Transcript Basic Physics Topics

Welcome to the
SI-Tee
Sunday, April 10, 2016
Accelerators and Ion
Sources
CHARMS Basic Physics Topics series
November 2nd, 2005
Outline
1. Accelerators
2. Ion Sources
(This is logically reverse order, but it is easier to
present things this way)
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Accelerators and Ion Sources
2
Accelerators – basic ideas

Charged particles can be accelerated in the electric field.

Examples from the nature – electrostatic discharge, αand β-decays, cosmic rays.

Rutherford's experiments with α-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 → 2·4He

CW was widely used as injector until the
invention of RFQ
Fermilab 750 kV C-W
preaccelerator
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Van de Graaff
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
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 @ IReS Strasbourg

25 MV Tandem @ ORNL

18 MV Tandem @ 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.
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
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 = ωrfCVrf2 –

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 @ GSI – HI

GELINA @ 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 CockroftWalton as injectors
2 MeV RFQ @ Idaho State Univ.
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Cyclotron

The cyclotron frequency
of a non-relativistic
particle is independent
of the particle velocity:
ω0 = eB0 / γm ≈ 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.
2
ωE
Bz 

ec
0
2
 ωρ 
1 

 c 
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 @ CERN (28, 450, 7000 GeV)

SIS @ GSI
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CERN Accelerator Complex
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GSI
The Present and the Future
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Welcome to the
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Sunday, April 10, 2016
Ion Sources
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
Field ionization
Plasma beam
Duoplasmatron
Sputter
Laser
Electron beam ionization
Hollow cathode
Pigatrons
Multifilament
Arc discharge
Multipole confinement
Pennings
Charge exchange
Cyclotron resonance
Surface plasma
Magnetrons
RF plasma
source: http://linac2.home.cern.ch/linac2/seminar/seminar.htm#intro
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Plasma ion sources

Ionization is actually a process of creation of a
plasma

Plasma ion source: Ionization mechanism: eˉ-eˉ
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
Penning Ion Gauge
(PIG) Ion Source
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
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 @ GSI
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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

MUCIS used @ GSI
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Large volume of uniform
and dense plasma possible
(2.5 cm – 1m size)
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Electron Cyclotron
Resonance IS (ECRIS)


Vapor held in a cavity with high magnetic field
Microwaves with frequency that coincides with eˉ 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 @ 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)
EXTRACTION
ELECTRODE
Negative surface ion
source also in use
Surface Ion-Source
http://isolde.web.cern.ch/ISOLDE/
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Sputter Ion Source
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
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 > φ

High field emission (fine point cathode)

Photo emission: λ < hc/φ
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Welcome to the
SI-Tee
Sunday, April 10, 2016
The End
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