Particle trap
A device used to confine charged or neutral particles where their interaction with the wall of a container must be avoided. Electrons or protons accelerated to energies as high as 1 teraelectronvolt (1012 electronvolts) are trapped in magnetic storage rings in high-energy collision studies. Other forms of magnetic bottles are designed to hold dense hot plasmas of hydrogen isotopes for nuclear fusion. At the other end of the energy spectrum, ion and atom traps can store isolated atomic systems at temperatures below 1 millikelvin. Other applications of particle traps include the storage of antimatter such as antiprotons and positrons (antielectrons) for high-energy collision studies or low-energy experiments. See Antimatter, Nuclear fusion, Particle accelerator, Plasma (physics), Positron
Charged-particle traps
Charged particles can be trapped in a variety of ways. An electrostatic (Kingdon) trap is formed from a thin charged wire. The ion is attracted to the wire, but its angular momentum causes it to spiral around the wire in a path with a low probability of hitting the wire.
A magnetostatic trap (magnetic bottle) is based on the fact that a charged particle with velocity perpendicular to the magnetic field lines travels in a circle, whereas a particle moving parallel to the field is unaffected by it. In general, the particle has velocity components both parallel and perpendicular to the field lines and moves in a helical spiral. In high-energy physics, accelerators and storage rings also use magnetic forces to guide and confine charged particles. A tokamak has magnetic field lines configured in the shape of a torus, confining particles in spiral orbits. This type of bottle is used to contain hot plasmas in nuclear fusion studies. Another type of bottle uses a magnetic mirror.
The radio-frequency Paul trap uses inhomogeneous radio-frequency electric fields to confine particles, forcing them to oscillate rapidly in the alternating field (see illustration). If the amplitude of oscillation (micromotion) is small compared to the trap dimensions, the trap may be thought of as increasing the (kinetic) energy of the particle in a manner that is a function of the particle position. The particle moves to the position of minimal energy and is therefore attracted to the center of the trap where the oscillating electric fields are weakest. At the center of the trap, the fields are exactly zero, and a single, cold ion or electron trapped there is essentially at rest with almost no micromotion.
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The Penning trap, with the same electrode configuration as the Paul trap, uses a combination of static electric and magnetic fields instead of oscillating electric fields.
Neutral-particle traps
Uncharged particles such as neutrons or atoms are manipulated by higher-order moments of the charge distribution such as the magnetic or electric dipole moments.
Magnetic traps of neutral particles use the fact that atoms usually have a magnetic dipole moment on which the gradient of a magnetic field exerts a force. The atom can be in a state whose magnetic energy increases or decreases with the field strength, depending on whether the moment is antiparallel or parallel to the field. A magnetic field cannot be constructed with a local maximum in a current-free region, but a local minimum is possible, allowing particles seeking a weak field to be trapped.
Laser traps use the strong electric fields of the laser beam to induce an electric dipole moment on the atom. A laser field tuned below the atomic resonance polarizes the atom in phase with the driving field; the instantaneous dipole moment points in the same direction as the field. Thus the energy of the atom is lowered if it is in a region of high laser intensity. The high-intensity trapping region is formed simply by focusing the beam of a laser. See Laser
Magnetooptic hybrid traps use, instead of the dipole forces induced by the laser field, the scattering force that arises when an atom absorbs photons. An inhomogeneous magnetic field separates the magnetic substates of an atom in a position-dependent manner. These states interact differently with circularly polarized light. It is possible to arrange a combination of laser beams with proper polarizations to create net scattering forces that drive the atom into the region of zero magnetic field. Such a trap requires much lower laser intensities and weaker magnetic fields. See Laser cooling
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