main IP
for by_2 pattern
0.1
PC
2x /x
1
IP
PC
2x /x
HER - PC Y tune shift vs main IP
for by_2 pattern
IP
0.1
0.01
0.01
HER 0mrad
HER 1mrad
HER 2 mard
HER 3.5 mrad
HER 4 mrad
HER 5 mrad
0.001
0.0001
5.5
6
6.5
7
7.5
 * (mm)
y
8
8.5
9
HER 0mrad
HER 1mrad
HER 2 mard
HER 3.5 mrad
HER 4 mrad
HER 5 mrad
0.001
0.0001
9.5
5.5
6
6.5
7
7.5
 * (mm)
8
8.5
9
y
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
9.5
LER PC tune shifts in by_2 pattern
vs. y* and f
LER - PC X tune shift vs main IP
for by_2 pattern
0.1
PC
2x /x
1
IP
PC
2x /x
LER - PC Y tune shift vs main IP
for by_2 pattern
IP
0.1
0.01
0.01
LER 0mrad
LER 1 mrad
LER 2 mard
LER 3.5 mrad
LER 4 mrad
LER 5 mrad
0.001
0.0001
5.5
6
6.5
7
7.5
 * (mm)
y
8
8.5
9
LER 0mrad
LER 1 mard
LER 2 mard
LER 3.5 mrad
LER 4 mrad
LER 5 mrad
0.001
0.0001
9.5
5.5
6
6.5
7
7.5
 * (mm)
8
8.5
9
y
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
9.5
Summary
•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)
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 DAFNE 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 DAFNE 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 DAFNE.
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 subgroup 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
DAFNE2. The same design could be modified to suit the
LER ring needs.
A. Gallo, The Strong RF Focusing: a possible approach to get short bunches at the IP
0,1
Hourglass effect
Squeezing the
vertical beam size
by reducing the
vertical -function
is effective only if
the bunch length is
also reduced to
about the * value.
 y*
1 mm
0,08
5 mm
0,06
0,04
2 cm
0,02
s (m)
0
-0,02
-0,01
0
0,01
Bunch length
0,02
Strong RF Focusing (SRFF)*
Modulation of bunch length
along the ring with a minimum at the IP
Bunch Length
RF
RF
RF
IP
s (m) IP
0
20
40
60
80
100
120
* A. Gallo, P. Raimondi and M. Zobov :”Strong RF Focusing for Luminosity Increase”
DAFNE Technical Note G-60, 18/8/2003
High RF voltage
+
Magnetic lattice which correlates longitudinal
position with energy deviation (high momentum
compaction)
Longitudinal phase space
From RF to IP
IP
Energy
spread
RF input
RF center
RF output
Bunch length
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)