An Introduction to Spallation Neutron Sources

Transcription

1 An Introduction to Spallation Neutron Sources Wei, Jie Yangzhou, China August 3, 2006 Copyright, 1996 Dale Carnegie & Associates, 1 Inc.

2 Class Outline I. SNS and high power accelerators II. Basic concepts and parameters III. Accelerator physics & systems IV. China Spallation Neutron Source Home work August , 2

3 August , 3 Class I SNS and high power accelerators

4 Class I Outline What is spallation neutron source? Synchrotron light source features Spallation neutron source features Applications Types of high intensity accelerators CW, long pulse, short pulse Examples of spallation neutron source SNS, J-PARC, ISIS August , 4

5 August , 5 What is spallation neutron source? High energy (~GeV) protons hit a heavy-metal target A proton heats-up the entire metal atom Energy released evaporates spallation neutrons 1 proton 20 ~ 30 neutrons (high flux) Moderated to energies suitable for experiments 5 MW SNS ~ 60MW high-flux reactor ~ 1x10 15 n/cm 2 s average neutron flux Number of neutron ~ proton energy (0.2 ~ 10 GeV)

6 August , 6

7 August , 7 Synchrotron light source features Electron beam, low radio-activation Widely developed user community Electron linac, booster (optional), main ring Beam lines around the main ring Long store, top-off

8 Spallation neutron source features Proton beam, noticeable radio-activation High penetration, sensitive to light elements, pulsed mode/time of flight, high resonance cross section H - linac, proton synchrotron or accumulator Beam lines from heavy metal target Rapid cycling August , 8

9 SNS s and Light sources Complementary user facilities SLS & SINQ (Swiss) 1 MW cw SNS (1997) 2.5 GeV Swiss Light Source (2001) ISIS and DIAMOND (UK) 160 kw pulsed SNS (1985) 3 GeV light source (2007) August , 9

10 ISIS and DIAMOND (RAL, UK) August , 10

11 August , 11 ISIS schematic Synchrotron Instruments Target Instruments Instruments Linac RFQ Ion source

12 August , 12 ISIS facility Replacement value ~ 200M (~$350M) Operational budget ~ 30M (~$50M) per year ~300 staff ~700 experiments per year ~1700 users per year

13 August , 13

14 August , 14 SINQ

15 August , 15 High-power proton applications Spallation neutron source Neutrino-factories Accelerator driven sub-critical nuclear power generation (ADS) Transmutation of nuclear waste Accelerator production of tritium Muon storage rings Proton radiography White neutron source applications High-intensity radioactive beams

16 Types of high power accelerators CW facilities Driven by a high intensity proton cyclotron 1.2 MW SINQ (PSI) driven by 590 MeV cyclotron Long (ms) pulse facilities Driven by a high intensity proton linac 1 MW LANSCE (LANL) driven by 800 MeV linac Short (µs) pulse facilities Driven by a combination of linacs and rings Partial energy linac and rapid-cycling synchrotron(s): ISIS (RAL) driven by 70 MeV linac/800 MeV RCS J-PARC driven by 400 MeV linac/3 GeV RCS/50 GeV MR Full-energy linac and an accumulator ring: PSR (LANL) driven by 800 MeV linac/accumulator SNS (ORNL) driven by 1 GeV linac/accumulator August , 16

17 World map ISIS, 160 kw, 1985 CERN CNGS, 0.5 MW IPNS, 7 kw, 1981 KEKS, 2 kw, 1980 SINQ, 1 MW cw, 1997 LANSCE, 80 kw, 1988 J-PARC, 1 MW, 2007 SNS, 1.4 MW, 2006 AGS, 0.13 MW, MW, FNAL NuMI 0.13 MW PEFP, MW FNAL PD MW August , 17 spallation neutron sources other high intensity projects

