STATUS REPORT AND FUTURE PLANS FOR THE PEP-II B FACTORY
Alan S. Fisher, SLAC, Stanford, CA 94309, USA
Abstract

Table 1. Some PEP-II Design Parameters.

The PEP-II B Factory at SLAC has been in operation
with the BABAR detector since the summer of 1999. The
peak currents and luminosity steadily increased throughthe end of the 2000 run on October 31. By that time,
0.75 A of electrons (the design current) routinely
collided with 1.4 A of positrons in 657 bunches, to givea peak luminosity of 2.6×1033 cm-2·s-1. We delivered an
integrated luminosity of up to 184 pb-1 per day and1033 per week; BABAR logged a total of 25 fb-1. Threedays of machine development at the end of the run
raised the peak luminosity to 3.29×1033, exceeding thedesign goal of 3.0. BABAR then logged data for an hour,
starting with a peak of 3.20. Next, we achieved thedesign value for positron current, 2.14 A, operating
without collisions.
The major limitation has been growth in the size ofthe positron beam in the low-energy ring (LER) due toelectron clouds and multipacting. Since the arcs benefit
from antechambers and a TiN coating with low
secondary emission, our efforts have concentrated on
the straights, where we added solenoid windings. Eachstraight wound allowed higher LER current without
blow-up, and a consequent increase in luminosity.
Measurements of the bunch-by-bunch luminosity
showed that the effect becomes significant by about thetenth bunch in a train, but clears after a short gap.
Careful control of the fill pattern has thus been essential
in raising luminosity.
For the 2001 run, which began in February, we added
a third LER RF station, to collide with more LER
current and more bunches, and also tried reducing βx*
from 50 to 35 cm, with the goal of reaching a luminosity of 5×1033 by year’s end. In 2003, we plan to add afourth LER and sixth HER RF station to reach 1×1034.

Circumference [m]RF frequency [MHz]Harmonic number
Colliding bunches
Current [mA]
Beam energy [GeV]
CM energy [GeV]
βx*, βy* [cm]εx*, εy* [nm]σx, σy, [µm]σz [mm]Tune shift
Aspect ratio at IP (v/h)
Crossing angle

LER
HER
2199.322
476.00
3492
1658
2140
750
3.119
8.973
10.58
50, 1.5
50, 1.5
49, 1.5
49, 1.5
157, 4.7
157, 4.7
12.3
11.5
0.03
0.03
0 (head on)

1 INTRODUCTION
The PEP-II B Factory, a 2.2-km asymmetric collider
at the Stanford Linear Accelerator Center[1], was built
in collaboration with the Lawrence Berkeley[ 2 ] andLawrence Livermore[3] National Laboratories to study
CP violation by tracking decays of B mesons moving in
the lab frame. At a single interaction point (IP), 9-GeV
electrons in the high-energy ring (HER) collide at zerocrossing angle with 3.1-GeV positrons in the lowenergy ring (LER). Table 1 lists some PEP design
parameters[4]. The first collisions were observed in July
1998, when the rings were commissioned without the
BABAR detector. After BABAR was installed in May
1999, commissioning of the full system began, andphysics runs followed shortly afterward. By the end of
1999, the luminosity had passed 1×1033 cm-2·s-1.

2 THE 2000 RUN
Table 2. PEP-II Records, as of March 2001. The data incorporates the newluminosity calibration discussed in Section 2.1.
Peak LER current [mA]
Peak HER current [mA]
Number of full buckets
Peak luminosity [1033 cm-2·s-1]
Peak luminosity with BABAR [1033 cm-2·s-1]
Integrated luminosity per shift [pb-1]
Integrated luminosity per day [pb-1]
Integrated luminosity delivered per week [pb-1]
Integrated luminosity delivered per month [pb-1]
Total luminosity integrated by BABAR [fb-1]

