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A beam dump is a device that absorbs a beam. This may be a beam of photons ("light") such as a laser beam, or a beam of electrically charged particles.

An optical beam dump is an optical element used to absorb a beam of light. For low power systems, this can be as simple as a piece of black velvet glued onto a stiff backing, but higher power beam dumps must often be designed carefully to avoid back-reflection, overheating, or excessive noise.

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.

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 powers of the beams to be absorbed can run into the megawatts.

Materials used for such beam dumps include blocks of copper, aluminum, 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 dirt serves as a radiation shield.

The most challenging beam dump design to date is that of the Large Hadron Collider, which must dissipate 725 MJ of beam energy in the 90 μs circulation time, which equates to a power of 8 TW.