266 Ion traps
in the radial direction; there is no axial micromotion because it is a d.c.
electric field along thez-direction. We will see that after cooling the
ions sit very close to the trap axis where the a.c. field causes very small
perturbations. In the later discussion of laser cooling the trapped ions
are considered simply as harmonic oscillators (neglecting micromotion).12.4 Buffer gas cooling
Trapping of ions requires a vacuum, but the presence of a small back-
ground of helium gas at a pressure (∼ 10 −^4 mbar) gives very effective
cooling of hot ions. The ions dissipate their kinetic energy through col-
lisions with the buffer gas atoms and this quickly brings the ions into
thermal equilibrium with the gas at room temperature. For ions that
start off above room temperature the buffer gas cooling actuallyreduces
the perturbations on the ions. Any slight broadening and shift of the
ions’ energy levels by the collisions is outweighed by the reduction in the
ions’ micromotion—the ions stay closer to the trap centre where they
see smaller a.c. fields.
Buffer gas cooling can be compared to having a vacuum flask that has
partially ‘lost its vacuum’—hot coffee in the flask cools down because
the low pressure of gas between the walls provides much less thermal
insulation than a good vacuum. It follows from this argument that
ions can only be cooled far below room temperature in a good vacuum,
e.g. a pressure of 10−^11 mbar in the laser cooling experiments described
later (otherwise collisions with the hot background gas atoms heat the
ions). This case corresponds to having a good vacuum flask; dewars in
the laboratory work on the same principle to keep things such as liquid
helium at temperatures much colder than the surroundings.
Buffer gas cooling finds widespread application in instruments that
need to operate reliably over long periods, such as the mercury ion clock
developed at NASA’s Jet Propulsion Laboratory in Pasadena, Califor-
nia. The linear Paul trap contains a cloud of mercury ions and mi-
crowaves drive the transition at 40 GHz between the two hyperfine levels
in the ground state of the ions. By reference to the resonance frequency
of the ions, the electronic servo-control system maintains the frequency
of the microwave source stable to 1 part in 10^14 over long periods. This
ion trap provides a very good frequency reference and has been used
for navigation in deep space, where the accurate timing of signals trans-
mitted to and from the probe determine its distance from a transmit-(^19) See Berkeland et al. (1998) for a ter/receiver on Earth. (^19) Paul traps with buffer gas cooling are also used
description of a state-of-the-art laser-
cooled mercury ion frequency standard.
to give long ion storage times in some commercial mass spectrometers.
There is a method analogous to buffer gas cooling that works at much
lower temperatures. Insympathetic coolinga trap confines two species
of ions at the same time, e.g. Be+and Mg+. Laser cooling one species,
as described in the next section, e.g. the Be+ions, produces a cloud
of cold ions that acts as the ‘buffer gas’ to cool the Mg+ions through
collisions. The ions interact through their strong long-range Coulomb