Incorporating HPRF in a Linear Cooling Channel an

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 June 3, 2010 EUROnu Meeting—Strasbourg June mtg: Zisman 3, 2010 EUROnu

Outline • Introduction • HPRF issues • Hybrid channel strategy • Initial evaluation • Window thickness optimization • Use of Be isolation windows • Window material comparison • Li. H “optimization” • Pressure comparison • Comments on implementation • Possible implementations • Implementation issues • Hardware R&D • Summary June 3, 2010 EUROnu mtg: Zisman 2

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, 2010 EUROnu mtg: Zisman 3

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 Li. H 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, 2010 EUROnu mtg: Zisman 4

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 2 a 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 Li. H E — reduce Li. H 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, 2010 EUROnu mtg: Zisman 5

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, 2010 EUROnu mtg: 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 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, 2010 EUROnu mtg: 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) Isolation window o it helps substantially (as seen by physicist) Isolation window (as seen by engineer) June 3, 2010 EUROnu mtg: 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 o even 17 windows looks acceptable – is this too good to be true? June 3, 2010 EUROnu mtg: 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 Al. Be. Met windows as time permits — Al is okay in terms of hydrogen embrittlement; not yet sure about other materials June 3, 2010 EUROnu mtg: Zisman 10

Li. H “Optimization” (1) • Looked at 34 atm performance for various Li. H 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, 2010 EUROnu mtg: Zisman 11

Li. H “Optimization” (2) • Carried out same exercise for 10 atm scenario — again, emittance and transmission optimize differently June 3, 2010 EUROnu mtg: Zisman 12

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, 2010 EUROnu mtg: Zisman 13

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 Li. H; 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, 2010 EUROnu mtg: Zisman 14

Possible Implementation (A) • Proposed concept with buffer vacuum illustrated here Cavity must be a pressure vessel! Gas only in cavity and beam pipe; permits cryogenic operation if needed June 3, 2010 EUROnu mtg: Zisman 15

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 June 3, 2010 EUROnu mtg: Zisman 16

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 Li. H 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, 2010 EUROnu mtg: Zisman 17

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 Li. H 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, 2010 EUROnu mtg: Zisman 18

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, 2010 EUROnu mtg: Zisman 19

Hardware R&D Program (2) • Need to verify materials properties (compatibility with H 2 atmosphere) — Be RF windows, if used — Li. H absorbers — vacuum vessel components, especially isolation windows June 3, 2010 EUROnu mtg: Zisman 20

Summary • Continuing to look at implications of using HPRF in linear cooling channel • New “hybrid” approach (GH 2 and Li. H) 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, 2010 EUROnu mtg: Zisman 21

Acknowledgments Thanks to: Wing Lau (Oxford) for guidance on isolation window design Steve Virostek (LBNL) for discussions of implementation issues June 3, 2010 EUROnu mtg: Zisman 22