Accelerators and Ion Sources

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Accelerators and Ion Sources

• CHARMS Basic Physics Topics series
• November 2nd, 2005

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Outline

• Accelerators
• Ion Sources
• (This is logically reverse order, but it is
  easier to present things this way)

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Accelerators basic ideas

• Charged particles can be accelerated in the
  electric field.
• Examples from the nature electrostatic
  discharge, a- and ß-decays, cosmic rays.
• Rutherford's experiments with a-particles
• Discovery of the nucleus in 1911
• First artificial nuclear reactions
• Inspiration for high-voltage particle
  accelerators
• Muons and pions were discovered in cosmic-ray
  experiments with emulsions.
• Everyday life TV-set, X-ray tubes...

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Types of Accelerators Used in Science

• Electrostatic Cockroft-Walton, Van de Graaff
• Induction Induction linac, betatron
• Radio-frequency accelerators LINAC, RFQ,
  Cyclotron, Isochronous cyclotron,
  Synchrocyclotron, Microtron, Synchrotron

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Cockroft-Walton

• High voltage source using rectifier units
• Voltage multiplier ladder allows reaching up to
  1 MeV (sparking).
• First nuclear transmutation reaction achieved in
  1932 p 7Li ? 24He
• CW was widely used as injector until the
  invention of RFQ

Fermilab 750 kV C-W preaccelerator
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Van de Graaff

• Voltage buildup by mechanical transport of charge
  using a conveyor belt.
• Builds up to 20 MV

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Tandem Van de Graaff

• Negative ions accelerated towards a positive HV
  terminal, then stripped of electrons and
  accelerated again away from it, doubling the
  energy.
• Negative ion source required!
• Examples
• VIVITRON _at_ IReS Strasbourg
• 25 MV Tandem _at_ ORNL
• 18 MV Tandem _at_ JAERI
• 20 MV Tandem in Buenos Aires

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Induction linac

• Creation of electric field by magnetic induction
  in a longitudinal evacuated cavity in magnetic
  material

• Very high intensity beams (up to thousands of
  Amperes)

N. C. Christofilos et al., Rev. of Sci. Inst. 35
(1964) 886
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Betatron

• Changes in the magnetic flux enclosed by the
  circular beam path induce a voltage along the
  path.

• Name derived from its use to accelerate electrons
• To the left Donald Kerst with two of the first
  operational betatrons (2.3 and 25 MeV)

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RF Accelerators

• High voltage gaps are very difficult to maintain
• Solution Make the particles pass through the
  voltage gap many times!
• First proposed by G. Ising in 1925
• First realization by R. Wiederöe in 1928 to
  produce 50 kV potassium ions
• Many different types

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RF LINAC basic idea

• Particles accelerated between the cavities
• Cavity length increases to match the increasing
  speed of the particles
• EM radiation power P ?rfCVrf2
• the drift tube placed in a cavity so that the EM
  energy is stored.
• Resonant frequency of the cavity tuned to that of
  the accelerating field

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RF LINAC phase focusing

• E. M. McMillan V. Veksler 1945
• The field is synchronized so that the slower
  particles get more acceleration

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LINAC Examples

• SLAC 3 km, 50 GeV electrons, 2.856 GHz
• UNILAC _at_ GSI HI
• GELINA _at_ IRMM Geel 150 MeV electrons

GELINA maquette
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RF Quadrupole

• Simultaneous generation of a longitudinal RF
  electric field and a transverse focusing
  quadrupole field

• Low-energy, high-current beams
• Compact
• Replacing Cockroft-Walton as injectors

2 MeV RFQ _at_ Idaho State Univ.
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Cyclotron

• The cyclotron frequency of a non-relativistic
  particle is independent of the particle
  velocity?0 eB0 / ?m eB0 / m
• E. O. Lawrence in 1929

