WE6RFP027
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1. the beam power being delivered to the dump a beam profile position monitor to measure the beam density and position at the entrance to the dump a dipole corrector to steer the beam to the center of the dump and some additional beam loss monitors The beam dump itself is also being redesigned Our goal is to design and build a forced air cooled dump to completely eliminate the complications e g radiolysis resin beds etc that come with water cooled systems The conceptual design calls for removing the water cooled portion of the dump thus leaving a 38 x 38 x 331 cm hole in the existing large stack of steel and concrete shielding and replacing it with an AJT graphite and AISI A36 steel assembly The steel 1s necessary since the space available is not sufficient to fully stop a 1 3 GeV proton beam by using only graphite Two isometric CAD images of the new design are shown in Figs 2 3 The new dump will be designed for 5 kW twice the rating of the old dump Recirculated air is pumped into the outer jacket of the new assembly The reentrant vacuum window is also cooled by this air A heat exchanger located in the tunnel and cooled by the magnet water cooling loop will remove the heat from the dump Profile A N BCM S BLM Dipole corrector Figure 1 Momentum dump beam line upgrade The new dipole corrector and beam diagnostics are shown in red SCRAPER ELECTRONICS The original design of the three HEBT collim2. Proceedings of PAC09 Vancouver BC Canada WE6RFP027 PERFORMANCE OF AND UPGRADES TO THE SNS COLLIMATOR SYSTEMS M A Plum A A Abdou L Jacobs J Janney P J Geoghegan S McTeer I Popova P Ferguson A Zhukov Oak Ridge National Laboratory Oak Ridge TN USA Abstract As the Spallation Neutron Source SNS beam power is increased the collimator systems are becoming correspondingly more important The High Energy Beam Transport HEBT transverse collimators are now routinely used during neutron production We are in the process of redesigning the HEBT momentum collimation system due to problems with gas production from radiolysis The Ring collimators are designed for two stage operation but to date they are mainly used in one stage mode In this paper we will discuss the status the operational performance and upgrades to the collimation systems INTRODUCTION With a design proton beam power of 1 4 MW delivered to the neutron spallation target even beam losses that amount to of a small fraction of a percent become significant and cause high activation levels that can impede accelerator maintenance activities The SNS collimator systems 1 are designed to alleviate this problem by cleaning beam halo and tails that can cause beam loss by stripping it off and depositing it into well shielded enclosures The SNS has collimator systems in the HEBT the Ring and the Ring to Target RTBT beam lines In the straight ahea
3. ator systems had just beam loss monitors to estimate how much beam was intercepted by the scrapers and sent to the collimators dump This is an indirect and inaccurate Accelerator Technology Subsystems T19 Collimation and Targetry Proceedings of PAC09 Vancouver BC Canada measurement and it does not allow one to determine how much beam 1s individually intercepted by the scrapers Figure 2 The new momentum dump including the square housing that slips inside of the shielding Recirculated air is pumped into the outer port and returns on the inner port Figure 3 Cross section of the new momentum dump Green shows the reentrant vacuum window brown shows the graphite and magenta shows the steel These scrapers have now been modified to be electrically isolated and the signals caused by secondary electron emission are collected by electronics interfaced to the control system and the machine protection system The waveforms can be observed in the control room and the user can determine how much beam power is intercepted by each scraper Also if the signal level exceeds a pre determined threshold indicating that too much beam power is being intercepted the machine protection system will turn off the beam This function is performed in an analog front end that is independent of software and timing To mitigate noise issues twisted pair type cable is used to carry the reference noise line along with signal line The refe
4. d portion of the HEBT shortly after the linac two collimator systems each rated for up to 2kW of beam power scrape beam tails that may be present in the transverse phase space Each system employs four top bottom left and right independently controlled thin 25 mg cm carbon scrapers to strip the tails of the H beam to H Once the beam polarity of the tails is changed they take a different path through the magnetic lattice and are absorbed in heavily shielded water cooled cylindrically symmetric collimators Further along the HEBT beam line at a point of high dispersion in the 90 deg arc a third set of left right scrapers intercept the high and low momentum tails of the beam In this case the newly created H particles are bent by the arc dipoles in the direction opposite to the main H beam into a short beam line leading to a water cooled beam stop capable of absorbing up to 2 5kW of beam power In the Ring where the beam species is H simple charge exchange stripping is no longer an option so instead the beam tails are intercepted by thick water cooled tungsten scrapers The intercepted beam is ORNL SNS is managed by UT Battelle LLC for the U S Department of Energy under contract DE AC05 000R22725 Accelerator Technology Subsystems T19 Collimation and Targetry scattered and collected in three water cooled heavily shielded cylindrically symmetric collimators much like the first two HEBT units each
5. er line with circular symmetry on the plane of the disk from radii 2 3 cm to 5 3 cm and with the 1 GeV protons directed parallel to the beam line axis The energy deposition was calculated on the basis of one incoming proton and then normalized to the full beam intensity This model indicates that for 2kW of beam power absorbed by a collimator system 160 watts is deposited in the water Measurements taken in the neutron production target deionized water cooling systems indicate that 0 4 to 0 5 molecules of hydrogen are produced per 100 eV of energy absorbed in the water This combined with the particle tracking simulations for the HEBT collimators allows an estimate of the hydrogen gas creation to be 0 5 0 7 I h for 2kW absorbed by a HEBT collimator system This gas generation rate is comparable to that estimated for the momentum collimator system However gas can be more effectively vented from the HEBT transverse collimator cooling water system and to date it has not caused any operational problems 2848 Proceedings of PAC09 Vancouver BC Canada DESIGN OF NEW MOMENTUM COLLIMATOR SYSTEM The new momentum collimator system will include an upgrade of the beam diagnostics between the carbon scrapers and the beam dump The initial design had just beam loss monitors to estimate how much beam was being scraped and delivered to the dump As shown in Fig 1 the new design will include a beam current monitor to directly measure
