The prefire chamber was a component used in Federation-issued handheld phaser rifles, ship-mounted phaser cannons and phaser arrays.
In phaser rifles, energy flows from the prefire chamber to the emission aperture. These, along with the rapid nadion pulse, beam control assembly, safety interlock, beam width intensity controls, energy cell usage and discharge crystal were all investigated by Geordi La Forge and Data on an apparently fraudulent, Romulan-made, Starfleet-issued phaser rifle in late 2367. Upon further investigation of the wave pattern on the emission beam, they reviewed the pulse frequency out of prefire chamber and the initial output spike. (TNG: "The Mind's Eye")
By adjusting the prefire chamber of the USS Enterprise-D, in early 2368, La Forge was able to give the phasers the focus they needed to disable the heavily shielded polarity coil generator, in use aboard a Tamarian starship, in as few shots as possible. This task was completed in "a few hours." (TNG: "Darmok")
Temperature issues could also arise in the prefire chamber. In 2372, Captain Kathryn Janeway requested that Commander Chakotay coordinate with the phaser maintenance crew, aboard USS Voyager, to "[s]ee if they solved that problem with the prefire chamber temperature." (VOY: "Resolutions")
In attempt to increase the phaser relays efficiency, Worf and Nog downloaded a new operating system into the USS Defiant's computer core in 2373. After re-initializing the interface program, they charged the prefire chamber and recalibrated the emitter stage, but found the relay efficiency continued to read steady with the same throughput of 148.7. With a lack of the desired results, Worf had Nog return to the engine room and re-download the operating system, only this time one file at a time. (DS9: "Soldiers of the Empire")