The PEP-II Lower Pressure HER vacuum chamber
S. DeBarger, S. Metcalfe, J. Seeman, M. Sullivan, U. Wienands, D. WrightStanford Linear Accelerator Center, SLAC, CA 94309, USAAbstract
This new vacuum chamber has been installed from 12 to21 meters upstream of the BaBar detector in the PEP-IIHigh Energy Ring (HER) to reduce lost particle
backgrounds. The backgrounds from HER now dominate
the backgrounds in the BaBar detector and the presentvacuum pressure is 1 x 10-9 Torr. The new chamber will
increase the pumping significantly by adding 18 x 2000 l/s
titanium sublimation pumps to the existing 5 x 440 l/s ion
pumps, and is expected to reduce the pressure by about afactor of five. Features of the chamber include improved
water cooling, improved vacuum conductance throughcopper RF screens featuring over 15,000 small squareholes and the ability to sublimate titanium while the beamis still on.
1. INTRODUCTION
It is critical to keep the pressure as low as possible in theregion of PEP-II [1] occupied by this vacuum chamber inorder to minimize the production of bremsstrahlung
scattered beam gas particles which cause BaBar detector
backgrounds and lead to radiation damage. The chamberthat has just been removed had 6 ion pumps, approx. 1mapart which proved inadequate for the task, having a total
pumping speed of 2640 l/s. The chamber area cooled bywater was increased to fully accommodate the powerful
SR fans (5.5 kW) generated by the High Energy Ring(HER) as it goes through the upstream B2 and B3
magnets and from the Low Energy Ring (LER) as it goes
through the two B1 magnets near the collision point.
2. THE NEW CHAMBER DESIGNWe chose a design that incorporated a plenum chamber
underneath the beam chamber in order to maximize thescreen area (and hence conductance) and to maximize thenumber of Titanium Sublimation Pumps (TSP’s) thatwould fit in between 5 of the existing ion pumps (which
remain to handle the non getterable gasses). The final
pump count was 18 x 2000 l/s TSP’s and 5 x 440 l/s ion
pumps = 38200 l/s. Ideally, we would have chosen tomake the chamber out of copper to maximize heat transfer
but this proved impractical to manufacture within the time
available, so stainless steel was chosen, with 15 copper
RF screens brazed in to the underside of the chamber.

Figure 1: Typical cross sections through the chamber through ‘X’ and ‘Z’ axes.

The overall length of the chamber is 882.22 cm. It was
desirable but impractical to manufacture this in one pieceso it was decided to make the chamber in 3 separate
sections, each with a length of 288.74 cm.

other to just break through (the inner slot will be parallelto the beam direction), followed by forming the plate on atool to the required radius. The resulting vacuum
conductance of the screens is 24500 l/s, almost 10 times
greater than the previous chamber.