6. king well RADIOLYSIS The general definition of radiolysis is the dissociation of molecules by ionizing radiation In our case we have the dissociation of water molecules in the cooling water system by the 1 GeV proton beam All of our collimator systems absorb the beam power in a water cooled bed of stainless steel balls and a typical path of a charged particle hitting the collimator will include some water Radiolysis products include hydrogen hydrogen peroxide oxygen and oxygen containing radicals A gas sample taken in December 2008 from the HEBT transverse collimator cooling water system showed elevated levels of H 0 45 vs the naturally occurring 0 0002 in air and depressed levels of O 13 2 vs 20 9 in air The increased level of H3 1s consistent with the radiolysis process The depressed level of O can be attributed to the sampling method which introduced air possible recombination of hydrogen and oxygen within the water loop and the complicated water chemistry Particle tracking simulations using MCNPX 2 have also been performed to estimate the fraction of the beam power that is deposited in the HEBT transverse collimator cooling water The collimator geometry model was developed according to design drawings except that the particle bed was represented by set of coincident cylinders of steel and water in a volume ratio of about 65 35 The beam source was represented by a disk source perpendicular to the beam cent
7. rated for 2kW of beam power Finally in the RTBT two more collimator systems again much like the HEBT units but in this instance without any scrapers are used to protect the neutron spallation target by intercepting any beam particles that may have distorted trajectories due to extraction kicker misfires EARLY OPERATING EXPERIENCE During the initial months of low beam power operations the collimator systems were seldom pressed into service since they showed negligible improvement in beam loss However as the beam power was increased to greater than about 300 kW the HEBT collimator systems began to yield significant improvements in downstream beam loss especially in the injection dump beam line where we are most sensitive to beam halo and tails The momentum collimator system gave the biggest improvements but in April 2008 a pump in the water cooling system for this collimator failed due to a concurrent pressure and temperature excursion A post mortem analysis of the event showed that the over pressurization was caused by a combination of excessive beam power and the inability to effectively vent the gases created by radiolysis The pump was replaced but after discovering the radiolysis gas venting issue the momentum dump was removed from service since radiolysis is a problem at any beam power The momentum dump is now being re designed more on this later After the momentum dump the next most effective collimators are the
8. rence signal is amplified separately and subtracted from the scraper signal The electronics also allows the application up to 250 V of negative HV bias to the scraper plate thus increasing the secondary electron emission and rejecting any electrons Accelerator Technology Subsystems T19 Collimation and Targetry WE6RFP027 emitted from other scrapers in case of several scrapers being inserted simultaneously An example set of waveforms is shown in Fig 4 Based on measurements such as this one the beam power typically intercepted during neutron production operations ranges from zero to a few hundred Watts well within the design capacity of 2 kW for each collimator system The bandwidth of the system is 10 kHz The electronics are calibrated by comparing upstream and downstream beam current monitor signals with the scraper signals under carefully controlled conditions HEBT_Diag Scrpo2 800 1000 1200 Pos mm Chrg uC HVBias V MPS Trip uA Scrpo02R 21 54 9 09e 06 100 7 00e 07 MPS Setup Figure 4 Example waveforms from the HEBT scraper electronics Two of the four scrapers are inserted far enough to intercept some beam FUTURE PLANS We plan to install the new momentum dump in 2010 Based on the short time we were able to make use of the original dump we expect to see immediate improvements in the downstream beam loss especially in the injection dump beam line area The ring scrapers have so far been rarel
9. upstream HEBT transverse phase space collimator systems These systems are now in routine use They mainly improve the beam losses in the injection dump beam line sometimes by as much as a factor of 25 They sometimes also improve beam loss in the HEBT arc However there are also times when they yleld very little beam loss improvement depending on the quality of the beam from the linac The ring collimator system is designed to operate as a two stage system First the beam tails are intercepted by the scrapers then the scattered beam is intercepted by the collimators However the system can also be used as a single stage system where the scrapers are retracted and the limiting aperture becomes the collimators themselves This mode maximizes the dynamic aperture in the ring and this is the mode that we typically use On the occasions where the scrapers have been brought into play we did not observe a significant improvement in beam 2847 WE6RFP027 loss so it is simpler for now to use this system in single stage mode As we further increase the beam power we anticipate that we will begin using the scrapers The purpose of the RTBT collimator systems is to protect the target from errant beams caused by extraction kicker misfires and because there are no adjustable components these collimators have been used as originally planned since the first day of beam to target and with the possible exception of radiolysis issues they are wor
10. y used during the typical neutron production runs because they have not significantly improved the overall beam loss in the ring However as we continue to increase the beam power we will monitor the performance of this system to determine how best to use it to control the beam loss We expect that it greatest value will be at the highest beam intensities REFERENCES 1 N Catalan Lasheras and D Raparia PAC 01 p 3263 2001 N Catalan Lasheras et al PRSTAB vol 4 010101 2001 2 L S Waters ed MCNPX User s Manual Version 2 1 5 Los Alamos National Laboratory NM TPO E83 G UG X 00001 Nov 1999 2849
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