Beam dump

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search

A beam dump is a device designed to absorb the energy of photons or other particles within an energetic beam. Examples include absorption of laser emissions, and absorption of particles produced within particle accelerators.

Optical beam dumps[edit]

Optical beam dump suited for several watts of laser light

An optical beam dump is an optical element used to absorb a beam of light. Major design concerns in a beam dump typically include the management and reduction of back reflections and scattering as well as the dissipation of heat generated by absorption. For low-power systems and less demanding applications, the device can be as simple as a piece of black velvet or flock paper glued onto a stiff backing, but higher-power beam dumps must often incorporate more elaborate features to avoid back-reflection, overheating, or excessive noise. Dumping the beam with a simple flat surface may scatter unacceptably large amounts of light for some applications, even though the direct reflection may be effectively reduced. To minimize scattering, it is common to use deep, dark cavities lined with an absorbing material to dump the beam. A particularly simple and relatively inexpensive approach is to use a stack of razor blades with the sharp edges facing the beam, so that the spaces between the blades form very deep cavities from which little light escapes.

A commonly available type of beam dump suitable for most medium-power lasers is a cone of aluminum with greater diameter than the beam, anodized to a black color and enclosed in a canister with a black, ribbed interior. Only the point of the cone is exposed to the beam head-on; mostly, incoming light grazes the cone at an angle, which eases performance requirements. Any reflections from this black surface are then absorbed by the canister. The ribs both help to make light less likely to escape, and improve heat transfer to the surrounding air.

Extremely high-power beam dumps have been made using water with controlled amounts of colored salts (e.g., copper(II) sulfate) to give a moderate absorbance of the beam. The water is circulated through a long pipe with a window at one end, and chilled using a heat exchanger.

Charged-particle beam dumps[edit]

The purpose of a charged-particle beam dump is to safely absorb a beam of charged particles such as electrons, protons, nuclei, or ions. This is necessary when, for example, a circular particle accelerator has to be shut down. Dealing with the heat deposited can be an issue, since the energies of the beams to be absorbed can run into the megajoules.

Materials used for such beam dumps include blocks of copper, aluminium, carbon, beryllium, and tungsten. The block often has a long conical hole, where the beam hits it, so as to spread the heating over a larger region of the block. If the beam to be absorbed is high-powered, these blocks may be up to several meters long and water-cooled. Pools of mercury have also been used.

If the particles in the beam are energetic enough, induced radioactivity, production of neutrons by spalling, radiation embrittlement, and production of secondary particles can also be issues. Considerable quantities of material may be put around the beam dump, especially "downstream" of the beam dump, to serve as radiation shielding. Also, the beam may be carefully not aimed at anything critical (such as people or particle detectors), and angled down into the earth before it hits the beam dump, so that soil serves as a radiation shield.

The most challenging beam dump design to date is that of the Large Hadron Collider. Each of the two beam dumps, in case, for instance, of a dipole magnet quench, must be able to dissipate 362 MJ (approximately 100 kilowatt-hours, enough to power the average American home for over three days) of beam energy in the 90 μs circulation time, which equates to a power of 4 TW.