Ion pump (physics)

"Ion pump" redirects here. For a protein that moves ions across a plasma membrane, see Ion transporter. An ion pump is not to be confused with an ionic liquid piston pump or an ionic liquid ring vacuum pump.

An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump capable of reaching pressures as low as 10−11 mbar under ideal conditions.[1] An ion pump ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3–7 kV, which allows the ions to accelerate into and be captured by a solid electrode and its residue.

Penning trap

The basic element of the common ion pump is a Penning trap.[2] A swirling cloud of electrons produced by an electric discharge is temporarily stored in the anode region of a Penning trap. These electrons ionize incoming gas atoms and molecules. The resultant swirling ions are accelerated to strike a chemically active cathode (usually titanium).[3] On impact the accelerated ions will either become buried within the cathode or sputter cathode material onto the walls of the pump. The freshly sputtered chemically active cathode material acts as a getter that then evacuates the gas by both chemisorption and physisorption resulting in a net pumping action. Inert and lighter gases, such as He and H2 tend not to sputter and are absorbed by physisorption. Some fraction of the energetic gas ions (including gas that is not chemically active with the cathode material) can strike the cathode and acquire an electron from the surface, neutralizing it as it rebounds. These rebounding energetic neutrals are buried in exposed pump surfaces.[4]

Both the pumping rate and capacity of such capture methods are dependent on the specific gas species being collected and the cathode material absorbing it. Some species, such as carbon monoxide, will chemically bind to the surface of a cathode material. Others, such as hydrogen, will diffuse into the metallic structure. In the former example, the pump rate can drop as the cathode material becomes coated. In the latter, the rate remains fixed by the rate at which the hydrogen diffuses.

Types

There are three main types of ion pumps: the conventional or standard diode pump, the noble diode pump and the triode pump.[5]

Standard diode pump

A standard diode pump is a type of ion pump employed in high vacuum processes which contains only chemically active cathodes, in contrast to noble diode pumps.[5]

Noble diode pump

A noble diode pump is a type of ion pump used in high-vacuum applications that employs both a chemically reactive cathode, such as titanium, and an additional cathode composed of tantalum. The tantalum cathode serves as a high-inertia crystal lattice structure for the reflection and burial of neutrals, increasing pumping effectiveness of inert gas ions.[5] Pumping intermittently high quantities of hydrogen with noble diodes should be done with great care, as hydrogen might over months get re-emitted out of the tantalum.

Applications

Ion pumps are commonly used in ultra-high vacuum (UHV) systems, as they can attain ultimate pressures less than 10−11 mbar.[1] In contrast to other common UHV pumps, such as turbomolecular pumps and diffusion pumps, ion pumps have no moving parts and use no oil. They are therefore clean, need little maintenance, and produce no vibrations. These advantages make ion pumps well-suited for use in scanning probe microscopy and other high-precision apparatuses.

Radicals

Recent work has suggested that free radicals escaping from ion pumps can influence the results of some experiments.[6]

See also

References

  1. 1 2 "Ion Pumps" (PDF). Agilent.
  2. ↑ Cambers, A., "Modern Vacuum Physics", CRC Press (2005)
  3. ↑ Weissler, G.L. and Carlson, R.W., editors, Methods of Experimental Physics; Vacuum Physics and Technology, Vol. 14, Academic Press Inc., London (1979)
  4. ↑ Moore, J.H.; Davis, C. C.; Coplan, M.A.; Greer, S. (2003). Building Scientific Apparatus. Westview Press. ISBN 0-8133-4006-3.
  5. 1 2 3 The pumping of helium and hydrogen by sputter- ion pumps part II
  6. ↑ J. Zikovsky, S. A. Dogel, A. J. Dickie, J. L. Pitters, R. A. Wolkow (2009). "Reaction of a hydrogen-terminated Si(100) surface in UHV with ion-pump generated radicals". Journal of Vacuum Science and Technology A 27 (2): 248. doi:10.1116/1.3071944.

Sources

External links

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