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• Types of ion sources

• Sources of negative ions• Discharge sources• Laser ion sources• Field emission ion sources

• Sources of highly charged ions:ECR, EBIS, EBIT

Ion sources

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• Cs vapor from an oven fills an enclosed volume between the cooledcathode and the heated ionizing surface; some vapor condenses on thecathode surface. • Cesium atoms are ionized by the hot surface. • Cs ions are accelerated towards the cathode, sputtering it. Some materials will preferentially sputter negative ions; others, neutral orpositive particles which pick up electrons as they pass through thecondensed Cs layer / vapor, producing negative ions.

Sputter ion sourceto produce negative ions

Negative ion sources

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Plasma ion source

•Can produce both positive and negative ions and largeemission currents (up to mA), but mostly singly charged ions

electrons start discharge

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The ion current depends on the plasma density. Constrictingthe discharge with a magnetic field increases: • the plasma density,• the potential difference across the restriction • the energy of the ionizing electrons. Plasma escapes through a small aperture in the anode and expands. Ions are extracted afterwards.

Plasmatron sources

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Radiofrequency, electrodeless discharge source• no contamination due to electrode material• higher stability

Radiofrequency ion sources

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Inductively coupled plasma (ICP)

Applications• optical emission spectroscopy (ICP-OES):Emission lines are observed and analyzed• mass spectroscopy (ICP-MS):the masses of the ions produced are measured

Differentialpumping

Ions transfered tomass spectrometer

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Laser ion sources

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Principle of ablation laser ion sources

Plasma burst

Plasma expansion

and thinning

chargeseparation

ionextraction

and guiding

10 ns20 mJ

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Liquid metal ion source: Field emission

Field emission: Fowler-Nordheim law

Atomic size resolution!

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Formation of the Taylor cone

Electrostatic forces deform the liquid surface and produce an „atomically“ sharp tip (Taylor cone).

Simulation of time evolutionof the tip shape

t →

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Electrospray molecular sources

• Liquid flows through a needle at high voltage relative to a nearby surface.• The electric field at the tip causes the liquid to be “fissioned” into fine electrically charged droplets.• Ionization results from desorption of ions from them.

needlespray

fission and atomization

• Ions from delicate organic molecules can be prepared

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Motivation: Atomic physics in very strong fields

Methods of ionization

Sources of highly charged ions:•Electron cyclotron resonance ions sources (ECRIS)•Electron beam ion sources (EBIS)•Electron beam ion traps (EBIT)

Sources of highly charged ions

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Series expansion in non-perturbative QED: coupling constant Zα ≈ 1

Many virtual photons, each interaction Z× stronger than in H

Physics of highly charged ions

• QED: most accurate field theory. But: Theory of (non-perturbative) QEDin high fields still under construction.• Highly charged ions reduce the many-electron contributions while scaling up QED and relativistic effects.• Few-particle QED is still nearly unexplored• Nuclear size effects strongly influence electronic structure

• Relativistic effects grow with Z4, • Quantum electrodynamics (QED) also scales with Z4

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Schrödinger Dirac QED

n=1

p3/2

s1/2, p1/2

n=2

p1/2

s1/2

p3/2

115 keV

132.3 keV131.8 keV

+radiation fieldEn = Ry. Z2/n2

1s Lamb shift

n=3

s1/2

+ relativity

Highly charged ions are strongly affected by relativistic and QED effects

for U92+ 464 eV

Lamb shift

Physics of highly charged ions

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Ionization potential rises from 10 eV to 130000 eV

0 10 20 30 40 50 60 70 80 90 100

0.01

0.1

1

10

100barium

tungsten

krypton

neon

argon

uraniumIo

niz

atio

n p

ote

ntia

l (ke

V)

Ion charge state q+

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Production by sequential electron impact ionization

As the ion charge state goes up:

• growing ionization potential:

10 eV → 130000 eV• diminishing cross section:

10-16 cm2 → 10-24 cm2

continuum

130 keV

31 keV

12 keV

n=1

n=2 n=3

beam electronwith energy Ek

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• The plasma ionization degree depends on the balancebetween electron production and losses to the walls.

