THE PEP-II Movable Collimators
S. DeBarger, S. Metcalfe, C. Ng, T.G. Porter, J. Seeman, M. Sullivan, U. Wienands,
Stanford Linear Accelerator Center, SLAC, CA 94309, USA
which cause BaBar detector backgrounds and lead toradiation damage.

Abstract
Three movable collimators have been manufactured for
installation in the PEP-II LER and HER beamlines
upstream of BaBar to improve backgrounds in BaBar bya factor of 2. Each collimator has a pair of horizontallyopposed, water cooled jaws with RF finger seals allaround the edge of the jaws, these seals are the only
sliding parts inside the vacuum chamber. Each jaw
travels independently through a distance of 16.5 mm(LER) or 21mm (HER) and is supported above thecollimator from motorized slideways with position
feedback. The larger HER collimator has a titanium
sublimation pump incorporated into the underside of thecollimator, pumping through RF screens in the bottom ofthe chamber. Water cooled fixed ramps protect theleading and trailing edges of the jaws.
1. INTRODUCTION
The new collimators are part of a package of measuresthat are being introduced in the PEP-II [1] beamlines tocapture the bremsstrahlung scattered beam gas particles

2. COLLIMATOR DESIGN
Dispersion strengthened copper, known by the tradename Glidcop [2] AL-15, was chosen for all the surfaces
exposed to high heat loads because its of high strength atelevated temperature [3] and hence its ability to tolerate
up to 200 W/linear cm of SR power during operation.
In order to provide the most rigid structure, the body wasmachined from a single billet of Glidcop with Glidcop
transitions attached to each end of the body. Thetransitions are octagonal at the outer end, to match theexisting beam pipe, and rectangular at the inner end, tomatch the collimator body, where the aperture is 5.69 cmwide x 5 cm high for the Low Energy Ring (LER) whilst
the High Energy Ring (HER) collimator is 8.33 cm wideto accommodate the larger beam profile. The transition
also incorporates water cooled side ramps. Stainless steel
end flanges are fitted for connection to the beamline andare fitted with a standard RF omega seal.

Figure 1: Cutaway view of the HER collimatorEach Glidcop jaw is gold plated and water cooled over its
entire exposed surface. They can be positioned
independently, with a resolution of 0.0025 cm; the jaw
travel is 1.65 cm (LER) 2.1 cm (HER). The bar attachedto the back of the jaw has 3 functions: 1) it provides
physical support for the jaw, 2) it contains the supply andreturn water passages, 3) it contains the air vents thatwould be used to identify a water leak within the jaw.
Internally, the jaw contains brazed in stainless steel
water diverters which ensure even cooling of the surface.
There are no water to vacuum brazed joints, instead an
air gap was fitted, which vents to 3) above. Convection
analysis indicated that the water cooling requirements
would be 2 gpm for each of the jaws and 0.5 gpm foreach of the ramps.

There was insufficient room for a TSP underneath theLER collimators so a NEG pump and a 440l/s ion pump
was fitted in the drift chamber immediately upstream of
the collimators.
The area to the side of the collimators has very little
space available, so the jaws are supported by slideways
mounted above the collimator via rigid arms that connect
the slideways to the back of the jaw. A bellows unit
makes the vacuum seal. A constant force coil spring
system was fitted to counterbalance the effect of vacuumat the bellows, ensuring that the motor sees the sameforce whilst operating in either direction.
To provide support and fine adjustment of the collimatorassembly, a standard SLAC designed Sukiennicki
support system, was fitted. This design uses rod ends
with differential nuts with separate adjustments for eachdirection of motion.

figure 2: Assembling fingers on a jaw.
The RF fingers are screwed on to the jaws, simplifying
replacement in the event of a damaged finger. Muchdevelopment work went into the type and thickness ofplating to be used on the sliding surfaces of finger andbody. The chosen combination, which, under testing [4],ran for the equivalent of 10 years without failure,
involved silver plating the body 0.018 mm and rhodium
plating the fingers 0.0025 mm. Prior to rhodium plating,the fingers were electropolished to remove all sharpedges caused by the EDM process then nickel flashed.
Other than the RF fingers, there are no moving/sliding
parts in the vacuum.
The HER collimator, which is currently being
manufactured, will have a single Titanium Sublimation
Pump (TSP) lying horizontally in a plenum underneaththe collimator. The TSP cartridge (Thermionics Sierra
Inc, model SB-275-3) carries 3 Ti heating elements
connected to a common power socket outside thechamber. A mask is fitted above the heating elements toensure that Ti would not be deposited in the path of thebeam, allowing regeneration to take place while the beamis still on. The total Ti covered surface will be 2200 cm2.

