Incorporating HPRF in a Linear Cooling Channel: an Update Michael S. Zisman Center for Beam Physics Accelerator & Fusion Research Division Lawrence Berkeley National Laboratory and Juan Gallardo Advanced Accelerator Group Physics Department Brookhaven National Laboratory EUROnu Meeting—Strasbourg June 3, 2010

EUROnu mtg:Zisman2 Outline Introduction HPRF issues Hybrid channel strategy Initial evaluation Window thickness optimization Use of Be isolation windows Window material comparison LiH “optimization” Pressure comparison Comments on implementation Possible implementations Implementation issues Hardware R&D Summary

June 3, 2010EUROnu mtg:Zisman3 Introduction We have evidence that vacuum RF cavity gradient performance degrades in a strong magnetic field —alternative approach of HPRF does not o though it has other potential issues It seems prudent to begin investigating the technical aspects of implementing HPRF in a linear cooling channel —minimizes changes in cooling channel layout and hardware MCTF

June 3, 2010EUROnu mtg:Zisman4 HPRF Issues Many differences between HPRF and “standard” linear cooling channel —energy loss distributed rather than limited to discrete absorbers —loss medium gaseous rather than liquid hydrogen or LiH o likely requires some modularity for safety reasons —must match gradient to energy loss, even if max. gradient can be higher o cannot take full advantage of high maximum gradient MCTF

June 3, 2010EUROnu mtg:Zisman5 Hybrid Channel Strategy Primary purpose of HPRF is to avoid degradation from magnetic field —use gas only to deal with this task o requires much lower pressure than to reach material limit For the Study 2a case, we need gradient of ~15 MV/m —from HPRF test cavity, expect this to require only ~10 atm at room temperature o or ~2.5 atm at 77 K —need eventually to confirm with 201-MHz cavity At this pressure, GH 2  E is ~¼ of LiH  E —reduce LiH thickness by 25% to maintain same overall  E o not exactly right due to different beta weighting –but, a reasonable starting point for re-optimizing channel performance

June 3, 2010EUROnu mtg:Zisman6 Initial Evaluation (1) Looked at performance of proposed “hybrid” channel (Gallardo) —results encouraging, but not yet optimized o not much change in performance between gas-filled hybrid (red line) and vacuum (black line) channels –isolation window does have a substantial effect 

June 3, 2010EUROnu mtg:Zisman7 Initial Evaluation (2) Took quick look at effect of adding even one more Ti isolation window (Gallardo) —it hurts! —maintenance can be accommodated with gate valves o safety considerations may dictate more subdivisions –need to explore using lower Z window material  hydrogen embrittlement must be evaluated for each choice

June 3, 2010EUROnu mtg:Zisman8 Window Thickness Optimization Initial estimates used flat windows (uniform thickness) —engineering guidance (Lau) says that window can be thinner in the middle —implemented in ICOOL (crudely) o it helps substantially Isolation window (as seen by engineer) Isolation window (as seen by physicist)

June 3, 2010EUROnu mtg:Zisman9 Use of Be Isolation Windows Since Ti (or stainless steel) cause losses, look at using Be windows —use design concept from previous slide o even 17 windows looks acceptable –is this too good to be true?

June 3, 2010EUROnu mtg:Zisman10 Window Material Comparison To make sure we were not fooling ourselves, ran cases with both Be and Ti —the difference is obvious Will next look at Al and AlBeMet windows as time permits —Al is okay in terms of hydrogen embrittlement; not yet sure about other materials

June 3, 2010EUROnu mtg:Zisman11 LiH “Optimization” (1) Looked at 34 atm performance for various LiH thicknesses —emittance reduction and transmission do not optimize simultaneously o isolation windows play a role —for NF, transmission is the more important quantity to optimize o probably true for MC also in early stages of cooling

June 3, 2010EUROnu mtg:Zisman12 LiH “Optimization” (2) Carried out same exercise for 10 atm scenario —again, emittance and transmission optimize differently