Goal
2140
750
1658
3.0
3.0
45
135
785
3300

Achieved
2140
920
1658
3.29
3.20
66
184
1033
3820
26

2.1 Highlights
The 2000 run, from January throughOctober, achieved a number of records,
as Table 2 shows. In the machinedevelopment days at the end of the run,the luminosity reached a peak of 3.29
×1033 cm-2·s-1, exceeding the design
goal of 3.0. BABAR then took data for ashort while at 3.20. This luminosity
was achieved with currents of 1550
mA in LER and 800 mA in HER,Table 3. Parameters for Record Luminosity.
LER
Peak luminosity [1033 cm-2·s-1]
Current [mA]
Number of bunchesΣx,y at low current [µm]IP beam sizes, x, y (average) [µm]
Horizontal tune shift
Vertical tune shift

HER
3.29

1550

800
692
210, 6.7
147, 5
0.069
0.060
0.055
0.028

filling only 692 bunches and so suggesting that higher
luminosities will be possible in the future. The parameters for this luminosity are summarized in Table 3.
During October, the integrated luminosity, measured
over a shift, day, week or month, all exceeded thedesign goals. Also, in single ring operation, the LER
current was raised to the design value of 2140 mA, andthe HER reached 920 mA, exceeding the design valueof 750.
This luminosity is a bit higher than values reported
previously. The BABAR group recently refined theircalibration of our luminosity monitor, based on adetailed off-line study of e+e–→e+e– and e+e–→µ +µ –decays. This correction, which adds 6% to theluminosity, is incorporated here.

2.2 Thermal and Vacuum Issues
As the currents increased during the run, thermal
limitations became apparent in several components. A
number of bellows required modest air cooling. Moreseverely, the connectors and directional couplers on thelongitudinal feedback kickers overheated, at one point
burning some of the Heliax-cable insulation. New
couplers and additional cooling were added immediately, with more improvements added during thedowntime (Sec. 3.4); temperatures there are now closely
monitored with thermocouples at numerous points.
Two permanent-magnet dipoles (B1), located inside
BABAR on either side of the IP, bring the beams into
head-on collision. They create hot synchrotron fans thattravel outside the detector to cooled chamber surfaces
10 to 15 m away. The hottest fan, from the electrons,
strikes the long “high-power dump” chamber at grazing
incidence over several meters. This water-cooled,rectangular, copper chamber began to leak at one corner,
where it was brazed to a stainless-steel vacuum flange.
The hot fan caused one side of the chamber to expand
more than the opposite side; over many cycles, thestress cracked the braze. While building a replacement
chamber with an improved design, we patched the leakto finish the run. A metal “boot” was epoxied to theregion outside the leak and evacuated with aturbomolecular pump, in order to maintain acceptablevacuum inside the beampipe. However, we limited theelectron current to a maximum of 600 mA until the final

weeks of the run, in order to avoid further damage. The
chamber was replaced during the downtime between the2000 and 2001 runs.
For the longer term, the most worrisome thermal
constraint has been found on the bellows at the forward(downstream electron) end of the “support tube”—the40-cm-long beryllium beampipe enclosing the IP andrunning between the inboard ends of the two B1
magnets. Some power from the beam goes into a mode
trapped by a synchrotron-radiation mask just outboardof the support tube, and this power appears to leakthrough the shielding fingers of the bellows. The waterlines for cooling the support tube enter from the otherend and turn around a few centimeters away from thebellows, reducing the cooling there. The bellows is deep
inside the detector, tightly surrounded by the siliconvertex tracker (SVT), making it inaccessible until thescheduled access to the SVT in two years. The most we
can do until then is to force a small flow of dry air along
the support tube. The benefit is limited, since a radial
ion pump outboard of the bellows on each side blocks
the flow of air along the tube wall.
Measurements of the dependence of the bellows
temperature on the currents in the two rings show alargely quadratic behavior, indicative of heating fromhigh-order modes rather than synchrotron radiation. The
LER also has a linear term. We interpret this as achange in bunch length with current, since we also
found a linear dependence on bunch length by varying
the LER’s RF voltage. The cross term from the productof the HER and LER currents, responsible for about
10% of the heating, varied with a scan of the arrival
phase of the HER relative to the LER at the IP. The
spatial periodicity suggested a trapped mode with afrequency of 5.4 GHz.

