Particle Trap

I

Introduction

Particle Trap, device that confines charged or neutral elementary particles (the particles that make up atoms) or atoms away from containing walls. Because of their extremely small size—about 1 angstrom, or 1 ten-billionth of a meter—atoms and elementary particles have both classical and quantum characteristics, meaning they behave according to the laws of both classical mechanics and quantum mechanics (see Quantum Theory). Trapping atoms and elementary particles demonstrates their existence and facilitates study of them. See also Field (physics); Electromagnetic Radiation; Atom.

Particle traps use a variety of methods to trap particles, depending on the nature of the particle. Neutral particles (with no net charge) behave differently than particles with an electric charge. Regions of space affected by electricity or magnetism (called electric or magnetic fields) can change the path of a charged particle, but they have a much weaker effect on neutral particles. Therefore, the design of traps for neutral particles is quite different from that of traps for charged particles.

II

Neutral Particle Traps

Neutral particle traps use either magnetic fields or electromagnetic radiation (radiating energy, such as light or infrared radiation) to trap particles. A trap that uses a magnetic field is called a magnetic trap. Two types of traps use electromagnetic radiation. The radiation trap, or magneto-optic trap, uses the tiny forces generated by photons (packets of electromagnetic energy) hitting atoms to collect the atoms in a trap. The dipole trap, or laser trap, uses a laser (a beam of electromagnetic radiation) to make an atom susceptible to an electric field, which then holds the atom in the trap.

A magnetic trap consists of a spherical magnetic field set up by two magnets. Atoms are like tiny, weak magnets, with a north pole and a south pole. This magnetism defines a property of the atom called its magnetic dipole moment. Depending on this moment, the magnetic energy level of an atom will either increase or decrease with the strength of a magnetic field that surrounds it. Only atoms with magnetic energies that decrease as the strength of the magnetic field surrounding them decreases get trapped in magnetic traps. (Atoms with magnetic energies that increase as the strength of the field decreases are not trapped.) Atoms seek the lowest magnetic energy state possible, so atoms with the appropriate magnetic dipole moment will seek a region in which the magnetic field is weak. The weakest magnetic field within the magnetic trap is in the center of the trap, so the atoms congregate in the center of the trap. The stronger magnetic fields around them keep them from escaping.

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Radiation or magneto-optic traps use lasers—beams of light in which all of the electromagnetic waves that make up the light have the same orientation, with all the troughs and crests of the waves aligned—to herd atoms into the center of a trap. First, atoms are exposed to a magnetic field that separates them into two groups by their magnetic properties. These properties determine to which energies of laser light the atoms will react. Though the laser’s electromagnetic radiation has no mass, it does have a speed (the speed of light), and this speed, combined with the radiation’s energy, gives the radiation a momentum that it can transfer to objects that it strikes. This momentum is called radiation pressure. The radiation pressure of a system of lasers can herd atoms into a trap and keep them there.

Laser or dipole traps use a laser with a specific frequency to turn atoms into dipoles—objects with a negatively-charged end and a positively-charged end. Depending on the laser that the trap uses and the initial state of the atoms, the energy of these dipoles either decreases as an electric field gets stronger or decreases as an electric field gets weaker. The atoms will seek a low energy level, and so they will move to either the strongest or weakest part of the electric field. In a trap with dipoles whose energy decreases as the strength of the electric field decreases, a second laser beam with a hole in it will set up an electric field with a weak region in the center in which the atoms will be trapped. In a trap designed for atoms whose energies decrease as the strength of an electric field increases, a focused laser beam will provide a small, strong electric field in which the atoms will be trapped.

III

Charged Particle Traps

Particle traps for charged particles use electric or magnetic fields to control the motion of particles. A moving charged particle will change its direction in an electric field or magnetic field if its initial motion is not aligned with the field. Some examples of simple traps are electrostatic traps and magnetostatic traps.

Electrostatic traps consist of a thin wire with an electric charge. A particle with an opposite charge will be attracted to the wire, but its initial motion will cause it to spiral around the wire instead of hitting it.

