Introduction ISIS accelerator and target general
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Introduction ISIS accelerator and target general

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ISIS Introduction to the accelerator and target.General information.

ISIS Introduction to the accelerator and target.General information.

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  • 1. FELIX QVI POTVIT RERVM COGNOSCERE CAVSAS Introduction to ISIS accelerator and target David Findlay Accelerator Division ISIS Department Rutherford Appleton Laboratory
  • 2.
    • ISIS is large facility for making measurements on condensed matter samples using neutrons, so need lots of neutrons
    • Three kinds of “traditional” elementary particles:
        • Electrons (in atom, ~electron-volts)
        • Protons (in (hydrogen) atom, ~electron-volts)
        • Neutrons (in nucleus, ~ mega electron-volts)
    • Many more resources required for producing neutrons than electrons or protons
  • 3. Electron source Proton source
  • 4. Neutron source
  • 5.
    • ISIS is spallation neutron source
    • — used for studying molecular structure of matter
    • Two key questions:
      • Where are the atoms? (structure)
      • How are they connected together? (dynamics)
  • 6. Structure Atomic motions Paracetamol
  • 7.
    • Interatomic spacings typically ~few Å (1 Å  0.1 nm)
    • Need uncharged probe with wavelength ~1 Å
    • Practical choices: neutrons, X-rays
      • X-rays: 1 Å  12 keV
      • Neutrons: 1 Å  0.1 eV
    • Dynamics: typical energies ~meV
    • Neutrons have just the right mass to satisfy both requirements simultaneously
    • Neutrons also sensitive to magnetism, since they carry magnetic moment
  • 8. Measurements are made on condensed matter samples on ISIS by neutron scattering Just as an object can be seen by suitably collecting scattered optical photons, so a condensed matter sample can “seen” by suitably collecting scattered neutrons Source Sample Detector neutrons
  • 9. Neutron source is pulsed Neutron energies measured by time of flight t = 0 Source Detector time t E = ½ m (l/t)² length l
  • 10.  =  /p (reduced de Broglie wavelength)  pc =  c/   = 1 Å   = 0.159 Å  c = 197 MeV.fm = 1970 eV.Å (e²/  c = 1/137)  pc = 12.4 keV Neutrons: p²c² = 2mc²E, m = 938 MeV  E(neutron) = 80 meV X-rays: pc = E  E(X-ray) = 12 keV Cf. dynamics: typical energies ~meV
  • 11. ISIS is accelerator -driven neutron source 800 MeV protons, 200 µA, 160 kW on tungsten target ~2×10 16 neutrons/second (mean) from spallation Uses three cascaded particle accelerators
  • 12.
    • Particle accelerators:
    • Accelerate elementary or not so elementary particles ( e.g. e – , p, H – , d, heavy ions)
    • Must be charged particles — neutral particles cannot be accelerated
      • e.g. neutrons, used on ISIS, are produced as secondary particles from primary protons
    • Particles accelerated by electric field, not magnetic field, but magnetic fields used to guide particles being accelerated
    • F = q E F = q v×B
    E F v B F
  • 13.
  • 14.
    • Throughout world: >10000 particle accelerators
    • ~50% industrial, ~40% radiotherapy
    • ~100 at ~1 GeV and above
    • Output energies range between:
      • ~100 keV (e.g. ion implanter), and ~10 TeV (CERN LHC (large hadron collider))
    • ISIS accelerator
      • 800 MeV protons, 200 µA
      • 160 kW on to tungsten target
      • ~2×10 16 neutrons/second from spallation
      • Also muons (protons into thin graphite target  pions  muons)
  • 15. Extremes of accelerator range
  • 16.
    • Accelerate using electric field
    • Clearly for 100 keV can use 100 kV DC power supply unit
    • But can scarcely use 10,000,000,000,000 V DC power supply unit for LHC
    • Instead, for high energies, use oscillating radio frequency (RF) fields, and pass particles repeatedly through these fields
    • RF fields produce bunched beams
      • — lots of bundles of charge in a long line
    DC RF ns – µs spacing
  • 17.
    • Every accelerator needs a source of particles
    • Electron accelerators: electron gun
      • ( cf. back of TV tube)
    • Accelerators for other stable particles:
