Penning trap

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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.

Penning traps use a strong homogeneous axial magnetic field to confine
particles radially and a quadrupole electric field to confine
the particles axially. The static electric potential can be generated using a
set of three electrodes: a ring and two
endcaps. In an ideal Penning trap the ring and endcaps are hyperboloids of revolution.
For trapping of positive (negative) ions, the endcap electrodes are kept at a
positive (negative) potential relative to the ring. This potential produces a saddle
point in the centre of the trap, which traps ions along the axial direction.
The electric field causes ions to oscillate (harmonically in the case of an
ideal Penning trap) along the trap axis. The magnetic field in combination with
the electric field causes charged particles to move in the radial plane with a
motion which traces out an epitrochoid.

The orbital motion of ions in the radial plane is composed of two modes at
frequencies which are called the magnetron and the modified
cyclotron frequencies. These motions are similar to the deferent and
epicycle, respectively, of the Ptolemaic model of the
solar system. The sum of these two frequencies is the cyclotron
frequency, which depends only on the ratio of electric charge to mass and on the strength
of the magnetic field. This
frequency can be measured very accurately and can be used to measure the masses
of charged particles. Many of the highest-precision mass measurements (masses of
the electron,
proton, 2H,
20Ne and
28Si) come from Penning traps.

There is a technique called Fourier
transform ion cyclotron resonance mass spectrometry, or FTICR. This
technique measures the potential induced by a cloud of ions as a function 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.

Buffer gas cooling, resistive cooling, 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, moving
image charges in
the electrodes are made to do work through an external resistor, effectively
removing energy from the ions. Laser cooling can be used
to remove energy from some kinds of ions in Penning traps. This technique
requires ions with an appropriate 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 process
dominates the cooling of electrons in Penning traps, but is very small and
usually negledgable for heavier particles.

Using the Penning trap can have 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.
Also, 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 can be responsible
for heating and decoherence effects and
these effects scale as a high power of the inverse distance between the ion and
the electrode.

[edit]
Trivia

On a February 2007 episode of Late Night
with Conan O'Brien, in a sketch, Conan O'Brien and Jim Carrey discussed the
physics of electrons bound in a Penning trap. When the band leader, Max
Weinberg, attempted to correct them on the matter, he was proven wrong, and
declared an "ass." [1]

The paper they reference is here. [2]

[edit]
External links

Nobel Prize in Physics 1989

The High-precision Penning
Trap Mass Spectrometer SMILETRAP in Stockholm, Sweden

Retrieved from "http://en.wikipedia.org/wiki/Penning_trap"

Categories: Measuring
instruments | Atomic
physics

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This page was last modified 21:12, 1 May 2007.

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