June 3, 2010EUROnu mtg:Zisman13 Pressure Comparison Looked at best cases for 34 and 10 atm —about the same o infer that desired performance can be obtained over a broad range of parameters

June 3, 2010EUROnu mtg:Zisman14 Comments on Implementation Modular system, with independent gas supplies and isolation windows, seems feasible —if low-Z isolation windows are okay Materials issues must be carefully considered —hydrogen embrittlement must be evaluated for all structural materials o also Cu, Be, and LiH; Al and Be-Cu alloys are particularly resistant Operating at LN temperature reduces P by factor of ~4 —but complicates engineering of channel o insulating vacuum, cooling of RF cavities, differential contraction,... —not convinced this is worth the trouble

June 3, 2010EUROnu mtg:Zisman15 Possible Implementation (A) Proposed concept with buffer vacuum illustrated here Gas only in cavity and beam pipe; permits cryogenic operation if needed Cavity must be a pressure vessel!

June 3, 2010EUROnu mtg:Zisman16 Possible Implementation (B) A more “MICE-like” version is illustrated here Gas fills entire vessel; likely incompatible with cryogenic operation Cavity can be a thin-walled vessel

June 3, 2010EUROnu mtg:Zisman17 Implementation Issues (A) Pressure-vessel code issues must be dealt with for cavities and beam pipe —walls must be thick enough to withstand pressure RF window must be pressurized on both sides to 34 atm —Moretti believes this can be done with special epoxy “plug” o used successfully in MTA tests of 805-MHz HPRF cavity Vent/fill line design must avoid  P on LiH windows Cryogenic operation probably possible —need to insulate fill/vent lines outside vacuum area —need to accommodate differential contraction (e.g., between sections) o usually use bellows for this, but may not be possible with 34 atm of gas On the plus side, can likely keep hydrogen zone contained within apparatus

June 3, 2010EUROnu mtg:Zisman18 Implementation Issues (B) Cavity and tuners could be similar to MICE implementation Bellows connections between sections may not be permitted Vent/fill line must avoid  P on LiH windows Cryogenic operation more difficult —would require a vacuum-insulated outer layer —warming individual sections would be problematical unless bellows are permissible Outer vessel is a (substantial) pressure vessel Area outside containment vessel probably a hydrogen zone —special requirements for electrical equipment, lights and switches, hydrogen sensors,... —use of benign gas (e.g., N 2, CO 2 ) in outer containment area possible o filling and emptying become trickier

June 3, 2010EUROnu mtg:Zisman19 Hardware R&D Program (1) Cooling channel concept similar to that tested in MICE —if simulation tools are vetted, no need for muon beam experiment HPRF tests already planned will answer primary question, whether the gas will stand up to the intense ionizing radiation without shorting out the cavity —this is key remaining issue o if not, must check whether additives (e.g., SF 6, CO 2 ) will solve problem Testing a 201 MHz cavity in implementation B should be done to pin down required pressure to get 16 MV/m —if no frequency effect, may need only ~10 atm o which makes containment easier —could be accommodated with shorter (and thicker) MICE RFCC vacuum vessel

June 3, 2010EUROnu mtg:Zisman20 Hardware R&D Program (2) Need to verify materials properties (compatibility with H 2 atmosphere) —Be RF windows, if used —LiH absorbers —vacuum vessel components, especially isolation windows

June 3, 2010EUROnu mtg:Zisman21 Summary Continuing to look at implications of using HPRF in linear cooling channel New “hybrid” approach (GH 2 and LiH) looks feasible —assuming HP gas option tolerates intense ionizing radiation o to be tested in MTA...hopefully this year Looked briefly at issues of two alternative implementation schemes —both would be challenging —low-Z isolation flanges look benign Cryogenic operation would reduce P by a factor of ~4, but at the expense of many engineering challenges —probably not cost effective for hybrid approach o and less necessary

June 3, 2010EUROnu mtg:Zisman22 Acknowledgments Thanks to: Wing Lau (Oxford) for guidance on isolation window design Steve Virostek (LBNL) for discussions of implementation issues