International Scoping Study Accelerator Working Group Summary Report

International Scoping Study Accelerator Working Group Summary Report Michael S. Zisman Center for Beam Physics Accelerator & Fusion Research Division Lawrence Berkeley National Laboratory International Design Study Meeting–CERN March 29, 2007

Introduction • Nu. Fact 06 marked formal culmination of the ISS — completed a one-year exploration of an optimized Neutrino Factory design o carried out by international team (participants from Europe, Japan, U. S. ) — goal: study alternative configurations to arrive at baseline specifications for a system to pursue further • Work was quite successful — but, we have not done everything o final report not yet completed, but “abridged version” is now available • Communications via NF-SB-ISS-ACCELERATOR e-mail list — worked well and should be continued for IDS March 29, 2007 IDS: CERN - Zisman 2

Neutrino Factory Ingredients • Proton Driver — primary beam on production target • Target, Capture, Decay — create , decay into • Bunching, Phase Rotation — reduce E of bunch “Front End” • Cooling — reduce transverse emittance • Acceleration — 130 Me. V 20– 40 Ge. V • Decay Ring — store for ~500 turns; long straight section March 29, 2007 ISS Baseline IDS: CERN - Zisman 3

NF Design: Driving Issues • Constructing a muon-based NF is challenging — muons have short lifetime (2. 2 s at rest) o puts premium on rapid beam manipulations – requires high-gradient NCRF for cooling (in B field) – requires presently untested ionization cooling technique – requires fast, large acceptance acceleration system — muons are created as a tertiary beam (p ) o low production rate – target that can handle multi-MW proton beam o large muon beam transverse phase space and large energy spread – high acceptance acceleration system and decay ring — neutrinos themselves are a quaternary beam o even less intensity and “a mind of their own” March 29, 2007 IDS: CERN - Zisman 4

Challenges • Challenges go well beyond those of standard beams — developing solutions requires substantial R&D effort o R&D should aim to specify: – expected performance, technical feasibility/risk, cost (matters!) We must do experiments and build components. Paper studies are not enough! March 29, 2007 IDS: CERN - Zisman 5

Accelerator WG Organization • Accelerator Working Group program managed by “Accelerator Council” — R. Fernow, R. Garoby, Y. Mori, R. Palmer, C. Prior, M. Zisman — met mainly by phone conference • Aided by Task Coordinators — — — Proton Driver: R. Garoby, H. Kirk, Y. Mori, C. Prior Target/Capture: J. Lettry, K. Mc. Donald Front End: R. Fernow Acceleration: S. Berg, Y. Mori, C. Prior Decay Ring: C. Johnstone, G. Rees March 29, 2007 IDS: CERN - Zisman 6

Accelerator Study Aims • Study alternative configurations; arrive at baseline specifications for a system to pursue √ — examine both cooling and no-cooling options • Develop and validate tools for end-to-end simulations of alternative facility concepts (not completed) — correlations in beam and details of distributions have significant effect on transmission at interfaces (muons have “memory”) — simulation effort ties all aspects together • Develop R&D list as we proceed √ — identify activities that must be accomplished to develop confidence in the community that we have arrived at a design that is: o credible o cost-effective — until construction starts, R&D is what keeps the effort alive March 29, 2007 IDS: CERN - Zisman 7

Accelerator Study Approach • Did trade-off studies including designs from all regions — also scientists from all regions (but uncorrelated) o ensured common understanding of, and buy-in for, the results — examined possibilities to choose the best ones • This “regional mixing” was actively fostered — recommend this approach continue during IDS phase • ISS was partly a team-building exercise — number of Neutrino Factory facilities likely built worldwide 1 – voluntarily working together toward a single design increases odds of some facility being built — for AWG, this worked pretty well March 29, 2007 IDS: CERN - Zisman 8

Proton Driver Questions • Optimum beam energy √ — depends on choice of target — fairly broad • Optimum repetition rate √ — depends on target and downstream RF systems o find that 50 Hz is reasonable compromise for cases studied • Bunch length trade-offs √ • Hardware options √ — examined candidate machine types for 4 MW operation o FFAG (scaling and/or non-scaling) o Linac (SPL and/or Fermilab approach) o Synchrotron (J-PARC and/or AGS approach) — ultimately decided to specify only parameters, not hardware March 29, 2007 IDS: CERN - Zisman 9

