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)
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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