Bibliothèque d'Applications

A radio frequency quadrupole ion trap utilizes a radio frequency quadrupole (RFQ) in order to trap an ion beam. A continuous high energy ion beam is first cooled and converted into a spatially confined bunch, which can then be released with minimal heating of the beam. The device is also commonly referred to as a Paul trap where an oscillating AC voltage is used to contain the beam radially. The axial confinement of the particles is achieved by using a DC voltage along the length of the quadrupole designed to create a potential well that can contain the beam.

The RFQ is often used to guide particle beams with kinetic energies in the order of 10-100 eV. In order to efficiently trap these particles, the kinetic energies are reduced through a process called buffer-gas cooling. The high energy charged beam is made to pass through a buffer gas, often consisting of a low molecular mass inert gas such as He or Ar. The highly energetic ions undergo elastic collisions with the light and slow moving buffer gas molecules, thus reducing the overall kinetic energy of the particle beam. The RF electric fields help contain the collisional scattering in the radial direction.

The device can be operated in two configurations: trapping and extraction. In the trapping configuration, the axial DC voltage contains a minima which helps in trapping the cooled beam. The cooled beam is therefore collected near the minima as a bunch of particles. This bunch can then be extracted by altering the DC voltage to help guide the beam out of the quadrupole. This two-stage operation thus allows for a continuous beam to be converted to a pulsed beam.