Optimum Energy • Optimum (calculated) energy for high-Z targets is broad, but drops at low-energy –: 6 – 11 Ge. V +: 9 – 19 Ge. V We adopted 10 ± 5 Ge. V as representative range March 29, 2007 IDS: CERN - Zisman 10

Bunch Length Dependence • Investigated by Gallardo et al. using Study 2 a channel — decrease starts beyond 1 ns bunch length o 1 ns is preferred, but 2 -3 ns is acceptable – such short bunches harder to achieve at low beam energy — stronger sensitivity to bunch length than seen in Study 2 o not yet understood in detail (different phase rotation and bunching) March 29, 2007 IDS: CERN - Zisman 11

FFAG Proton Driver Rees, Prior 10 Ge. V non-scaling FFAG n = 5, h = 40, radius = twice booster radius = 127. 576 m 3 Ge. V RCS booster mean radius = 63. 788 m n=5, h=5 Bunch compression for 5 bunches: Longitudinal bunch area = 0. 66 e. V-s 1. 18 MV/turn compresses to 2. 1 ns rms 180 Me. V Hˉ linac Achromatic Hˉ collimation line Add h = 200, 3. 77 MV/turn for 1. 1 ns rms Works for n = 3 also

SPL Scheme • Proposed scheme increases energy to 5 Ge. V — requires accumulator and compressor rings to deliver baseline bunch parameters o delivery system for 2 ns bunches has been worked out March 29, 2007 IDS: CERN - Zisman 13

Target/Capture/Decay • Optimum target material √ — studied by Fernow, Gallardo, Brooks, Kirk o targets examined: C (5, 24 Ge. V); Hg (10, 24 Ge. V) – re-interactions included o Hg (24 Ge. V) is nominal Study 2/2 a “benchmark” case • Target limitations for 4 MW operation — consider bunch intensity, spacing, repetition rate √ — limits could come from target. . . or from beam dump • Superbeam vs. Neutrino Factory trade-offs — horn vs. solenoid capture √ o can one solution serve both needs? March 29, 2007 IDS: CERN - Zisman 14

Target Material Comparisons (1) Results from H. Kirk March 29, 2007 IDS: CERN - Zisman 15

Target Material Comparisons (2) • Results — Hg at 10 Ge. V looks best thus far • Power handling capability of solid target materials is still an issue — R&D on solid target options is in progress (Bennett) — C at 4 MW still looks hard o would require frequent target changes • Can required short bunches be produced at E ~ 5 Ge. V? — important for Neutrino Factory but not for Superbeam • Results all based on MARS predictions — need experimental data to validate March 29, 2007 IDS: CERN - Zisman 16

Baseline Target System • Neutrino Factory solenoid capture system Tapers from 20 T, 15 cm to 1. 75 T, 60 cm over 20 m March 29, 2007 IDS: CERN - Zisman 17

Front End • Compare performance of existing schemes (KEK, CERN, U. S. -FS 2 a) — use common proton driver and target configuration(s) √ — consider possibility of both signs simultaneously √ — final conclusions require cost comparisons, which will come later • Evaluate implications of reduced VRF √ — take Vmax = 0. 75 Vdes and 0. 5 Vdes • Evaluated trade-offs between cooling and downstream acceptance √ • Look at polarization issues √ — still some issues left if bow-tie ring option considered March 29, 2007 IDS: CERN - Zisman 18

Cooling Channel Comparisons (1) • Palmer has looked at all current designs — FS 2, FS 2 a, CERN, KEK channels • Performance of FS 2 a channel is best — includes benefits of both sign muons — only FS 2 a (with both signs) meets Nu. Fact 99 goal of 1021 useful decays per year March 29, 2007 IDS: CERN - Zisman 19

Cooling vs. Acceptance • Evaluated trade-offs between cooling efficacy and downstream acceptance (Palmer) — increasing from 30 to 35 mm-rad halves the required length of cooling channel o at 45 mm-rad, no cooling needed • At present, A 30 mm-rad seems practical limit March 29, 2007 IDS: CERN - Zisman 20

Acceleration • Compared different schemes on an even footing — RLA, scaling FFAG, non-scaling FFAG o considered implications of keeping both sign muons o must consider not only performance but relative costs — brought scaling FFAG design to same level as non-scaling design • Look at implications of increasing acceptance — transverse and longitudinal o acceptance issues have arisen in non-scaling case – leading to exploration of a revised acceleration scenario March 29, 2007 IDS: CERN - Zisman 21