2.3 Multipacting and Electron Clouds
Last spring and summer, the current in many LER
ion pumps grew slowly as a function of beam current,
then crossed a threshold and increased at a much greater
rate. We also observed that the specific (normalized)
luminosity, which depends only on the sizes of theoverlapped beams at the IP, was found to drop
significantly at currents below those at which beambeam limitations should come into effect.
Although pump current is normally proportional topressure, much of this increased current dropped
immediately when the beam was aborted. The rest of
the signal, from gas desorbed by the beam, decayed
exponentially over tens of seconds. We attributed therapidly lost pump current to the collection of electrons
released by synchrotron radiation from the beampipe
walls. The rapid rise with current then was interpreted
as a multipactor process: the electrons are drawn into
the positron beam and accelerated across the beampipe,
releasing more electrons in a cascade that creates anelectron cloud. In some cases the process was clearlyresonant: as the beam current increased over threshold,the pump current first increased rapidly to a peak, thendecreased. The details of this resonance depend on thebunch current, bunch spacing, beampipe geometry, andwall cleanliness. A solenoid wrapped around thebeampipe adjacent to the ion pump reduced the electron
cloud by preventing photoelectrons from crossing
through the beam potential and ejecting more electrons
from the opposite wall.
At the same time, we observed that the LER beamsize, measured by the synchrotron-light monitor,
exhibited similar behavior, growing little with current
below a threshold, and then increasing rapidly. The
threshold was lower, and the growth somewhat more
rapid, with the beams in collision, but it was alsopresent in single-beam operation. We attribute it to theonset of the electron-cloud instability (ECI)[5] due tothe cloud created by multipacting. Two techniques havebeen helpful in controlling this effect: solenoidal fields
and short gaps in the fill pattern.
Since the arcs have much more synchrotron radiation
than the straights, they might be expected also to havedenser electron clouds. However, LER arc chambers are
made of aluminum extrusions with an antechamber.
Downstream of each dipole, a “photon stop” in theantechamber receives the hard synchrotron radiationfrom the bend. Secondary electrons are thus kept out of
the main chamber and the field of the positron beam. To
further suppress the ECI, the aluminum walls are coatedwith titanium nitride (TiN), to lower their secondaryemission coefficient. In contrast, the beampipes in
PEP’s straight sections are mostly stainless-steel
cylinders. The material and the circular geometry permit
the growth of the instability.
Consequently, we added our solenoids to thestraights. Solenoidal windings wrapped directly on the
tubes, with a typical field of 30 G, noticeably helped in
reducing the LER beam blow-up, even when only asingle straight was wrapped with about 100 m ofwindings in the drift spaces. We have now addedsolenoids to all six straights, for a total of
approximately 600 m. As each straight was wound, thethreshold for beam-size growth moved higher and theluminosity increased.
The electron cloud builds up quickly, but also decays
quickly, and so small gaps in the fill pattern can have a
big effect. PEP’s design called for filling every second
RF bucket (2.1 ns per bucket) around the entire ring
except for a 5% gap for both clearing ions and ramping
up the beam-abort kicker’s field. However, ourinjection control allows us to fill any specified bucket toany desired charge. As we gradually raised the beam
currents while optimizing luminosity, we began withfewer buckets, to keep a high charge per bunch until
limited by beam-beam effects. However, we kept touniform fill patterns; early in the year we filled every 8th(called a “by-8” fill), then every 6th, 4th, and finally 3rd

