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FELIX QVI POTVIT RERVM COGNOSCERE CAVSAS

Introduction to ISIS accelerator and target

David FindlayAccelerator DivisionISIS DepartmentRutherford Appleton Laboratory

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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, ~megaelectron-volts)

Many more resources required for producing neutrons than electrons or protons

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Electron source

Proton source

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Neutron source

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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)

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Structure

Atomic motions

Paracetamol

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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

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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

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Neutron source is pulsed

Neutron energies measured by time of flight

t = 0

Source

Detector

time tE = ½ m

(l/t)²

length l

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= /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

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ISIS is accelerator-driven neutron source

800 MeV protons, 200 µA, 160 kW on tungsten target~2×1016 neutrons/second (mean) from spallation

Uses three cascaded particle accelerators

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EF

vB

F

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 acceleratede.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 = qE F = qv×B

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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 µA160 kW on to tungsten target~2×1016 neutrons/second from spallation

Also muons (protons into thin graphite target pions muons)

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Extremes of accelerator range

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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

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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)AZ (radioactive beams, from proton

accelerator and thin target)

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Some big RF accelerators

Muons — have to be quick! t½ = 2.2 µs

UK Neutrino Factory

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Neutron generation on ISIS:

800 MeV protons, 200 µA, 160 kW on tungsten target~2×1016 neutrons/second (mean) from spallation

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H– ion source (17 kV)665 kV H– Cockcroft-Walton70 MeV H– linac800 MeV proton synchrotron

Extracted proton beam lineTargetModerators

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H– ion source (17 kV)•Hydrogen gas•Arc, ~50 A arc current•Plasma•Caesium to lower cathode work function

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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)

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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

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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

Synchrotron because strength of magnetic field and frequency, amplitude and phase of RF all have to be synchronised

Fifty 10 ms acceleration cycles per secondMagnetic fields: 0.17–0.71 tesla; Reff= 26.0 mRF: 1.3–3.1 MHz, ~0–150 kV per turn, ~1 MW max.

Ten-fold symmetry

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Everything synchronised to magnetic field

Biased sine wave — (660 + 400 cos (t)) amps

Megawatt resonant LC circuit

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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

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Target

~2.5×1013 (4 µC) protons per pulse on to tungsten target (50 pps)

~15–20 neutrons / proton, ~4×1014 neutrons / pulse

Primary neutrons from spallation:evaporation spectrum (E ~1 MeV) + high energy tail

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Moderators

But want meV, not MeV

Moderation — elastic nuclear scattering — low A

Three moderators:liquid hydrogen (20°K), methane (100°K), water (43°C)

Reflector

Moderators

Primary targetProtons

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Source

Sample

Detector

neutrons

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Future

•300 µA upgrade (from 200 µA)RFQ (radio frequency quadrupole 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

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DC accelerator

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RF accelerator

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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

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Second Target Station

10 ppsEvery fifth pulse200 kW ÷ 5 = 40 kW

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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, ...

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1 MW upgrade

Add 3 / 8 GeV synchrotron

Muons

Neutrons

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Further into future — 2½ and/or 5 MW upgrades