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The annular confinement beam (sometimes shortened to confinement beam) was an essential component of the transporter system. Generated by the primary energizing coils, the annular confinement beam confined the transporter matter stream when it was on its way to the target destination.

When deflector shields were active, an annular confinement beam could not penetrate the shield envelope, making transport almost impossible, except through the expert use of sensor windows. (TNG: "The Wounded")

The annular confinement beam also made it possible to transport objects and persons at warp, although the velocities of both vessel and target needed to be identical. (TNG: "The Best of Both Worlds")

In the event of an annular confinement beam failure or malfunction, the object in transport would suffer pattern degradation, causing severe injury and even death in the case of living creatures. Commander Sonak was killed in such an incident in the 2270s. (Star Trek: The Motion Picture)

The annular confinement beam could be intentionally breached by the transport subject during the initial stages of transport if their bodies contained certain material compounds. However, doing so resulted in a massive energy discharge as the beam failed, and could be lethal. (TNG: "The Hunted")

In 2368, Miles O'Brien was able to boost the confinement beam in order to beam down one person to the surface of Mab-Bu VI's moon that was ravaged by heavy electrical storms. Geordi La Forge estimated the chances for success at 50:50. (TNG: "Power Play")

The only time that the annular confinement beam was disengaged was when a transport in progress was suspected to contain some explosives or other dangerous material which were not allowed on board a ship. The targeting scanners were then reset to a point in space, the transporter matter stream was rerouted to that point, and the annular confinement beam was disengaged, dispersing the material harmlessly into space.

In 2369, Miles O'Brien tried boosting the annular confinement field in order to retrieve a wounded Klingon who was trying to beam aboard Deep Space 9. He eventually succeeded. (DS9: "Dramatis Personae")

In 2371, a microscopic singularity explosion in the Sol system caused the USS Defiant's annular confinement beam to fluctuate. (DS9: "Past Tense, Part I")

That same year, after the crew of the USS Voyager observed unusual photonic activity in a protostar, they attempted to beam samples into two containers for analysis, but only one of them filled. Captain Kathryn Janeway traced the problem to a slight breach in the annular confinement beam and after it was fixed, the procedure was attempted again and both containers filled. Later, it was discovered that the breach caused the energy to leak into the transporter system. (VOY: "Heroes and Demons")

In 2374, by narrowing the confinement beam, Harry Kim was able to transport Tuvok and Seven of Nine from aboard the Hirogen ship that was being pulled into a black hole. (VOY: "Hunters")

According to the Star Trek Encyclopedia, 4th ed., vol. 1, p. 9, this system can be properly referred to by its abbreviation ACB.