Beam losses and collimation at the Oak Ridge Spallation Neutron Source by Mike Plum Beam losses and collimators in transfer lines workshop Lund May 13-14, 2014

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Table of contents SNS collimator and scraper systems overview MEBT scraping HEBT scraping and collimation Ring single stage vs double stage collimation RTBT collimation for target protection Matching and beam profiles in the SNS HEBT

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy SNS Accelerator Complex Front-End: Produce a 1-msec long, chopped, H - beam 1 GeV LINAC Accumulator Ring: Compress 1 msec long pulse to 700 nsec 2.5 MeV LINAC Front-End Accumulator Ring RTBT HEBT Injection Extraction RF Collimators 945 ns 1 ms macropulse Current mini-pulse Chopper system makes gaps Current 1ms Liquid Hg Target 1000 MeV

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Scrapers and collimator locations RTBT HEBT Injection Extraction RF Collimators In MEBT: Left-right, top-bottom scrapers In HEBT: Two pairs of left-right scrapers Two pairs of top-bottom scrapers Two collimators In HEBT: Left-right (high and low momentum) scrapers Followed by beam dump In Ring: Four scrapers (0, 45, 90, 135 deg.) Three collimators Most effective Occasionally used Scrapers almost never used Rarely used In RTBT: Two collimators Target protection

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy SNS collimators Rated for 2 kW and 2 full beam pulses Water cooling skids located in tunnel Total of 7 in HEBT, Ring, and RTBT

6Managed by UT-Battelle for the U.S. Department of Energy M. Plum – ESS Beam Losses and Collimation wkshp May 2014 HEBT collimator From N. Simos, EPAC02 Fixed aperture, stationary, water cooled Double-walled Inconel-718 Ring and RTBT collimators are similar One Ring collimator has elliptical aperture

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Scrapers HEBT scrapers: mm thick carbon-carbon (stripping) Ring scrapers: 5 mm thick tantalum mounted to water cooled copper block (scattering) MEBT scrapers: carbon-carbon wedge (stopping)

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy MEBT scraping experience Horiz. and vert. scrapers – Standard part of neutron production – Reduces linac and injection dump losses by up to ~60% – Effectiveness in loss reduction varies from source to source MEBT Emittance without scraping MEBT Emittance with scraping No scraping scraping Gaussian fit DTL profile, log scale HEBT profile, log scale Courtesy A. Aleksandrov x [mm] x’ [mrad] x [mm]

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Beam Charge (typically scrape ~3-4% of the beam) time Warm linac beam loss (~55% lower loss at this location) Ring Injection Dump beam loss (~57% lower loss at this location) Scrapers in Scraping at low beam energy (2.5 MeV) The effectiveness of the MEBT scrapers varies with the ion source and the machine lattice Courtesy J. Galambos Scrapers out

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy HEBT scrapers / collimator experience 4 pairs scrape transverse phase space, 1 pair scrapes longitudinal (mounted in dispersive section) In the early days of SNS this system was routinely used and it made significant reductions in beam loss – especially in the ring injection dump beam line Since that time scrapers were added to the MEBT, and they work even better The machine tune was also improved, and the injection dump beam line also received extensive modifications Today the HEBT scrapers / collimators are used to scrape just a small amount of beam tails and to make small improvements on the beam loss

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy The 4 scrapers cover roughly 3/8 of the beam aperture. Therefore, multiple-turns are necessary for particles to intercept the scrapers. QH10, QV11 scrapers 1 st collimator 2 nd collimator 3 rd collimator QH12, QV13 Ring scrapers / collimators Courtesy S. Cousineau

12Managed by UT-Battelle for the U.S. Department of Energy M. Plum – ESS Beam Losses and Collimation wkshp May 2014 Observations: More out-scattering from collimation system in the single stage system Collimating beam without scraping it lowers the efficiency, causes more beam loss in downstream arc (Courtesy S. Cousineau) Arc C Collimation Straight Single stage vs two stage collimation Courtesy S. Cousineau Collimation section beam loss monitors

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Ring collimation experience We almost always use the ring collimator system in single stage mode – The collimation efficiency is good enough in this mode and activation levels due to collimation inefficiency are low – We also prefer to maximize the acceptance of the ring – this allows the beam size to be maximum, which minimizes the stripper foil hits, which minimizes the beam losses in the location where our activation levels are the highest – This also simplifies operations – don’t need to adjust scraper positions Aside from the high activation levels in the ring injection area, and some modest activation levels in the collimation section, the activation levels in the rest of the ring are low, and the collimator system works well

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy RTBT collimation experience SNS has two collimators in the RTBT beam line to protect the target from off-center beams due to mis-fires of the 14 extraction kickers These are passive collimators with no accompanying scrapers or other movable devices They have operated trouble-free since the beginning