• Limitations relativistic effects break the
  isochronism ? Epmax 12 MeV

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Isochronous Cyclotron

• In order to restore the isochronism, the magnetic
  field needs to be shaped in function of the
  radius to match the change of the frequency with
  the particle energy.
• However, such configuration leads to vertical
  orbit instability ? restoration of the orbit
  stability using the Azimuthal Varying Field (AVF)
  L. H. Thomas (1938)

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Synchrocyclotron

• Instead of modifying the magnetic field, the
  radio frequency can be modulated ? pulsed beams
• Limit at 1GeV
• Example SC in CERN (600 MeV)

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Synchrotron

• Use of the phase-focusing principle in a circular
  orbit with a constant radius
• RF and magnetic fields are tuned to synchronize
  the particle revolution frequency and confine its
  orbit.
• Examples
• PS, SPS, LHC _at_ CERN (28, 450, 7000 GeV)
• SIS _at_ GSI

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CERN Accelerator Complex
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GSI The Present and the Future
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Ion Sources
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Ion Sources

• Very broad field with many applications
• Material science and technology (e.g. ion
  implantation)
• Food sterilization
• Medical applications
• Military applications
• Accelerators
• ...
• Beams of nanoamperes to hundreds of amperes
• Very thin to very broad beams (µm2 to m2)

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Types of Ion Sources (selection)
Surface ionization  Plasma beam
Field ionization  Duoplasmatron
Sputter  Hollow cathode
Laser  Pigatrons
Electron beam ionization  Multifilament
Arc discharge  Cyclotron resonance
Multipole confinement  Surface plasma
Pennings  Magnetrons
Charge exchange  RF plasma
source http//linac2.home.cern.ch/linac2/seminar/
seminar.htmintro
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Plasma ion sources

• Ionization is actually a process of creation of a
  plasma
• Plasma ion source Ionization mechanism e-e
  collisions
• Most widely used many different types
• Types differ according to plasma production and
  confinement mechanisms.

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Metal Vapor Vacuum IS (MEVVA)

• Electrostatic discharge between a cold anode and
  a hot cathode in a vacuum
• Evaporation and ionization of cathode atoms

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Penning Ion Sources

• Arc discharge in a magnetic field electrons
  confined radially by the magnetic field and
  axially by electrostatic potential well
• In cyclotrons it is possible to use the magnetic
  field of the accelerator
• One PIG is used _at_ GSI

Penning Ion Gauge (PIG) Ion Source
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Multi-Cusp Ion Source (MUCIS)

• Cusp-like magnetic field lines
• Most of the plasma volume in a relatively weak
  magnetic field

• Large volume of uniform and dense plasma possible
  (2.5 cm 1m size)

MUCIS used _at_ GSI
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Electron Cyclotron Resonance IS (ECRIS)

• Vapor held in a cavity with high magnetic field
• Microwaves with frequency that coincides with e
  cyclotron frequency in the field heat the
  electrons (and only electrons).
• No electrodes, no arc discharge very reliable,
  high currents
• 14 GHz, 0.5 T _at_ GSI, Dubna, LBNL, CERN

http//www.casetechnology.com/source.html
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Surface Ion Source

• Hot surface of a metal with high work function
  ionizes elements with low ionization potential
  (like alkalis)
• Negative surface ion source also in use

EXTRACTION ELECTRODE
Surface Ion-Source
http//isolde.web.cern.ch/ISOLDE/
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Sputter Ion Source

• Cesium vapor, hot anode, cooled cathode
• Some of the vapor gets condensed on the cathode,
  some gets ionized on the anode and accelerated
  towards the cathode where it sputters atoms from
  the cathode
• Produces negative ions of all elements that form
  stable negative ions

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Laser Ion Source

• Stepwise resonant excitation and photoionization
  of the atom
• Chemically selective wavelength tuned to the
  specific element
• Pulsed

http//isolde.web.cern.ch/ISOLDE/
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Electron Sources

• Thermionic emission escape of electrons from a
  heated surface. Condition Ee gt f
• High field emission (fine point cathode)
• Photo emission ? lt hc/f

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The End

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