Ion source

An ion source is an electro-magnetic device that is used to create charged particles. These are used primarily within mass spectrometers or particle accelerators.

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Mass spectrometry

In mass spectrometry, an ion source is a piece of equipment used to ionize analyte molecules and, if necessary, free them from the solid or liquid phase. Once the analyte ions are free to move electric fields will direct them into the mass analyzer.

There are several types of ion sources:

• Electron ionization (EI)
• Chemical ionization (CI)
• Ion attached ionization (IA) for fragmentation free ionization
• Fast atom bombardment (FAB)
• Field desorption (FD)
• Matrix-assisted laser desorption/ionization (MALDI)
• Atmospheric pressure chemical ionization (APCI)
• Atmospheric pressure photoionization (APPI)
• Electrospray ionization (ESI)
• Desorption electrospray ionization (DESI)
• Glow discharge (GD)
• Inductively coupled plasma (ICP)
• Microwave induced plasma (MIP)
• Thermospray ionization (TS)
• Direct analysis in real time (DART)

In particle accelerators

In particle accelerators an ion source creates a particle beam at the beginning of the machine, the Source. The technology to create ion sources for particle accelerators depends strongly on the type of particle that needs to be generated: electrons, protons, H^- ion or a heavy ion.

Electrons are generated with an electron gun, and there are many varieties of these.

Protons are generated with a plasma-based device, like a duoplasmatron or a magnetron.

H^- ions are generated with a magnetron or a Penning source. A magnetron consists of a central cylindrical cathode surrounded by an anode. The discharge voltage is typically greater than 150 V and the current drain is around 40 A. A magnetic field of about 0.2 tesla is parallel to the cathode axis. Hydrogen gas is introduced by a pulsed gas valve. Cesium is often used to lower the work function of the cathode, enhancing the amount of ions that are produced.

For a Penning source, a strong magnetic field parallel to the electric field of the sheath guides electrons and ions on cyclotron spirals from cathode to cathode. Fast H-minus ions are generated at the cathodes as in the magnetron. They are slowed down due to the charge exchange reaction as they migrate to the plasma aperture. This makes for a beam of ions that is colder than the ions obtained from a magnetron.

Heavy ions are generated with an electron cyclotron resonance ion source. The use of electron cyclotron resonance (ECR) ion sources for the production of intense beams of highly charged ions has immensely grown over the last decade. ECR ion sources are used as injectors into linear accelerators, Van-de-Graaff generators or cyclotrons in nuclear and elementary particle physics. In atomic and surface physics ECR ion sources deliver intense beams of highly charged ions for collision experiments or for the investigation of surfaces.

Theory of Operation

Gas flows through the ion source between the anode and the cathode. A positive voltage is applied to the anode. This voltage, combined with the high magnetic field between the tips of the internal and external cathodes allow a plasma to start. Ions from the plasma are repelled by the anode electric field. This creates an ion beam.^[1]

Ion Source Applications

• Surface cleaning and pretreatment for large area deposition
• Thin-film deposition
• Deposition of Thick Diamond-like carbon (DLC) Films
• Surface roughening of polymers for improved adhesion and/or biocompatibility^[2]

References

See also

• Cold cathode
• Hot filament
• RF antenna ion source