Non-scaling FFAGs (1) • Discovered dynamics problem related to dependence of revolution time on transverse amplitude (Machida, Berg) — larger amplitudes and bigger angles give longer path length o different flight times for different amplitudes lead to acceleration problems in FFAG – large-amplitude particles slip out of phase with RF and are not fully accelerated • Present conclusions — 30 mm-rad probably possible, but is already a stretch — cascading FFAG rings is harder than anticipated o two in series probably possible, but three in series looks iffy March 29, 2007 IDS: CERN - Zisman 22

Decay Ring • Design implications of final energy (20 vs. 40 Ge. V) √ — 25 and 50 Ge. V? • Optics requirements vs. beam emittance √ — arcs, injection and decay straight sections • Implications of keeping both sign muons √ • Implications of two simultaneous baselines √ • Both triangle and racetrack rings have been examined — recently started to re-examine “bow-tie” configuration March 29, 2007 IDS: CERN - Zisman 23

Decay Ring Geometry (1) • Triangle rings would be stacked side by side in tunnel — one ring stores + and one ring stores – o permits illuminating two detectors with (interleaved) neutrinos and antineutrinos simultaneously Rees March 29, 2007 IDS: CERN - Zisman 24

Decay Ring Geometry (2) • Racetrack rings have two long straight sections that can be aimed at a single detector site — could alternate storing + and – in one ring, or store both together — second ring, with both particles, would be used for another detector site • More flexibility than triangle case, but probably more expensive — can stage the rings if one detector is ready first — can point to two sites without constraints Johnstone March 29, 2007 IDS: CERN - Zisman 25

Decay Ring Geometry (3) • Triangle ring more efficient than racetrack ring for two suitable detector sites — for a single site, or sites in same direction from ring, racetrack is better Depth may be an issue for some sites, especially for racetrack with long baseline March 29, 2007 IDS: CERN - Zisman 26

R&D Program • Neutrino Factory R&D programs under way in — Europe under the auspices of BENE and UKNF — Japan, Nu. Fact-J supported by university, and some U. S. -Japan, funds o substantial scaling-FFAG results have come from this source — U. S. under the auspices of the NFMCC (DOE + NSF supported) • Several international experiments in progress — MERIT — MICE — EMMA (UK), electron model to study non-scaling FFAG performance • Proposals in preparation for new international efforts — high-power target test facility (CERN), to provide dedicated test-bed for next generation of high-power targets March 29, 2007 IDS: CERN - Zisman 27

Decisions on Baseline (1) • Proton Driver — specified parameters, not design o implicitly assumes liquid-metal target Parameter Energy (Ge. V) Beam power (MW) Value 10 ± 5 4 Repetition rate (Hz) 50 No. of bunch trains 3, 5 a) Bunch length, rms (ns) 2 ± 1 Beam durationb) ( s) March 29, 2007 40 a)Values ranging from 1– 5 possibly acceptable. spill duration for liquid-metal target. b)Maximum IDS: CERN - Zisman 28

Decisions on Baseline (2) • Target — assume Hg target; look at Pb-Bi also • Front End — bunching and phase rotation o use U. S. Study 2 a configuration — cooling o include in baseline; use U. S. Study 2 a configuration — keep both signs of muons o “waste not, want not” • Acceleration — used mixed system o linac, dog-bone RLA(s), FFAGs March 29, 2007 IDS: CERN - Zisman 29

Decisions on Baseline (3) • Decay Ring — adopt racetrack o keep alive triangle as alternative – depends on choice of source and baselines o energy 20 to 50 Ge. V March 29, 2007 IDS: CERN - Zisman 30

Summary • Made good progress toward consensus on a single optimized Neutrino Factory scheme • Must continue to articulate need for an adequately-funded accelerator R&D program — being encouraged to do this in an international framework o IDS is one ingredient — synergy with Muon Collider R&D is an advantage (in U. S. ) • It was a privilege to work on the ISS with such a talented and dedicated group — my thanks to: o Program Committee (Dornan, Blondel, Nagashima/Long) o Accelerator Council and task leaders (slide 6) o all members of Accelerator Group (see NF-SB-ISS-ACCELERATOR list) March 29, 2007 IDS: CERN - Zisman 31