Carter had been part of the team at Area 51 that had developed the Mark IX, also known as the "gatebuster." A naquadria-enhanced nuclear warhead capable of a multi-gigaton detonation, the Mark IX was designed solely for the purpose of vaporizing stargates and anything else in a 100-mile radius, and attempts to disable it, physically, electronically, or otherwise, result in immediate detonation.
SG-1 brought one of the devices aboard the Prometheus during the mission to Kallana after word was received that Kallana's stargate had remained active for more than two days as a force field expanded across the planet's surface. Their intention was to destroy the stargate in order to sever the connection and destroy the force field.
Mitchell and Carter used a pair of keys to simultaneously activate the warhead in standby mode before beaming it down to the planet's surface. Once on the planet, Mitchell switched the device to mode two and informed the Prior on the planet that the weapon would be detonated if the stargate was not disengaged within 30 "Earth minutes." The Mark IX's fuse is a wee bit twitchy in mode two, and when ha'tak vessels under the command of Gerak began firing on the force field, Mitchell reset the device to mode one and the team was extracted.
As the Mark IX detonated, the force field entered another expansion phase sufficient to envelop the entire planet. By providing almost 70% of the power needed to complete the force field, SG-1 had unwittingly played into the hands of the Ori, and the planet collapsed into a micro-singularity intended as a beachhead for invasion.
The Mark IX gatebuster was later used to sever the connection between the second Ori supergate and the black hole in the Pegasus Galaxy. Daedalus beamed the warhead directly behind the event horizon of the stargate in the Pegasus Galaxy so that the force wouldn't be absorbed by the active wormhole but the gate would be destroyed and the wormhole connection severed.
Cross Reference: Area 51, Kallana, Naquadria, Stargate
Episode Reference: Beachhead, The Shroud