18 Accelerators at the power frontier August , 18

19 August , 19 Intensity Livingston chart

20 August , 20 SNS: six lab collaboration

21 August , 21 SNS in comparison At 1.4 MW, ~8x ISIS, the world s leading pulsed spallation source The peak neutron flux will be ~20 100x ILL The world s leading facility for neutron scattering A short drive from HFIR, a reactor source with a flux comparable to ILL The SNS will begin operation in 2006

22 August , 22 SNS budget (2004)

23 August , 23 J-PARC schematic layout Japan Proton Accelerator Research Complex Similar cost, similar schedule (due 2006 ~ 2007) Ring clusters with expandable energy range; multipurpose (Courtesy J-PARC) January, 2005

24 J-PARC in phases KEK Portion JAERI Portion Phase 1 + Phase 2 = 1,890 Oku Yen (= $1.89 billions if $1 = 100 Yen). Phase 1 = 1,527 Oku Yen (= $1.5 billions) for 7 years. JAERI: 860 Oku Yen (56%), KEK: 667 Oku Yen (44%). August , 24

25 High-power applications by type CW and long pulse applications High average power, high current proton source, high duty factor (~10% or higher) to minimize mechanical shock, ~1 GeV to reduce power deposition in window & cost Transmutation of nuclear waste Accelerator production of tritium Accelerator driven subcritical (ADS) reactor power generation Short pulse applications High peak power, H- ion source for accumulation, pulsed high intensity secondary beam generation (duty factor < 10-4 ) Neutrons, Kaons, neutrinos, muons for neutrinos, muons for muon collider, radioactive isotope (ISOL) Radioactive isotope by fragmentation August , 25

26 August , 26 Class II Basic concepts and parameters

27 Class II Outline Bunch time structure Primary parameters Ion species; Kinetic energy Repetition rate Pulse intensity; Bunch length Emittances Beam evolution parameters Beam loss budget Controlled loss, uncontrolled loss August , 27

28 August , 28 Major SNS parameters Proton beam power on target 1.4 MW Proton beam kinetic energy on target 1.0 GeV Average beam current on target 1.4 ma Pulse repetition rate 60 Hz Protons per pulse on target 1.5x10 14 protons Charge per pulse on target 24 µc Energy per pulse on target 24 kj Proton pulse length on target 695 ns Ion type (Front end, Linac, HEBT) H minus Average linac macropulse H- current 26 ma Linac beam macropulse duty factor 6 % Front end length 7.5 m Linac length 331 m HEBT length 170 m Ring circumference 248 m RTBT length 150 m Ion type (Ring, RTBT, Target) proton Ring filling time 1.0 ms Ring revolution frequency MHz Number of injected turns 1060 Ring filling fraction 68 % Ring extraction beam gap 250 ns Maximum uncontrolled beam loss 1 W/m Target material Hg Number of ambient / cold moderators 1/3 Number of neutron beam shutters 18 Initial number of instruments 5

29 Time structure (pre-compression) August , 29

30 Time structure (post compression) August , 30

31 August , 31 Beam power Characterizing the power of a highintensity accelerator P = energy & average current Average current of facility I E k = Repetition rate I f N N p e Number of particles per pulse Raise energy, increase repetition rate, increase pulse intensity

32 Ion species H - ion is used for short pulse applications Allows multi-turn accumulation to enhance pulse intensity Controls beam profile Demands a powerful H - ion source Complication with electron stripping under gas scattering and under magnetic field Gas scattering: requiring relatively high vacuum Magnetic stripping: limits maximum magnetic field Proton beam is usually used for high-intensity cw or long pulse applications in the absence of rings August , 32

33 Kinetic energy Range largely determined by applications & experiments E.g GeV for neutron spallation Within a given range, a higher output energy implies a higher output beam power, relatively cheap to achieve for a RCS (linearly proportional) alleviated heating on target due to longer stopping length higher magnet field, higher ramping power, more difficult field quality control A higher injection energy implies reduced space-charge effects due to electro-magnetic force cancellation more probably magnetic stripping demanding lower field, longer magnet, more injection space higher cost of the injector accelerator August , 33