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy SNS collimator experience - radiolysis Momentum collimator beam dump failed in 2008 due to a combination of excessive beam power and the inability to effectively vent the gases created by radiolysis – This led to a concern about radiolysis in our other collimator systems – Estimate ~0.5 – 0.7 l/h of hydrogen gas production for 2 kW absorbed by a HEBT collimator system – Gas sample showed elevated levels of hydrogen gas in the system – We have been monitoring these systems for signs of radiolysis-related issues, but they have been working well Suggest that new collimator systems that have (scattered) beam passing through water consider the issue of radiolysis

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy SNS collimator experience (cont.) Beam tube inside collimators has a double Inconel-718 wall separated by a gap filled with pressurized helium for more effective heat transfer and to allow for leak checking – However, concerns over pressure increase due to heating and maintenance (e.g. how to check and maintain helium) led to never filling them with helium, so we just use air

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Beam loss mitigation: matching Conventional wisdom: It is best to match the beam Twiss parameters at the lattice transitions (e.g. one FODO lattice to another) Good advice for perfect beam distributions – but what about distributions that have different Twiss parameters for the core and the tails of the beam? Initial set up using the design parameters is a good place to start, but need empirical adjustments to, e.g., quad magnets and RF phase and amplitudes to minimize the beam loss (SNS, LANSCE, PSI, TRIUMPF)

18Managed by UT-Battelle for the U.S. Department of Energy M. Plum – ESS Beam Losses and Collimation wkshp May 2014 Mis-match in the linac and transport line Well matched / high loss beam at beginning of HEBT Mis-matched / low loss tune at beginning of HEBT The low- loss tune is mis- matched in HEBT and also other areas Vert. size Horiz. size rms beam size Distance along HEBT WireAnalysisFmt _ pta.txt

19Managed by UT-Battelle for the U.S. Department of Energy M. Plum – ESS Beam Losses and Collimation wkshp May 2014 Some H − beam profiles in the HEBT 1/10 Profiles measured May 5, 2014 Non-Gaussian tails / halo appear beginning at ~1/10 of peak Solid lines show Gaussian fits to the data Horizontal Vertical

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Proton beam profiles The SNS linac can be set up to accelerate protons for accelerator physics experiments – A stripper foil just downstream of the RFQ creates the H + beam – Transverse matching is adjusted with MEBT quads, all RF phases are shifted by 180 deg This is useful for comparing H + and H − beam dynamics Proton beam distributions at the exit of the linac may be useful for ESS design and simulation studies – Note: linac not well optimized for the following measurements

21Managed by UT-Battelle for the U.S. Department of Energy M. Plum – ESS Beam Losses and Collimation wkshp May 2014 Some sample H + beam profiles in the SNS DTL Horizontal Vertical Profiles measured Dec. 22, 2013 Floor in profiles due to RF electrons? Non-Gaussian tails / halo appear beginning at ~1/50 of peak Solid lines show Gaussian fits to the data 1/50

22Managed by UT-Battelle for the U.S. Department of Energy M. Plum – ESS Beam Losses and Collimation wkshp May 2014 Some sample H + beam profiles in the SNS HEBT Horizontal Vertical Profiles measured Dec. 22, 2013 Non-Gaussian tails / halo appear beginning at ~1/10 of peak Solid lines show Gaussian fits to the data 1/10

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Summary Scrapers and collimators are an important part of beam loss control at SNS – Beam loss reductions up to ~60%, depending on the location and the state of the ion source / linac Scraping and collimation continue to evolve at SNS – Installed vertical MEBT scrapers summer 2013 – Considering adding scrapers to beginning of SCL Radiolysis should not be overlooked in collimator designs Proton beam distributions at exit of linac show beam tails starting ~10% of peak – similar to H − distributions

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Thank you for your attention!

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Back up slides

26Managed by UT-Battelle for the U.S. Department of Energy The schematic view of the HEBT collimators 01L,R QH04 QV05QH06 QV07 QH08QV09 scraper collimator 01U,D 02L,R 02U,D

M. Plum – ESS Beam Losses and Collimation wkshp May Managed by UT-Battelle for the U.S. Department of Energy Scrapers HEBT scrapers Thin carbon-carbon 25 mg/cm 2 or ~0.125 mm thick Only needs to strip electrons off the H − particles Ring scrapers 5 mm thick tantulum mounted to water cooled copper block Thick enough to deflect protons

BEAM SNS has a two-stage collimation system. Job of the scrapers is to intercept the halo beam and project it to very high amplitude onto the collimators. High impact on the collimators decreases likelihood of outscatter. In a single-stage system, there are no scrapers. Particles directly hit collimators and have higher probability of outscatter. Outscattered particles are usually outside of the dynamic aperture, will be lost. Scraper system scraper collimator Without scrapers, we would have poor collimation efficiency. Two Stage Betatron Collimation System (Courtesy S. Cousineau)

29Managed by UT-Battelle for the U.S. Department of Energy M. Plum – ESS Beam Losses and Collimation wkshp May 2014 More H + beam profiles in the SNS HEBT Horizontal Vertical Profiles measured April 6, 2014 Non-Gaussian tails / halo appear beginning at ~1/10 of peak Solid lines show Gaussian fits to the data 1/10