Magnetostatic traps use a magnetic field to contain particles. The magnetic fields in the most common magnetostatic traps are ring-shaped or egg-shaped. A charged particle moving perpendicular to the direction of a magnetic field will move in a circle, while a charged particle moving parallel to the direction of a magnetic field will be unaffected by it. Most particles are not moving exactly perpendicular or exactly parallel to the field when they enter the trap, so inside the trap they move in a spiral around the magnetic field lines while maintaining their direction of motion parallel to the magnetic field. In a ring-shaped magnetic field, the field makes the particle the move around the ring in an even spiral. The earliest ring-shaped magnetostatic trap was the cyclotron, a type of particle accelerator. In an egg-shaped magetostatic trap, the field makes the particle move in tighter and tighter spirals down to one end of the trap, then reflect back in gradually bigger spirals to the widest part of the trap. It will continue on in smaller and smaller spirals to the opposite end of the trap and reflect back again.

Two more complicated types of charged particle traps are the Paul and Penning traps. The Paul trap uses oscillating electric fields (fields that change in strength in a periodic way) to hold particles in place. The Penning trap uses a stable electric field and a stable magnetic field to hold particles.

German physicist Wolfgang Paul developed the Paul trap in the 1950s. The Paul trap uses a ring-shaped electrode around its center and two smaller end-cap electrodes, one above and one below the ring electrode. All three electrodes are connected in an electrical circuit. An alternating current—an electric current that varies cyclically over time—is applied to the ring electrode, setting up an oscillating electric field inside the trap. The oscillation is at just the right frequency so that charged particles in the trap always experience a push from the electric field toward the center of the trap. The electric field is weakest at the center, so particles that reach the center tend to stay there. The Paul trap is very good at holding single particles or small numbers of particles and can hold a single particle almost completely still.

German American physicist Hans Dehmelt developed the Penning trap in the late 1950s. His trap used a vacuum discharge gauge developed by Dutch scientist Frans Michael Penning, hence the name. The Penning trap has the same arrangement of electrodes as the Paul trap, but the Penning trap uses direct current—electric current that is constant over time—and also includes a magnetic field, aligned with the vertical axis of the electrodes. The electric field that the current sets up is enough to keep a particle in the center of the trap steady, but it is not enough to bring the particle to the center or to hold it there against other forces. The magnetic field stabilizes the trap by making the particles in the trap adopt a complicated oscillating circular motion in the center of the trap. The particles gradually lose energy and come near rest at the center of the trap. The Penning trap can be used for storing thousands of particles or for bringing a single particle to rest.

IV

Using Particle Traps

Scientists use particle traps to store particles, to cool particles, and to provide tiny samples of particles that can be studied for long periods of time. A scientist may use a particle trap to store particles to be used in a particle accelerator or for other experiments. If a trap initially contains a large number of particles, collisions between the particles can be thought of as giving the sample its temperature. If scientists use the trap to evaporate some of the particles by allowing them to escape from the trap, fewer collisions occur and the temperature of the remaining sample will decrease. Scientists have been able to reach temperatures very near absolute zero (-273° C, -459° F) by cooling the traps with liquid helium and evaporating most of the initial sample of particles.

Scientists can observe trapped particles by analyzing the light they scatter. A trapped atom will absorb incident light and reradiate it hundreds of millions of times each second. Each atom has some probability of jumping into a long-lived unreceptive state, so it will “blink off” occasionally and appear dark. If the whole trap appears dark, it is evidence that a single atom is in the trap, since it is extremely unlikely that two or more atoms would go dark at the same time.

Particle traps have allowed much more accurate measurements of the properties of many particles, especially the electron. They have also facilitated the study of antimatter (matter made up of particles that are, in a sense, mirror images of the particles that make up normal matter). Penning traps have allowed scientists to trap and study antiprotons and to build an antimatter atom, anithydrogen. The stability of ions (electrically charged atoms) in particle traps has produced atomic clocks with a precision about 10,000 times better than that of the cesium clocks currently used. The traps provide the possibility for building an atomic clock with an accuracy 100 million times as great as the current cesium clocks.