34 Repetition rate Requirements from the user / target: Hz Time resolution & power per pulse needs -> lower rate Total beam power -> higher rate Rapid-Cycling Synchrotrons: sensitive to repetition rate Demands a strong power supply Demands a high radio-frequency (RF) voltage Demands RF shielding to avoid heating on vacuum chamber while allowing image charge to circulate (impedance control) Demands lamination to avoid heating in magnets Accumulators: less sensitive comparing with RCS More demanding on the pre-injector (ion source output, linac klystron power ) Higher injection energy, lower extraction energy Higher ring intensity (instabilities) and beam loading August , 34

35 August , 35 Pulse intensity Proportional to output beam power as high as possible Usually limited by space charge constraints, instability threshold, instability growth Ring average current I = Nef s Ring peak current parabolic: I I Gaussian: Bunching factor gap parabolic: B Gaussian: I 3π 1 = I = I 2φ B I = 2πσ φ Empirically: ~0.5 (accumulator); ~0.35 (RCS) I 1 C L 2 2π = B C 3 6

36 Bunch length Range largely determined by applications & experiments E.g. ~ 1 ns for neutrino factory proton drivers Spallation neutron: not sensitive to bunch length Beam gap needed for beam extraction; maintained by the RF system For low harmonic number: control bunch area/bucket area ratio For high harmonic number: missing bunches Choice of RF harmonic number Hardware availability at a particular RF frequency Consideration of possible coupled-bunch instability Needs for beam-gap cleaning August , 36

37 August , 37 Emittances Transverse emittance constant of acceleration: xdp x p x ~ βγx' Preservation of normalized emittance often needed for downstream applications; damping usually not practical Controlled emittance enlargement is usually used to alleviate space-charge effects; constraints from magnet aperture and power supply Longitudinal emittance constant of acceleration: φdw Often limited by the available momentum acceptance W E E 2 ; = β hω E s p p

38 August , 38 SNS beam evolution parameters Front End Linac Ring IS/LEBT RFQ MEBT DTL CCL SCL (1) SCL (2) HEBT Ring RTBT Unit Output Energy MeV Relativistic factor β Relativistic factor γ Peak current x10 4 9x10 4 ma Minimum horizontal acceptance πmm mr Output H emittance (unnorm., rms) πmm mr Minimum vertical acceptance πmm mr Output V emittance (unnorm., rms) πmm mr Minimum longitudinal acceptance 4.7E E E E E-04 19/π πevs Output longitudinal rms emittance 7.6E E E E E E-06 2/π πevs Controlled beam loss; expected 0.05 a N/A 0.2 b N/A N/A N/A N/A 5 c 62 d 58 e kw uncontrolled beam loss; expected f <1 1 <1 W/m Output H emittance (norm., rms) πmm mr Output V emittance (norm., rms) πmm mr Note a) corresponding to 27% chopped beam b) corresponding to 5% chopped beam c) beam loss on the transverse and momentum collimators d) including total 4% of beam escaping foil and 0.2% beam loss on collimators e) including 4% beam scattered on the target window f) corresponding to 20% beam loss averaged over RFQ length

39 Significance of exposure to radiation 1 Sv = 100 Rem US occupational limit 50 msv per year DOE laboratory guideline 12.5 msv per year Hands-on maintenance: 1 msv/hour 50 hours of work per year August , 39

40 Radio-activation & beam loss Hands-on maintenance: no more than 1 msv/hour residual activation (4 h cool down, 30 cm from surface) ~ 1 Watt/m uncontrolled beam loss ~ 10-6 beam loss per tunnel meter at 1 MW operation < 10-4 beam loss in a 200 m accumulator; ~ 10-3 in a 200 m rapid cycling synchrotron Beam loss [W/m] FE DTL Uncontrolled loss during normal operation CCL SCL HEBT August , 40 Length [m] RING High rad areas RTBT

41 August , 41 Class III Accelerator physics and systems

42 Class III Outline Layout: linac & ring structure Beam loss & cleaning Ring lattice Ring magnet, power supply, vacuum, RF, diagnostics Ring injection & extraction Space charge Impedances & collective effects Electron cloud August , 42