by October. (We preferred not to use by-2 too quickly
because bunch n then experiences a parasitic nearcrossing with bunch n±1 on either side of the IP, justoutboard of the B1 magnets.)
Our luminosity monitor has a mode that sweeps agate across the fill pattern to provide a display of theluminosity from each bunch. We observed a strong drop(≈60%) in luminosity from the first to the last buckets,and this drop had a time constant of about 100 buckets.
By inserting various “microgaps” in the fill pattern, we
determined that removing even 6 to 12 buckets allows a
noticeable recovery. The luminosity significantly
improved after adjusting the total number of filled
buckets, the number of and length of gaps, and thelength of the individual trains. The record luminosity
was achieved with a by-3 fill in which 10 bunches werefilled, followed by 6 left empty (bucket numbers 0, 3,
6…27; 48, 51…), for a total of 692 bunches.
The bunch-by-bunch luminosity and current monitors
also provide insight into the relative strengths of thetwo beams. The first LER train, following the main gap,
is better focused than subsequent ones because it hasn’t
encountered a thick electron cloud. As a result, there
can be significant loss of electrons from the first HER
train. After top-off, this train begins with the highest
luminosity, which drops as electrons are lost. To
counteract this, we gradually increase the charge per
LER bunch in the first train with a programmed ramp.

2.4 Other Issues
Considerable effort went into learning how todecouple the rings and compensate for the twist given
to the beams by BABAR’s solenoidal field. The mosthelpful technique was to generate a large orbit wave
with a corrector in one plane, then to observe thecoupling into the other plane with the beam-position
monitors (BPMs). Two correctors 90º apart in betatronphase are needed per plane. Careful comparisons withthe model can localize errors in the compensation
provided by skew quadrupoles in the region of the IP.
A number of feedbacks were added during the run tokeep the luminosity high. The IP loops dither the
HER’s position (x, y) and angle (x′, y′) at the IP tomaximize the luminosity monitor’s signal. The
HER/LER combined angle loops move both rings
together at the IP to optimize pointing at the luminosity
monitor, about 10 m away. Other loops maintain beam
position in sextupoles and in the pick-ups for transverse
feedback.
HER and LER orbit loops use BPMs in the arcs oneither side of the IP to determine a kick near the IP thatcompensates for thermal motion of the magnet supports(“rafts”) next to BABAR, mostly due to diurnal
temperature variations. Since that time, we have hadsome success in reducing the motion by using ourtemperature-controlled cooling water to limit
temperature changes in the rafts on either side of BABAR,

and we are trying to determine if these loops remain
helpful.
Backgrounds in BABAR remain a problem. Radiationdetecting PIN diodes inside BABAR, near the SVT, abort
the beams several times a day when the levels exceed asafe dose. We have been learning how to tune the rings
to reduce backgrounds. Improvement should be seen
later in the 2001 run (once sufficient scrubbing hastaken place) due to our downtime work on the IP-areavacuum chambers.
The HER is also subject at times to sudden drops inlifetime, often accompanied by bursts of radiation at the
SVT diodes, leading to a beam abort. We tentatively
attribute this to dust particles falling into the beam, buthave not been able to trace the source of the dust to
individual vacuum chambers. At times the lifetime
remains low (and the backgrounds high) for several
minutes, which may be caused by a plasma fromionized dust. At times shaking the HER (with sweptfrequency sinusoidal motion excited through thetransverse-feedback kickers) restores the lifetime, butmore often we are forced to abort the beam.

3 2000–2001 DOWNTIME WORKThe PEP shutdown—from November 1 throughFebruary 2—allowed us to make several improvements
addressing the issues discussed above.

