Penning trap
From Wikipedia, the free encyclopedia
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 to 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. The invention of the Penning trap is attributed to Hans Georg Dehmelt who shared the Nobel Prize in Physics in 1989 for this work.
[edit] Principle
Penning traps use a strong homogeneous 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. The endcaps are kept at the same electrostatic potential, while a potential difference is applied between the ring and the endcaps. 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 field 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, 20Ne and 28Si) come from Penning traps.
Using the Penning trap has two 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. 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.
Buffer gas, resistive, and laser cooling are techniques to remove energy from ions in a Penning trap. Buffer gas cooling relies on collisions between the ions and neutral gas molecules that bring the ion energy closer the energy of the gas molecules. In resistive cooling, the moving "image" charges in the electrodes are made to do work, effectively removing energy from the ions. Laser cooling can be used to remove energy from some kinds of ions in Penning traps; it does requires ions with the right kind of electronic structure. Radiative cooling is the process by which the ions lose energy by creating electromagnetic waves by virtue of their acceleration in the magnetic field. This technique dominates the cooling of electrons in Penning traps, but is very small for heavier charged particles.
There is also an analytical technique which uses a Penning trap called Fourier transform ion cyclotron resonance mass spectrometry, or FTICR. This technique measures the potential induced by a cloud of ions as a funciton of time and extracts the spectrum of cyclotron frequencies (and thus mass-to-charge ratios) of the ions from the time dependence of the potential.