43 August , 43 Linac structure (SNS example) Ion source Low energy beam transport (LEBT) Radio-frequency quadrupole linac (RFQ) Medium energy beam transport (MEBT) Drift tube linac (DTL) Coupled cavity linac (CCL) Superconducting RF linac (SCL) Front End LBNL RT Linac LANL Linac Controls HEBT BNL RFQ DTL CCL Interm. beta High beta H- Injector Chopper 2.5 MeV MHz 86.8 MeV 186 MeV 805 MHz, 5.0 MW RF Power SRF Linac JLAB 1000 MeV 2 HEBT Cavities 2000P-03548/jhb

44 August , 44 Front end

45 August , 45 DTL

46 August , 46 CCL

47 August , 47 SCL (β=0.61)

48 August , 48 SCL (β=0.81)

49 August , 49

50 Source of beam loss High radio-activation at injection, extraction,collection AGS: up to 100 msv/hour at localized area High beam loss FNAL Booster (25-40%): ramp tracking, debunchingrecapturing, transition, aperture! AGS/Booster (20 30%): pushing record intensity ISIS (~15%): injection capture, initial ramp PSR (0.3% Full energy accumulation): injection loss (1) space-charge tune shift (0.25 or larger) & resonance crossing (2) limited geometric/momentum acceptance (3) premature H- and H0 stripping and injection-foil scattering (4) errors in magnetic field and alignment (saturation, fringe, ramp ) (5) instabilities (resistive wall, electron-cloud instability ) (6) accidental beam loss (e.g., malfunction of the ion source/linac & misfiring of ring extraction kickers) (7) beam-halo loss during fast extraction. August , 50

51 August , 51 Multiple beam cleaning/protection Beam gap cleaning LEBT chopping (25 ns) MEBT chopping (10 ns) Ring BIG cleaning Scraping & collimation MEBT scraper HEBT transverse scraper HEBT momentum scraper Ring transverse two-stage collimator Ring momentum collimator (optional for AR) RTBT collimator

52 August , 52 Beam-loss localization Ring primary scraper Sacrifice collimation region for the rest Multi-stage system, efficiency above 90% Needs a large vacuum-pipe aperture and a long straight section (Courtesy H. Ludewig et al) collimator in SNS

53 August , 53 Secondary collector design Length enough to stop primary protons (~ 1 m for 1 GeV beam) Layered structure (stainless steel particle bed in borated water, stainless steel blocks) to shield the secondary (neutron, γ) Fixed, enclosing elliptical-shaped wall for operational reliability Double-wall Inconel filled with He gas for leak detection

54 August , 54 Scraping in beam transport Normalized phase space X x β x X ' dx α xx + β xx' = dµ β x µ ( s) = s ds' β ( s') Two pairs of scrapers at π/2 betatron phase advance Escaping radius at times scraper radius Channel aperture needs to be larger than the escaping radius 2

55 August , 55 Ring collimation optimization Secondary back-off distance H determined by a balanced consideration Chamber aperture Scraper adjustment range Primary scattering performance Phase advance optimized to minimize escaping secondary µ 1 = cos 1 A A + H µ = π 2 µ 1

56 Beam-gap cleaning Strip-line kicker at betatron frequency to kick-out particles in tens of turns Gated at beam gap, with rise/fall time much shorter that gap length Can be used for momentum cleaning if aperture is adequate August , 56

57 August , 57 European Spallation Source example ESS layout courtesy RAL Linac halo: adjustable foil scraper in HEBT Linac energy tail: scraping at highdispersion location in HEBT Linac gap residual: beam-in-gap kicker or momentum collection during initial ramping Linac malfunction: scraper in HEBT Ring halo: two-stage collimation

58 SNS Ring layout example August , 58 No energy ramping Long straight-section, large aperture Injection flexibility Collimation efficiency Four straight-sections for four functions Injection; RF; Collimation; Extraction Diagnostics allaround Dispersion-free injection Decoupled H, V, L

