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 NFMCC Meeting—U.-Mississippi January 14, 2010

NFMCCmtg:Zisman2 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 MCTF

January 14, 2010NFMCCmtg:Zisman3 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

January 14, 2010NFMCCmtg:Zisman4 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 ~34 atm at room temperature o or ~9 atm at 77 K —need eventually to confirm with 201-MHz cavity o Tollestrup has pointed out that 34 atm may be overkill 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

January 14, 2010NFMCCmtg:Zisman5 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 

January 14, 2010NFMCCmtg:Zisman6 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

January 14, 2010NFMCCmtg:Zisman7 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)

January 14, 2010NFMCCmtg:Zisman8 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 almost acceptable –is this too good to be true?

January 14, 2010NFMCCmtg:Zisman9 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

January 14, 2010NFMCCmtg:Zisman10 Pressure Dependence (1) It is not clear that 34 atm is really needed —10-15 atm may suffice for 15 MV/m Looked at performance effects vs. pressure —varied LiH thickness to (roughly) keep same energy loss —windows sized for 34 atm always —Study 2a value was ~0.06  /p

January 14, 2010NFMCCmtg:Zisman11 Pressure Dependence (2) Looked at optimizing LiH thickness at fixed pressure of 20 atm —safety window thickness also scaled to 20 atm case —transmission is not very sensitive to LiH thickness

January 14, 2010NFMCCmtg:Zisman12 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

January 14, 2010NFMCCmtg:Zisman13 Possible Implementation (A) Proposed concept with buffer vacuum illustrated here —compared with previous (NuFact09) talk, can conclude that Juan is better at drawing than I am Gas only in cavity and beam pipe; permits cryogenic operation if needed Cavity must be a pressure vessel!

January 14, 2010NFMCCmtg:Zisman14 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

January 14, 2010NFMCCmtg:Zisman15 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

January 14, 2010NFMCCmtg:Zisman16 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,...

January 14, 2010NFMCCmtg:Zisman17 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...eventually Looked briefly at issues of two alternative implementation schemes —both would be challenging —low-Z isolation flanges look benign o need to check AlBeMet and Al 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 (and less necessary)

January 14, 2010NFMCCmtg:Zisman18 Acknowledgments Thanks to: Wing Lau (Oxford) for guidance on isolation window design Steve Virostek (LBNL) for discussions of implementation issues