• To keep the electrons in the plasma, a strong multipole magnetic field surrounds the plasma volume.

• The increased path length due to the magnetic field increases the ionization probability.

• Improved ionization efficiency at lower pressure by reducing recombination.

Electron confinement: multicusp sources

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• Electrons rotate in a magnetic field B with frequencyω = e×B/m

and are heated by microwaves at that frequency. • Longitudinal confinement by magnetic fieldconfiguration.• Radial confinement by magnetic multipoles.• High electron temperature: multi-charged ionproduction at densities > 1012 electrons/cm3.• Used on cyclotrons and synchrotrons for theproduction of HCI beams.

Electron cyclotron resonance ion source

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Ar ions

• multiply to highly charged ions (Ar18+, Xe40+..) • currents up to 1 mA for O6+

• but rapidly falling currents for higher charge states

Electron cyclotron resonance ion source (ECRIS)

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• Hot plasma - configured with six cusps - is shaped by the sextupolemagnets surrounding the plasma chamber. • Solenoid magnets form the ends of the magnetic bottle, confiningthe plasma to the central region of the chamber.

Sextupole magnet and plasma confinment

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• An intense electron beam interacts with ions trapped in an electrostatic well. • Ions are confined radially by the space charge in the electron beam and axially by positive electrodes. • The ions can be investigated within the trap or expelled by lowering the potential of one end electrode.

Electron beam ion source (EBIS)

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Poisson‘s equation in cylindrical coordinates

Resulting potential with boundary conditions

Space charge potential: a line charge

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1000 2000 30000

20

40

60

80

100

120

140

center drift tube radiusspace charge potential

electron density

Spac

e ch

arg

e p

ote

ntia

l (V

)

Distance from axis (μm)

Ebeam

=2162 eV

Ibeam

=40 mA

Space charge potential of an electron beam

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0.1 1 10 100 1000

0

20

40

60

80

100

120

140

center drift tube radius

electron beam radius

space charge potential

electron density

Spac

e ch

arg

e p

ote

ntia

l (V

)

Distance from axis (μm)

Ebeam

=2162 eV

Ibeam

=40 mA

Space charge potential of an electron beam

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Electron Beam Ion Source (EBIS)

• Positive ions • High charge states up to U90+

• Small extracted currents

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• As electrons collide with the ions in the beam, they strip off electrons until the energy required to remove the next electron is higher than the beamenergy.

• The original LLNL EBIT (1986) used an electronbeam energy of about 30 keV to make neon-likeuranium (U82+).

• SuperEBIT achieved an electron beam energy of 200 keV, enough to make bare uranium (U92+).

The electron beam ion trap (EBIT)

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Production of HCI with an electron beam ion trap

radially: electron beamspace charge

total trap potential Utrap ≈ 200 V

(Utrap × ion charge) ≈ 10000 eV

15000 A/cm2

ne ≈ 1013 e-/cm3

axially: electrodes

I=450 mAE= 5 keV

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Section through the HD-EBIT I (1999)

HD-EBIT III(2006)

The original Livermore EBIT (1986)

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direct PI

E=hνEkin+Ebinding

resonant PI

K

L

E=hν

Eres

doubly excitedautoionizing

γ

interferenceFano profiles

K

L

Photoionization

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Photoionization

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Photoionization of ions

Merged ion-photon beam apparatus: ion beamand photon beam overlap and interact

Schlachter et al., LBNL

photon beam ion beam

photoionized ions

ion source

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110 120 130 140 150 160 170

0

1

2

3

4

Ion

yiel

d (a

. u.)

Photon energy (eV)

Ne4+

Photoionization of ions

Kjeldsen et al., Aarhus

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Measured and calculated cross sections for PI of C2+ ions. Solid curve: R-matrix results assuming 60% of ground state (gs) ions, 30% in the 3P0 and 5% each in the 3P1 and 3P2 metastable (ms) states in the primary beam.