59 August , 59 SNS ring lattice optics example First find the strengths of the two arc quadrupole families to get an horizontal phase advance of 2π and using the vertical phase advance as a parameter Then match the straight section with arc by using the two doublet quadrupole families and the matching quad at the end of the arc in order to get the correct tune without exceeding the maximum beta function constraints Retune arc quads to get correct tunes Always keep beta, dispersion within acceptance range and quadrupole strength below design values

60 Ring Lattice FODO/Doublet (hybrid) lattice 32 dipoles, 52 quadrupoles Chromatic correction with 20 arc sextupoles Orbit/coupling correction with correctors in the arcs Resonance correction in arc and straights August , 60

61 August , 61 J-PARC magnet example

62 August , 62 SNS magnet example 2000P-03563/jhb

63 J-PARC ceramic vacuum ducts Ti flange Ti sleeve RF Shield 1300 mm TiN coating Thickness : 15 nm August , 63 Brazing joint segment Every stripes are jumped over the joint area. Capacitor Capacitance : 330 nf

64 August , 64 SNS ring RF cavities

65 SNS Ring RF system components August , 65

66 August , 66 SNS ring diagnostics

67 August , 67 SNS ring injection layout

68 August , 68 3D phase space painting

69 August , 69 Space charge in linac

70 Space charge in ring w.p. (6.23,6.20) combined tune spreads at the end of accumulation: N=0.1* red (mainly chromatic spread and resulting loss due to resonances abovechromaticity correction with sextupoles or slight w.p. adjustment) N=1* pink (depressed by space charge) N=2* green (depressed by space charge) August , 70

71 August , 71 Space charge & halo formation

72 Tail from space charge & field error August , 72

73 Space charge tune shift Transverse tune shift: Based on cancellation between electric and magnetic field for relativistic particles Strong dependence on energy General Laslett tune shift August , 73

74 August , 74 SNS impedance budget (< 10 MHz) Key impedances were bench measured, as recommended. 1 incoherent and coherent part 2 25 Ω termination at PFN 3 measured inside vacuum vessel 4 ceramic pipe coated with 0.7 µm of copper and 0.1 µm of TiN 5 modes will be damped (peak values per cavity without damping) 6 without damping (at 17.8 MHz, contribution from 3 cavities), damping with glow bar 7 based on MAFIA simulations

75 August , 75 Impedance budget 50 MHz) 8 incoherent and coherent parts 9 measured inside vacuum vessel without feed-through 10 damped resonance at 17,6 MHz 11 possible high impedance around 170MHz (can be damped with lossy material ) 12 resonant frequency around 50MHz 13 peak value 35Ω at 35MHz

76 e-cloud: beam-induced multipacting August , 76 proton-electron yield Tertiary electrons. Lost proton e - Captured electron Tail of proton bunch: repels electrons Repelled electron Secondary electrons net energy gain Long proton bunch (~170m) Head of proton bunch: captures electrons electron-electron yield (Wang, Blaskiewicz, Furman, Pivi, ) Vacuum Chamber Wall

77 August , 77 Extraction kicker rise time/impedance Kicker waveform 200 ns Time (200 ns per box) Ferrite kicker inside vacuum pipe Optimize saturable inductor to effectively shorten rise time (200ns) Improved flat-top flatness (1%) PFN termination: lower impedance Increase magnet height to halve coupling impedance (same drive) Shield the terminating resistance, reducing cable reflection

78 Ring vacuum chamber coating Injection kicker ceramic chamber double coating Cu (~ 0.7 µm) for image current TiN (0.1 µm) for electron cloud Meets requirement: conductive coatings w/ end-to-end resistance of ~0.04Ω ± 50% (Henderson, Davino) Thickness uniformity < ± 30% Extraction kicker ferrite patterned TiN coating 0.1 µm TiN on 90% inner surface, with good adhesion Extraction kicker Injection kicker August , 78 Ceramic tube and anode screen

