Incorporating HPRF in a Linear Cooling Channel: an Update

1. Incorporating HPRFin 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

2. Introduction MCTF • We have evidence that vacuum RF cavity gradient performance degrades in a strong magnetic field • alternative approach of HPRF does not • though it has other potential issues • It seems prudent to begin investigating the technical aspects of implementing HPRF in a linear cooling channel NFMCCmtg:Zisman

3. HPRF Issues MCTF • 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 • likely requires some modularity for safety reasons • must match gradient to energy loss, even if max. gradient can be higher • cannot take full advantage of high maximum gradient NFMCCmtg:Zisman

4. Hybrid Channel Strategy • Primary purpose of HPRF is to avoid degradation from magnetic field • use gas only to deal with this task • 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 atmat room temperature • or ~9 atm at 77 K • need eventually to confirm with 201-MHz cavity • Tollestrup has pointed out that 34 atm may be overkill • At this pressure, GH2E is ¼ of LiH E • reduce LiH thickness by 25% to maintain same overall E • not exactly right due to different beta weighting • but, a reasonable starting point for re-optimizing channel performance NFMCCmtg:Zisman

5. Initial Evaluation (1) • Looked at performance of proposed “hybrid” channel (Gallardo) • results encouraging, but not yet optimized • not much change in performance between gas-filled hybrid (red line) and vacuum (black line) channels • isolation window does have a substantial effect  NFMCCmtg:Zisman

6. 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 • safety considerations may dictate more subdivisions • need to explore using lower Z window material • hydrogen embrittlement must be evaluated for each choice NFMCCmtg:Zisman

7. 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) • it helps substantially Isolation window (as seen by physicist) Isolation window (as seen by engineer) NFMCCmtg:Zisman

8. Use of Be Isolation Windows • Since Ti (or stainless steel) cause losses, look at using Be windows • use design concept from previous slide • even 17 windows looks almost acceptable • is this too good to be true? NFMCCmtg:Zisman

9. 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 NFMCCmtg:Zisman

10. 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 NFMCCmtg:Zisman

11. 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 NFMCCmtg:Zisman

12. 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 • 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 • insulating vacuum, cooling of RF cavities, differential contraction,... • not convinced this is worth the trouble NFMCCmtg:Zisman

13. 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 Cavity must be a pressure vessel! Gas only in cavity and beam pipe; permits cryogenic operation if needed NFMCCmtg:Zisman

14. Possible Implementation (B) • A more “MICE-like” version is illustrated here Cavity can be a thin-walled vessel Gas fills entire vessel; likely incompatible with cryogenic operation NFMCCmtg:Zisman

15. 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” • 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) • 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 NFMCCmtg:Zisman

16. 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,... NFMCCmtg:Zisman

17. Summary • Continuing to look at implications of using HPRF in linear cooling channel • New “hybrid” approach (GH2 and LiH) looks feasible • assuming HP gas option tolerates intense ionizing radiation • 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 • 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) NFMCCmtg:Zisman

18. Acknowledgments Thanks to: Wing Lau (Oxford) for guidance on isolation window design Steve Virostek (LBNL) for discussions of implementation issues NFMCCmtg:Zisman