Quadrupole ion trap

In a Paul trap, electrically charged particles are stored by means of an alternating electric field. Sometimes it is also referred to as a quadrupole ion trap, referring to the geometry of the field used. The German physicist Wolfgang Paul (1913-1993) received the Nobel Prize in Physics in 1989 for development.

Theory

The Paul trap is its classic design of three electrodes: one ring electrode and two end cap. These have hyperbolic inner surfaces. Between the ring electrode and end cap electrodes, an AC voltage with a high frequency (HF) is applied usually from 1 MHz to generate a quadrupole electric field in the interior of the case that applies a time periodic alternating force on the ions. The more the ions away from the center of the trap, the greater the stored power. The trajectories of the ions are described by the Mathieu differential equations (named after the mathematician Émile Léonard Mathieu ).

A cloud of ions in the interior of the case receives in the frequency of the alternating field alternating forces: focusing in the x-y plane ( of the ring electrode ) while defocusing force perpendicular thereto (Z- dimension), the phase change then a focus in the z- dimension defocusing in the xy plane. Both effects change with high frequency and produce an effective focus in all three dimensions, so a storage.

Mechanical analogue

An illustrative model by Wolfgang Paul facilitated the introduction of the principle: A ball would roll down from a stationary saddle surface. Rotating the surface, but the bullet can be stabilized. The further the ball moves away from the center, the steeper is the surface area and the greater is the restoring force.

Linear design

Practically a linear design of the Paul trap is often preferred. For this purpose, four metal rods are used which are parallel to each other in the rectangle. The bars each consist of three sections which are electrically separated by insulator rings. They are now operated similarly to the conventional structure having an alternating electric field, allowing a saving in the interior of the resulting rectangle. This design is simple and has the significant advantage that it contains at both longitudinal sides of an opening, can be easily engaged for example by a laser measurement, or the cooling of the particles, without interfering with the field geometry. It can also be used as the mass spectrometer (so-called quadrupole mass spectrometer ).

Use

The Paul trap provides a simple way to store charged particles dar. Since the stability of the paths depends on the mass -to-charge ratio of the ions, the Paul trap can be used for example for mass analysis in ion trap mass spectrometers. Furthermore, the Paul trap plays a major role in the first approaches to the realization of a quantum computer.