3.1 Solenoids for the Arcs
We have begun to wrap the far longer extent of thearcs, to determine whether the arcs contribute in part to
the ECI. Half of one arc was wound in January. The
work is more difficult because the antechamber gives
the beampipe a greater perimeter, and because a steel
support tube running above the aluminum chamber
makes wrapping difficult.
The beam chamber within the extrusion is an ellipse,
with a narrow opening on the +x side leading through aneck to the antechamber. With this geometry, it may not
be necessary to use a solenoidal field; instead, a horizontal dipole field oriented to direct stray electrons
through the neck could be sufficient to suppress theinstability. Such a field turns out to be much easier toarrange on our chambers. Long L-shaped forms can bewound in advance with coils running along the lengthof the L. The base of the L, with one side of the coil, isplaced in contact with the chamber, while the returning
side of the coil runs along the other arm of the L,
further from the beam. When placed above and below
the elliptical part of the beampipe, the pair of coils
produces a horizontal field at the beam. The polarity of
the field is reversed every 2 m to cancel the steering.
We are installing half an arc of these coils forcomparison with half an arc of solenoids.

3.2 Vacuum Improvements
The HER’s high-power dump chamber (Sec. 2.2) was
replaced, eliminating the major vacuum leak near the IP.
Starting 12 m away from the IP on the opposite(outgoing electron) side, a new chamber with largetitanium-sublimation pumps was installed to lower thepressure in a 9-m region that appears to be a significant
source of backgrounds.
The number of ion pumps was doubled in LER Arc 7,
following a program to increase the base pumping
speed in all the LER arcs, and so to obtain a higher
beam lifetime, which has typically been 1 hour whencolliding. (Ti-sublimation pumps in the antechambers
provide additional speed for the outgassing from thephoton stops.) The leaks that caused a somewhat
elevated pressure were fixed. A few minor leaks remain
due to difficulties with the tin-foil seals on thealuminum flanges.

3.3 Background Reduction
New horizontal collimators were installed in the LER,
12 and 25 m upstream of the IP. These have a
motorized jaw on both sides (±x).
A shielding wall was installed at the tunnel mouth, on
the forward side of BABAR, to reduce backgrounds due
to radiation from the tunnel. A similar wall waspreviously installed on the backward side.
We installed pulsed beam-separator magnets, whichshould provide cleaner injection by separating thebeams during fills and top-offs, and then rapidly
bringing them back into collision.

3.4 RF and Feedback
A third LER RF station was added, to provide
enough power for currents of up to 3 A. The cooling forthe connectors and couplers of the longitudinal feedback kickers was improved, as discussed in Sec. 2.2.

3.5 Survey
The entire ring was resurveyed. Errors in individualcomponents were found and corrected. In addition, we
adjusted for some settlement of BABAR by smoothly
adjusting the beamlines on either side to match, since
there was no time to reposition the detector.

4 STATUS OF THE 2001 RUN
The goal for the run, which lasts from February to theend of August, is to reach a peak luminosity of 5×1033
cm-2·s-1, and to deliver an integrated luminosity of 32
pb-1 over this time.
At this time (mid-March), the run has been difficult,probably because of many changes in too short a time.
In January, even before the run started, the RF
frequency was increased by 600 Hz, to better center thebeams in the rings. While this should be beneficial inthe longer term, we then weren’t able to return to thehorizontal orbits of October as we turned back on.
While the machine was still scrubbing, recoveringfrom the extensive vacuum work, we took theopportunity to do machine development, and lowered
βx* from 50 to 35 cm, first in the LER, and then thefollowing week in the HER.
With the tighter beam in LER, the positrons for the
first time dominated the electrons in beam-beam.
Electron beam loss was common, and there was little
margin for error in tune adjustment with changes incurrents. However, we were able to get to a peakluminosity of 2×1033. The subsequent decrease in the
HER βx* strengthened HER somewhat, but the tune
space remained difficult, and beam loss was
unacceptably frequent, precluding effective tuning.
Early in March we changed course, trying to revert asmuch as possible to the configuration of late October.
We did not, of course, undo the magnet repositioning
from the new survey, but we restored the RF frequency
and the βx* values. The magnet configurations werereloaded, and the ring orbits were steered back to the
saved orbits from October.
After these changes, it was easier to maintain the
beams in collision, providing time to tune the IP skew
quadrupoles and optimize luminosity. The peakluminosity is typically 2.2×1033 at this time.
Thermal constraints now limit the currents and hence
the luminosity. When new hot components are
discovered, we add more thermocouples and set thebeam-abort thresholds with caution. The result is that
thermal aborts are now more frequent than before, notbecause the machine is hotter, but because we arelooking in hotter places and have set lower limits. For
example, the connectors on the kicker chambers forlongitudinal feedback still get hot and now havenumerous thermocouples. The heating is more severe
with the by-3 fill pattern used in October (when wewere less aware of the problem), leading us to operate
mostly with by-2 now.
In addition to the hot components found last year, wehave recently discovered a high temperature that comesand goes erratically at a LER gate valve 12 m before theIP. The cause may be a LER synchrotron fan fromupstream that should be blocked by a mask. However,
the mask does not span the full height of the chamber,and small changes in the orbit may allow some of the
fan to hit the valve.
If the vertical angle of the HER at the IP is high, thenits B1 fan heats a large HER vacuum flange about 8 maway. Because this fan caused a leak in the past, thetemperature limit is now set low. The HER IP position
and angle feedback loops move HER to find the best
overlap with LER for luminosity. At times the
feedbacks appear to move the HER angle upward,
apparently following some change in the LER andcausing an abort. We suspect an intermittent problem