Figure 3: Sukiennicki adjustable support
Temperature monitoring is provided by 3 thermocouples
and a klixon on the collimator body and water returnlines. If any of these sensors detect a temperature of 66OC
(default setting) the beam abort system is triggered.
A polycarbonate cover is fitted over the top of thecollimator to protect the slideway components andbellows from dust and water.
For technical specs of collimators see [5]3. MANUFACTURE
A prototype jaw body was first manufactured fromaluminum to prove the complex CAM/EDM process. The
body and transition were hollowed out by a combinationof milling and EDM.
Prior to brazing, each component was plated to 0.013mm
with copper to minimize the diffusion of braze material
into the Glidcop Cu/Al2O3 matrix at elevatedtemperatures and to improve the vacuum sealing qualities
which can be a problem with Glidcop.

The vacuum furnace brazing was done in a 2 stage
process using standard SLAC procedures and tolerances.
The first braze used 35Au/65Cu filler alloy at 1025 OC,
soaking for 3 minutes and this was used to join thecooling bars to the body and the end flanges to theextensions. Next the assembly was brought together forthe second braze using 50Au/50Cu filler alloy at 985OC
for 4 minutes.
Assembly of the collimator was aided by the addition ofadjusting screws to finely position the jaws in the body.
Also, alignment tool holders were located on jaw arms
and the end flanges in order to calibrate the jaw position
relative to the body.
As part of the final cleaning process, the collimator andslideways (motors, LVDT s and switches are removed)
are subject to a 200 OC bakeout. In order to withstand this,the slideways, which were reconditioned having
previously been kept as spares for a wire scanner, werestripped down and lubricated with Nye Synthetic
Flouroether Grease type 899S-1 which is rated to 300 OC.
Also the plastic roller ball guides were replaced withaluminum guides.

figure 4: LER collimator installation

4. CONTROL SYSTEM
5. EARLY RESULTS
The PEPII Movable Collimators Control System is a
closed loop computer based system consisting of astepper motor drive system with LVDT s (Linear VoltageDifferential transducers) for positional feedback. Thestepper motors are Superior Electric Slo-Syn type M091FD-419E which are 4 phase, 6 wire, 150 oz-in DC
stepper motors, designed to operate at 5.3V driving 1.6A
per phase and are specially ordered with radiation
resistant wiring. They are operated in 200 steps/rev. fullstep mode driving a 2 mm/thread screw drive assembly.
This results in a step resolution of 10 µm/step. The drive
rate is set at 200 steps/sec. thus moving the collimators ata rate of 2mm/sec. The motors are driven from single
width CAMAC based stepper motor controllers, model
SMC-24B, manufactured by Joerger Enterprises. Theyare L/R drive type operating in full step, power down
mode. There are limit switches at both limits setapproximately 0.4mm from the stops to prevent the slide
assembly from being driven into the mechanical stops.
Software stops are set approximately 0.25mm before theswitches. The position of the collimators are fed back to
the control system by the use of Daytronic corporation
type DS500-S LVDTs which provide a linear voltage
output proportional to the collimator position over a 1
inch range. These are also specially ordered withradiation resistant wiring. The LVDTs are controlled byDaytronics dual LVDT conditioner cards type 10A302X8 specially modified for SLAC and are located in a
Daytronic 10K1 chassis. The output voltage from theconditioner card is taken directly from the card and fed
into a SAM (Smart Analog Module) which is a single
width CAMAC module.

Operation of the LER collimators has so far been
restricted to a single excursion of one of the jaws andthe results were very encouraging. We intend to wait forsome time until the beam has been fully conditioned
before operating the collimators.

6. REFERENCES[1] PEP-II Conceptural Design Report , SLAC report
418, LBL-PUB-5379, June 1993
[2] Glidcop is a registered trademark of OMG Americas
Corp., Research Triangle Park, NNC, USA[3] R.Valdiviez et al, The use of dispersion strengthened
copper in accelerator designs Los Alamos NationalLaboratory.
[4] Wear test. SLAC report, S.J.Metcalfe, May 2000,
http://www.slac.stanford.edu/grp/ad/me/coll_fin_wear.htm[5] J.Seeman et al, PEPII New High Power, Low
Impedance, Movable Collimator , P2298, EPAC2000,June 2000