Photoionization of C2+ ions: time-reversed recombination of C3+ with electrons, A. Müller et al., J. Phys. B: At. Mol. Opt. Phys. 35, L137 (2002)

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X-ray spectra of quasars indicate the presence of highly charged C, N, O , Ne and Fe ions

M. Sako et al., Astron. Astrophys. Lett. 365, L168 (2001)

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Around black holes or neutron stars, X rays generated by infalling matter photoionize the surroundings: Photoabsorption lines appear

Iron K-shell radiation is the last spectral signature of baryonicmatter before crossing the event horizon

Photoionized plasmas in astrophysics

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Date: 11 May 2010, Satellite: XMM-Newton; artist's impression of WHIM in the Sculptor WallCopyright: Spectrum: NASA/CXC/Univ. of California Irvine/T. Fang. Illustration: CXC/M. Weiss

Blazar H2356-309: Line-of-sight crosses the Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03

Observer

Photoabsorption lines due to warm-hot intergallactic medium

hot, photoionizedintergallactic medium

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Photoion extraction and charge analysis

After interaction with photons, ionsand photoions are extracted, massselected and detected

electrostaticdeflector

gun

trap

collector

photon beam:1013 photons/s

Wien filter

positionsensitivedetector

Fe14+

Fe15+

BE

monochromator

extracted ions

X-raydetector

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Fe14+ photoionization

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Comparison with theory for Fe14+

• Doppler shift depends on prediction• Hypothetical two-phase outflows in NGC 3783• Results confirm RMBPT calculations:no two-phase outflows

Magenta dots: our dataRed curve: HULLACBlack curve: RMBPT

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Doppler shift corrected based on experiment

1 5 . 0 1 5 . 5 1 6 . 0 1 6 . 5 1 7 . 0 1 7 . 5

- 1

0

Precise measurements of HCI X-ray absorption line positions and cross sections are possible with EBITs

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• Acceleration schemes for ions• Electrostatic accelerators• RF accelerators

Accelerators

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Van de Graaff principle

• Purely electrostaticacceleration• Ion source is installed at high voltage terminal• Potential is caused bycharging up the terminalwith a mechanical chargetransport chain

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Potential

0 Volt 0 Volt

10 MV

Energy

20 keV10.02 MeV

70.02 MeV

negative ionsC-

stripping positive ionsC6+

Tank with insulating gas (SF6)

Tandem van de Graaff accelerator (1930)

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MPI-K: 12 MV tandem accelerator

High voltageterminal

GroundCharge

Rubber conveyor beltor metal/insulatorchain (Pelletron)

van de Graaff principle

Tank (5.3 bar SF6)Terminal inside the tank

Negative ionsource

Van de Graaff accelerator

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In 1930 the New York Times announced that a "new apparatus to hurl particles at a speed of 37,000 miles per second in an effort to obtain a long-sought goal —the breaking up of the atom — was described here today by Professor Ernest O. Lawrence of the University of California."

The cyclotron

One of the original Lawrence cyclotrons

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The synchrotron• Ring with bending magnets and RF cavity synchronouslyaccelerate particle bunches• Magnetic focusing by quadrupoles and by radial fieldgradients in the bending magnets

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HF linear accelerator structures

Wideröe (1928)

• Deliver bunched beams• Use powerful RF generators• High voltage generated by

resonantly driven drift tubes

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Heavy ion linear accelerators at the GSI Darmstadt

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++

-

-

GSI

• RFQs do not use drift tubes but resonant waveguides at f ≅ 100-500 MHz

• Oscillating electric field and shape of electrodes induces an longitudinalaccelerating component in z direction

Radio-frequency quadrupoles as accelerators

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Ion accelerators: beam foil technique

To produce higher charge states in accelerators, ions in low charge statespass through a very thin foil where electrons are stripped.

Example:• ion source produces a beam of 20 keV Ne2+

• accelerated to: 20 MeV Ne2+

• after passing stripper: 20 MeV Ne10+

ion source accelerator stripper foil

storagering

Storage ring = synchrotron without acceleration

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• Ions interact 106 times per second with a collinear beam of cold electrons at nearly the same speed. • The transversal components of the ion motion are cooled.

IonsIons

I: 5-500 mAU: 10 - 200 kV

Electrons

• Momentum spread Δp/p : 10-4 – 10-5

• Beam diameter : 2 mm

Electron cooling in storage rings