with a BPM may cause a LER orbit feedback to move,
potentially causing either of these thermal problems.
Consequently, we are reviewing the performance of allloops. Some may need improvement, while others may
no longer be needed, since the drifts they were designed
to compensate may now be reduced.
We are also losing beam several times a day due totrips of the HER RF stations. In some events, the RF
may not have been the cause; instead, the high reflected
power appears to follow a beam abort from some othersource that fails to register with the abort system.
Once we have fully returned to October’s
performance, we plan to reintroduce the changes infrequency and βx*. When we commission the third LER
RF station, the shorter bunch length will permit us toreduce βy* from its present 12.5 mm; a 10-mm lattice isready to try with beam, and a 7-mm lattice is ready for amagnet test. We will also commission the pulsed
separator magnets.

5 PLANS FOR 2002 AND BEYOND
The medium-term goal is to reach a luminosity of1034 cm-2·s-1 by the end of 2003. We have begun
building two more RF stations (one per ring), to get thehigher current and shorter bunches this goal requires.
The new stations should be ready by the fall of 2002.
For the long term, we have begun studying paths to
3×1034 cm-2·s-1. Both rings would require morecurrent—1.5 A in the HER and 4 A in the LER—andthus more cooling and more RF power. Similarly, thebunch-by-bunch feedbacks would have to bestrengthened. We would further reduce βy* to about 6
mm, which would involve moving the IP quadrupoles
inward. We would then need to further shorten thebunch length, perhaps with a lower α or with thirdharmonic RF cavities. The new, tighter IP wouldchange from our head-on configuration to one with acrossing angle of perhaps ±3 mrad, allowing morebunches without the parasitic crossings.

5 ACKNOWLEDGMENT
I wish to thank the large number of people—theaccelerator and BABAR phyicists, machine operators,
and members of SLAC’s technical staff—whose efforts
have made PEP-II successful.

REFERENCES[1] U.S. Dept. of Energy contract DE-AC03-76SF00515.
[2] U.S. Dept. of Energy contract DE-AC03-76SF00098.
[3] U.S. Dept. of Energy contract W-7405-Eng-48.
[4] PEP-II: An Asymmetric B Factory, Conceptual Design
Report, LBL-PUB-5379, SLAC-418, CALT-68-1869,
UCRL-IC-114055, UC-IIRPA-93-01, June 1993.
[5] K. Ohmi and F. Zimmermann, “Head-Tail Instability
Caused by Electron Clouds in Positron Storage Rings,”
Phys. Rev. Lett. 85, 3821 (2000).