79 August , 79 Class IV China Spallation Neutron Source

80 Proposed projects in China China Spallation Neutron Source - immediate project with future upgrades Huge domestic demand from the user community Compliments light sources (3 in China) and reactors Bridges the technology towards future ADS Accelerator Driven Subcritical reactor projects future project under active R&D Compliments fast-breeder reactor (FBR) and pressured water reactors (PWR) Transmutation of waste from nuclear power plants Eventual power generation with a subcritial reactor No long-lifetime waste, more abundant fuel ( 238 U), higher safety/possibly lower cost, less proliferation problem August , 80

81 layout Linac: H- beam, 81 MeV (DTL) to 230 MeV (SCL) Rapid-cycling synchrotron: 1.6 GeV at 25 Hz August , 81

82 latest development Project responsibility: IHEP Accelerator, Target & instruments: IHEP, IPHY in collaboration Site to be in Guangdong upgrade August , 82

83 Challenges Physics: Space charge & halo, electron cloud, fringe field, impedance & instability, diagnostics Engineering: Rapid-cycling technology (power supply, ceramic vacuum chamber, RF shielding, RF system, magnet/coil ), high-intensity source, RFQ, Linac and transport, collimation, remote handling, coating, diagnostics Management (budget): SNS: US$1.4B + upgrade fund + operating fund J-PARC: US$1.89B + people BSNS: US$0.2B Primary challenges: Complete project scope with limited budget Reserve potential for future development in phases August , 83

84 Design philosophy Fit in China s present economical situation Total phase-i cost 1.2B CNY (+0.6 B CNY) An advanced facility with upgrade potential Phase I beam power goal: 120 kw; phase II: 240 kw Ultimate potential: 500 kw and optional 2nd target (minimum initial cost with super-conducting RF linac) Adopt mature technology as much as possible First high-intensity proton machine in China Closely collaborate with world leaders, develop domestic technology Keep final fabrication in China as much as possible August , 84

85 August , 85 primary parameters Phase I Phase II Phase II Beam power on target [kw] Proton energy on target [GeV] Average beam current [µa] Pulse repetition rate [Hz] Protons per pulse [10 13 ] Linac energy [MeV] Linac type DTL DTL DTL+SCL Target number or 2 Target material Tungsten Moderators H 2 O (300K), CH 4 (100K), H 2 (20K) Number of spectrometers 7 18 >18

86 RFQ & MEBT: success at IHEP RFQ Following the previous RFQ design (352MHz) 324 MHz, 4-vane, 3 MeV output energy MEBT & chopper Leaving space for chopper August , 86

87 Commissioning of ADS-RFQ RFQ 出入口 ACCT 信号 : 入口 I in =44mA 出口 I out =41mA. 由此, 传输效率为 92%. ( 调束 7 小时后的结果,0.4% 工作比 ) 黄色 :RFQ 腔内射频场信号, 凹部 : 束流负载 August , 87 蓝色 : 输入耦合器反射信号, 凹部 : 束流负载使反射下降

88 Rapid cycling synchrotron layout August , 88

89 Ring lattice: best in class Four-fold symmetry Separated functions FODO arc Easy correction Dispersion-free doublet straight long, uninterrupted straight for collimation & injection Missing-gap momentum collimation High efficiency August , 89

90 Ring magnet; domestic vendors Large aperture, laminated magnets with eddy-current cuts near the ends and plates Stranded coil: more discussion from KEK / JAEA visitors this month Measurement at 25 Hz to be developed August , 90

91 August , 91 Ring vacuum: new approaches Ceramic vacuum chamber Metallic brazing (J-PARC) or glass joint (ISIS)? Possible external wrap-on RF shielding Japan IMP is developing quadrupole duct Dipole duct: difficult path in Japan; cost unknown IHEP to try glass joint (dipole) with help from ISIS

92 Ring RF: proven technology Ferrite-loaded RF cavity MHz Test of ferrite rings supplied by BNL etc. Controls: feedforward, dynamic tuning, feedback, radial & phase loops August , 92

93 August , 93 as ADS test facility towards higher power towards higher energy