Ion trap

Atomic and molecular ion traps have a number of applications in physics and chemistry such as precision mass spectrometry, improved atomic frequency standards, and quantum computing.

This makes ion traps more suitable for the study of light interactions with single atomic systems.

The two most popular types of ion traps are the Penning trap, which forms a potential via a combination of static electric and magnetic fields, and the Paul trap which forms a potential via a combination of static and oscillating electric fields.

[2] Penning traps can be used for precise magnetic measurements in spectroscopy.

This may lead to a trapped ion quantum computer[3] and has already been used to create the world's most accurate atomic clocks.

The physical principles of ion traps were first explored by F. M. Penning (1894–1953), who observed that electrons released by the cathode of an ionization vacuum gauge follow a long cycloidal path to the anode in the presence of a sufficiently strong magnetic field.

[6] A scheme for confining charged particles in three dimensions without the use of magnetic fields was developed by W. Paul based on his work with quadrupole mass spectrometers.

[7] The ion trap must be delicately adjusted for maximum brightness.

[8][9] Any charged particle, such as an ion, feels a force from an electric or magnetic field.

Ion motion and confinement in the trap is generally divided into axial and radial components, which are typically addressed separately by different fields.

In both Paul and Penning traps, axial ion motion is confined by a static electric field.

Paul traps use an oscillating electric field to confine the ion radially and Penning traps generate radial confinement with a static magnetic field.

and amplitude proportional to the electric field strength and is confined radially.

, it can be shown that Since the electric field is given by the gradient of the potential, we get that Defining

-plane are a simplified form of the Mathieu equation, A standard configuration for a Penning trap consists of a ring electrode and two end caps.

However, as expected from Earnshaw's theorem, the static electric potential is not sufficient to trap an ion in all three dimensions.

To provide the radial confinement, a strong axial magnetic field is applied.

For the radial electric field produced by the electrodes in a Penning trap, the drift velocity will precess around the axial direction with some frequency

[16] Penning traps are well suited for measurements of the properties of ions and stable charged subatomic particles.

Precision studies of the electron magnetic moment by Dehmelt and others are an important topic in modern physics.

Paul traps are commonly used as components of a mass spectrometer.

The invention of the 3D quadrupole ion trap itself is attributed to Wolfgang Paul who shared the Nobel Prize in Physics in 1989 for this work.

Ions are trapped in the space between these three electrodes by the oscillating and static electric fields.

A static applied voltage results in a radial logarithmic potential between the electrodes.

[14] In a Kingdon trap there is no potential minimum to store the ions; however, they are stored with a finite angular momentum about the central wire and the applied electric field in the device allows for the stability of the ion trajectories.

[22] In 1981, Knight introduced a modified outer electrode that included an axial quadrupole term that confines the ions on the trap axis.

[23] The dynamic Kingdon trap has an additional AC voltage that uses strong defocusing to permanently store charged particles.

Though the idea has been suggested and computer simulations performed[25] neither the Kingdon nor the Knight configurations were reported to produce mass spectra, as the simulations indicated mass resolving power would be problematic.

Some experimental work towards developing quantum computers use trapped ions.

Units of quantum information called qubits are stored in stable electronic states of each ion, and quantum information can be processed and transferred through the collective quantized motion of the ions, interacting by the Coulomb force.

An ion trap, used for precision measurements of radium ions, inside a vacuum chamber. View ports surrounding the chamber allow laser light to be directed into the trap.
Trapped ion with axes of motion. The ion is shown with the radial confining electrodes of a linear Paul trap . Axial motion (red arrow) is parallel to the radial electrodes and radial motion takes place in the plane given by the green arrows. In a Paul trap, axial motion is confined by a static field and radial motion by the oscillating field. In a Penning trap, axial motion is confined by the static electric field and radial motion is confined by the static magnetic field.
The radial trajectory of an ion in a Penning trap; the ratio of cyclotron frequency to magnetron frequency is .
A linear ion trap component of a mass spectrometer
FTICR mass spectrometer – an example of a Penning trap instrument
Schematic diagram of ion trap mass spectrometer with an electrospray ionization (ESI) source and Paul ion trap.
Partial cross-section of Orbitrap mass analyzer – an example of a Kingdon trap.