Penning trap
|
|
Penning_trap.JPG
Penning traps are devices for the storage of charged particles using a constant magnetic field and a constant electric field. This kind of trap is particularly well suited for precision measurements of properties of ions and stable subatomic particles which have electric charge. Recently this trap has been used in the physical realization of quantum computation and quantum information processing as well. Currently Penning traps are used at CERN to store antiprotons. Any form of antimatter propolusion for interplanetary voyages will probably use these traps for the same purpose.
Penning traps use a strong axial magnetic field to confine particles radially and a quadrupole electric field to confine the particles axially. The magnetic field causes charged particles to move in spirals, and the electric field prevents particles from spiraling out of the trap along the magnetic field lines. The static electromagnetic potential can be generated using a set of three electrodes; a ring and two endcaps. The ring and endcaps are hyperboloids of reflection. A potential difference is applied between the ring and endcap. This potential produces a saddle point in the centre of the trap in which traps the ion only along the axial direction. In order to trap the ion in the radial plane a homogeneous magnetic feld is applied along the axis to force the ions into circular orbits and keep them in the trap.
The oscillations in the radial plane are composed of two frequencies which are called the magnetron and the cyclotron frequencies. The cyclotron frequency depends on the ratio of electric charge to mass and strength of the magnetic field. This frequency can be measured very accurately and can be used to measure the ratio of masses. Many of the highest-precision mass measurements (masses of the electron, proton, 2H, 12C, 20Ne and 28Si) come from Penning traps.
Using the Penning trap has several potential advantages over the radio frequency trap (Paul trap). Firstly, in the Penning trap only static fields are applied and therefore there is no micro-motion and resultant heating of the ion due to the dynamic fields. However laser cooling in the Penning trap is more complicated because one degree of motion (the magnetron motion) cannot be cooled directly.
Secondly the Penning trap can be made larger whilst maintaining strong trapping. The trapped ion can then be held further away from the electrode surfaces. Interaction with patch potentials on the electrode surfaces is responsible for heating and decoherence effects and these effects scale as a high power of the distance between the atom and the electrode.
There is also an analytical technique which uses a Penning trap called Fourier Transform Ion Cyclotron Resonance spectroscopy, or FTICR. A new Penning trap is currently being constructed at the University of Chicago Roanoke Campus.de:Penning-Falle