Stripper foils by Mike Plum Ring Area Manager Spallation Neutron Source Operation of Large Vacuum Systems Workshop Oak Ridge July 13, 2011 2Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Motivation Stripper foils are the best technology available today for low- loss multi-turn injection into storage rings and synchrotrons Key component at many facilities Spallation Neutron Sources (ORNL, J-PARC, ISIS, C-SNS, PSR) Colliders (BNL, CERN, FNAL, ) Ion beams (FRIB, ) Some cases are easy, and you can just buy the foils already mounted Some cases are very demanding & specialized, like SNS and J-PARC, where the foils must be fabricated in-house 3Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Motivation (cont.) This is an area of active development, with R&D programs at labs around the world, and with occasional workshops dedicated to this topic One of the biggest issues is stripper foil lifetime (e.g. SNS, FRIB, PSR, Project-X) Limits the maximum achievable beam power The stripper foil is like a fuse in the beam line if you over power it, the evaporates or breaks into pieces, and the beam shuts off At SNS, weve improved our stripper foils to the point where we can use a single foil for an entire run cycle at 1 MW (~4 months), but we prefer to change foils every month We do not know if our foils will hold up when we reach our design power of 1.4 MW, and there is even more uncertainty about foil lifetime for our power upgrade to 3 MW 4Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 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 5Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 SNS foils Photos by C. Luck NewIn useEnd of life (4 months later) 17 x 45 mm, ~0.34 mg/cm 2 or ~1 micrometer thick Stripping efficiency is ~97% (Balance between thick enough to strip and thin enough to minimize beam loss) 6Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Best foil technology today SNS Nanocrystalline diamond Self supporting (R. Shaw, ORNL) J-PARC and PSR Hybrid boron-carbon Requires fiber support (I. Sugai, KEK) PSR AC-DC arc discharge Requires fiber support (I. Sugai, KEK) 7Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 SNS diamond foils 10 m Develop with ultraviolet light Etch with Buffered Oxide Etch Strip resist with Acetone Etch Silicon with TMAH Strip oxide with Buffered Oxide Etch Positive Photoresist SiO 2 Silicon Substrate Patterning Process Thermal expansion mismatch diamond vs silicon Foils scroll upon release from Si wafer Foil corrugation method developed 50 Line/inch Foil: 254 m 10 m 20 mm 12 mm Courtesy R. Shaw 8Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Foil lifetime - temperature Primary limits on foil lifetime are temperature and thermal stress At 2200 deg. K the carbon sublimates (evaporates) at a rate of ~1 micrometer / hour (our foils are 1 micrometer thick to start with!) Vapor pressure P(T) = Ae (-B/T), so the sublimation rate is a strong function of temperature ~600 deg. temp fluctuations! (SNS, 1.5 MW, 1 GeV Courtesy Y. Zhang) 9Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Temperature Foil temperatures are hard to model Emissivity, heat capacity, and thermal conductivity are not well known at these high temperatures Energy deposition is not well known Need exact number of foil hits per injected proton (~8 or so at SNS) Need to know how much energy is deposited by H particle as it breaks up in passing through the foil Need to include effects like knock-out (delta ray) electrons caused by relativistic protons striking the foil (~28% effect?) Foil temperatures are hard to measure Need to know emissivity Fast function of time (peak temp only lasts for ~10s of microseconds) We have observed by eye that at ~800 kW, the SNS foils get white hot (>1700 K) We are developing a foil temperature measurement system at SNS 10Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Convoy electrons The SNS foil is in a strong magnetic field ~0.25 T, to control beam loss caused by the partially-stripped H 0 excited states The convoy electrons (the electrons stripped off the incoming H beam) must be carefully controlled At 1.5 MW beam power, the convoy electron power is 1.6 kW enough to melt the foil bracket, electron collector, etc. 11Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Bracket failures and convoy electrons 12 mm 32 mm 17 mm wide foil 29 mm Solved by mounting foils >24 mm horizontally from the bracket. All this melted aluminum evaporated and was deposited on the vacuum chamber walls, other foil brackets, and everything else in sight 12Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Bracket damage by reflected convoy electrons 15.5 Days Peak Power KW MWhrs to Target Reflected convoy electron damage Photo by Chris Luck Solved by changing from aluminum to titanium brackets 13Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Cathode spot in-vacuum breakdown Crater traces left by cathode spots (Picture taken with an electron microscope). FromVacuum arcs, also referred to as cathodic arcs, are high current discharges between cold electrodes. Typical currents are 100 Amperes or more while the voltage between anode and cathode is only about 20 Volts This leads to "micro-explosions," and one can observe microscopic craters left on the cathode surface. (FromSolved by changing from aluminum to titanium brackets, machining to a flatness specification, and hand-polishing the bracket arm and clamp 14Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Bracket pinching The generations 1 through 3 foil brackets were made of aluminum due to its ease of machining, good conductivity, light weight, and low radioactivation However, aluminum also has a low melting point and a high coefficient of thermal expansion ~8x higher than silicon Titanium screws created additional mismatch As the temperature increases in this arrangement, the stripper foil substrate is pinched between the clamp and the bracket arm, induces high tensile and compressive stresses in the silicon substrate, leading to fracture Now we use titanium brackets (also higher melting point) 15Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Laser stripping Laser stripping is the up and coming technology Laser stripping eliminates the beam loss caused by foil scattering (which accounts for the majority of the beam loss) and avoids the thermal foil problems Laser stripping has been demonstrated in just one series of experiments, at SNS, and only for ~10 ns A experiment to demonstrate 1 10 us stripping is in progress at SNS, but results are not expected for a few more years 90% stripping of ~870 MeV H beam (Danilov, PAC07) 16Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Future development Boron doped foils to get higher conductivity, to reduce the foil charging and vacuum breakdown problems Develop new corrugation patterns to avoid the twisting and curling problems as the foils age Work to achieve practical laser stripping injection system At SNS we need 1 ms, 60 Hz stripping (>1 million times higher duty factor than has been demonstrated to date) 17Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Thank you for your attention! 18Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Backup slides 19Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Characteristics of a good stripper foil Low atomic number to minimize radioactivation and beam loss caused by scattering Low vapor pressure / high melting point / able to get very hot Non hazardous Easy to fab Self supporting (SNS design calls for 3 free sides to reduce foil hits by the circulating beam) Thick enough to strip the beam, yet as thin as possible to reduce heating and scattering Some sort of carbon is typical Amorphous, AC-DC discharge, nano-crystalline diamond, diamond-like carbon 20Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 Arcs and sparks (vacuum breakdown) (no tab on this bracket) Clearest evidence to date of vacuum breakdown 21Managed by UT-Battelle for the U.S. Department of Energy M. Plum OLAV III- July 2011 SNS power ramp up to date October 2006 to present 1.08 MW Full design power is 1.4 MW Power reduced to save money