      • ion sources (ionisation, plasma)
    • Accelerators of unstable particles:
      • subsidiary accelerators
        • e + (from electron accelerator)
        • µ +,– (from  +,– from proton accelerator)
        • A Z (radioactive beams, from proton accelerator and thin target)
  • 18.
    • Some big RF accelerators
      • Muons — have to be quick! t ½ = 2.2 µs
    UK Neutrino Factory
  • 19. Neutron generation on ISIS: 800 MeV protons, 200 µA, 160 kW on tungsten target ~2×10 16 neutrons/second (mean) from spallation
  • 20.
      • H – ion source (17 kV)
      • 665 kV H – Cockcroft-Walton
      • 70 MeV H – linac
      • 800 MeV proton synchrotron
      • Extracted proton beam line
      • Target
      • Moderators
  • 21.
    • H – ion source (17 kV)
      • Hydrogen gas
      • Arc, ~50 A arc current
      • Plasma
      • Caesium to lower cathode work function
  • 22.
  • 23.
  • 24.
  • 25.
    • Cockcroft-Walton (665 kV, H – ions)
      • DC accelerator
      • 10-stage voltage multiplier (5.5 kHz)
    665,000 V is a high voltage, so large insulation spacings required (~2 m on basis of ~10 kV / inch rule of thumb)
  • 26.
  • 27.
  • 28.
  • 29. Linac (70 MeV, H – ions) 4-section (-tank) drift tube linac Acceleration by 202.5 MHz RF, not DC Each tank highly ~10 m long, ~1 m diameter. Highly resonant; Q ~50000 Hide particles inside drift tubes while sign of oscillating accelerating field wrong
  • 30.
  • 31.
  • 32.
  • 33.
  • 34.
  • 35.
    • Synchrotron (800 MeV, protons)
    • Circular machine
      • Magnets to bend particles round in circle
      • RF electric fields to accelerate particles
    • H – ions stripped to protons when injected
    • Synchro tron because strength of magnetic field and frequency, amplitude and phase of RF all have to be synchronised
    • Fifty 10 ms acceleration cycles per second
      • Magnetic fields: 0.17–0.71 tesla; R eff = 26.0 m
      • RF: 1.3–3.1 MHz, ~0–150 kV per turn, ~1 MW max.
    • Ten-fold symmetry
  • 36. Everything synchronised to magnetic field Biased sine wave — (660 + 400 cos (  t)) amps Megawatt resonant LC circuit
  • 37.
  • 38.
  • 39.
  • 40. All beam in synchrotron extracted in one turn  = v/c = 0.84, 163 m circumference  revolution time = 0.65 µs 4 µC ÷ 0.65 µs  6 A circulating current Extracted pulse ~0.4 µs long
  • 41.
  • 42.
  • 43.
  • 44.
  • 45.
  • 46.
  • 47.
  • 48. Target ~2.5×10 13 (4 µC) protons per pulse on to tungsten target (50 pps) ~15–20 neutrons / proton, ~4×10 14 neutrons / pulse Primary neutrons from spallation: evaporation spectrum (E ~1 MeV) + high energy tail
  • 49.
  • 50.
  • 51. Moderators But want m eV, not M eV Moderation — elastic nuclear scattering — low A Three moderators: liquid hydrogen (20°K), methane (100°K), water (43°C) Reflector Moderators Primary target Protons
  • 52.
  • 53.
  • 54.
  • 55. Source Sample Detector neutrons
  • 56.
  • 57.
    • Future
    • 300 µA upgrade (from 200 µA)
      • RFQ ( r adio f requency q uadrupole accelerator)
        • (gets ~50% more beam into linac)
      • Synchrotron second harmonic RF upgrade
        • (enlarges “RF buckets” in synchrotron so more charge can be accelerated)
    • Second Target Station (10 pps)
    • 1 MW upgrade (from 800 MeV to 3500 MeV)
    • 2½ and 5 MW upgrades
  • 58.
  • 59. DC accelerator
  • 60. RF accelerator
  • 61.
  • 62.
  • 63.
    • Second harmonic RF cavities for synchrotron
      • Four cavities ( cf. six fundamental cavities)
      • Fed with RF at twice frequency of fundamental
      • Enlarges area of phase space within which
        • stable acceleration of particles is possible
  • 64.
  • 65.
  • 66.
  • 67. Second Target Station 10 pps Every fifth pulse 200 kW ÷ 5 = 40 kW
  • 68.
  • 69.
  • 70. Second Target Station (TS2) — £100M, first beam 2007 Optimised for cold neutrons Cold neutrons  low energy / slow neutrons Consistent with low pulse repetition frequency — 10 pps ( cf. 50 pps to present target) Slow neutrons  long wavelengths — sensitive to longer range structure Polymers, surfactants, colloids, proteins, viruses, pharmaceuticals, ...
  • 71. 1 MW upgrade Add 3 / 8 GeV synchrotron Muons Neutrons
  • 72. Further into future — 2½ and/or 5 MW upgrades