A plenum design maximizes the number of pumps andarea of screen hence maximizing pumping speed,
however it reduces the chamber area that can be‘scrubbed’ by synchrotron radiation (SR) photons, sobeam conditioning is not expected to be as effective
although this should be more than compensated for by theincreased pumping speed.
A 2D analysis of the chamber and plenum identified highstress areas in the welds joining the plenum to thechamber so external ribs were added to stiffen up thestructure and to counteract the ‘bourdon tube’ effect ofthe pressure in the water jacket. Together with the internal
stiffening ribs between the RF screens, these measuresresult in a very rigid structure which shows no significant
deflection during vacuum and pressure testing.
The velocity of the water through the cooling jacket will
be relatively low, so a set of water diverter rods werewelded to the outside of the vacuum vessel before thejacket was attached in order to continuously mix thewarmer water at the sides with the cooler water at the top.
The attachments for the water supply and return in thewater jacket are adjacent to the SR masks to ensure
maximum water flow at these critical areas. These coppermasks are tapered from 0 to 0.35 cm high and brazed tothe inside of the vacuum vessel at appropriate locations toshield valves and bellows from the SR fans.
Thermocouples were placed at the edge of the water
jacket on both sides of the chamber at 2 m intervals.
These would abort the beam if a SR fan should stray fromits predicted path, below the water cooled area.
Thermocouples were also placed on the return coolingwater manifold.
The TSP cartridges (Thermionics Sierra Inc, model SB275-3) carry 3 Ti heating elements connected to acommon power socket outside the chamber. Eachcartridge is fitted with 7 shield plates which were placed
to ensure that Ti would not be deposited in the path of thebeam, allowing regeneration to take place while the beamis still on - see the shadow lines on Fig 1. The total Ti
covered surface is 131000 cm2. Water cooling coils are
fitted around the outside of this surface to improvepumping efficiency. The TSP’s are regenerated bypreheating at 35amps for 5 minutes and flashed at 44
amps for 3 minutes.
An RF screen design was required that maximized
vacuum and thermal conductance whilst being economic
to manufacture. Most designs did not meet the latter
requirement given that 15 screens were required, each 50
cm x 23 cm. The final solution involved slotting a flat
copper plate on both sides with one cut at 90 deg. to theFigure 2: The prototype screen.
A pair of RF screened, 20.3 cm ID, all metal gate valves,
with double acting pneumatic actuators were fitted at bothends of the third chamber. This new valve design wassupplied by VAT Inc, model 47146-CE77-X. The valves
were added to enable chamber 3 to be removed for future
maintenance without venting the entire beamline.
3. VALVE CONTROLSThe air to the VAT valves is controlled by electricallyactuated 24V 4-way pneumatic control valves located ona panel where the radiation levels are lower than thelevels at the valve itself. The air is nominally 100 psi“house air” with a local backup supply bottle. There is apressure regulator and air pressure switch on each panel.
The valves are equipped with IN and OUT position
switches, the status of which is reported to the control
system via the valve controller. The OUT switch status isalso reported to the Beam Abort Trigger System toprevent beam damage to the valve. The solenoids areactuated from a programmable multi-valve controller
capable of controlling up to 6 valves. Each valve can beprogrammed to close when the desired combinatorial
logic of up to 16 inputs, such as gauges or pumps, isrealized. The mode of operation for each valve iscontrolled by key actuated switches where the key can beremoved from any of 4 positions :1. CAMAC Control via the PEPII control system.
2. LOCAL Control from the front panel of the controller.
3. J-BOX Control from a key box located on thesolenoid panel near the valve.
4. CLOSE Valve closed

4. MANUFACTURING
The vacuum chamber proved difficult to manufacture,mainly as a result of distortion due to welding. Toolingjigs were manufactured to hold the chamber duringmachining and to straighten it after each stage of themachining/welding process. Also cleaning and stressrelieving was required prior to, and following, welding.
The first chamber to be delivered from the vendor was
accidentally pressure tested to a higher pressure than therequested 150 psi. Failure of the inner chamber occurredat 190 psi, causing a bubble along the weakest (thinnest)
section of the tube. This bubble was repaired by theapplication of heat, vacuum in the water jacket andconsiderable force along the fault line, however, the end
result was a chamber that was excessively bent andtwisted and we decided to make a new one.

From start of design to end of installation took exactly 12months: 5 months to complete the design and produce
shop drawings of the chamber, 6 months to manufactureand 1 month to install.
5. EARLY RESULTS
It will take some time to prove the effectiveness of thisnew chamber but the early signs are very encouraging.
The graphs below compare the recent startup with asimilar graph taken a year earlier. The latest graph shows
the same vacuum pressure even though it has had lessscrubbing time and more beam current.

Ultrasonic measurements of the vacuum chamber wall
thickness (from the inside - the outside was covered by
the water jacket) showed that there were variations due tothe tube manufacturing process. Unfortunately, one of theremaining chambers had it’s thinnest section in a highstress location and could have caused another, similar
failure so we decided to limit the water pressure in thechannel to 50 psi, requiring a pressure regulated watersupply with check valves and pressure relief valves toeliminate the possibility of overpressurising the system.
The vacuum furnace brazing process was carried outusing standard SLAC procedures. The screens and masks
were clamped into place on the chamber using a35Au/65Cu filler material and soaked for 4 minutes at1025OC. It was necessary to ensure that braze filler
material on the screens did not contaminate the adjacentarea where the plenum would subsequently be welded.

Figure 4: Year 2000 startup

The chamber is instrumented with three nude hot-filamentgauges (Varian UHV-24) which measure the pressure
distribution and which are used to calibrate a ResidualGas Analyzer (MKS Orion Compact). Two Pirani gauges(HPS) are used as interlock switches to permit opening of
the adjacent VAT valves.
Final preparation for assembly comprised glow dischargecleaning followed by a 200 OC bakeout for 4 days.

Figure 5: Year 2001 startup
6. REFERENCE

Figure 3: Upstream half of the assembled chamber.

[1] “PEP-II Conceptural Design Report”, SLAC report
418, LBL-PUB-5379, June 1993