The Goa'uld and other alien races have used a variety of force fields as barriers, restraints, and as a means of weapons deflection and personal protection.
Goa'uld vessels, including the ha'tak, alkesh, and teltac, are equipped with defense shields which offer protection from the weapons fire of other ships. These shields, powered by crystal technology, invisibly envelope the vessel and typically keep objects out but not in, allowing departing death gliders to pass through the shield. SG-1 first encountered the deflection capability of the ha'tak defense shields when Apophis's motherships were poised to destroy Earth, and his shields easily withstood the energy of a prototype naquadah-enhanced warhead. SG-1 was able to penetrate the shields protecting Anubis's mothership, however, by using a short controlled burst of the hyperdrive engines on the F-302 to enter subspace just long enough to bypass the shield and reemerge on the inside. Asgard ships are also protected by defense shields, even more powerful than those of the Goa'uld. The SGC has incorporated this technology in its own vessels, including the Prometheus and the Odyssey. A version of this technology was also used when Odyssey became trapped in a time dilation bubble and a protective field enveloped Teal'c as the ship around him was destroyed and sent backwards through time.
The Goa'uld personal force field shield is activated by a wrist device and forms a glowing yellow/orange barrier around an individual. It is unique in that it has a deflection capability that is directly proportional to the amount of kinetic energy directed against it. An individual protected by such a shield is able to fire a weapon, such as a ribbon device or staff weapon, out through the barrier. However, a weapon directed against it, such as a bullet, staff weapon, or zat'nik'tel, is deflected, whereas slower weapons such as an arrow, dart, or knife can penetrate it. SG-1 has encountered a number of Goa'uld System Lords, including Apophis, Heru'ur, Seth, Sokar, Cronus, Marduk, and Osiris, who have used the personal force field shield. Bounty hunter Odai Ventrell also used a similar personal shield activated by a wrist device. Force fields have been used as security barriers as well. The stargates on Erebus and Tartarus are protected by a Goa'uld force field which acts much like an iris, preventing anyone from entering or exiting the gate unless a deactivation code is transmitted first.
A Tok'ra modification of the personal force field shield was used in an attempt to capture a Kull Warrior. Designed to keep objects in rather than out, the shield was intended to trap the Warrior but to allow penetration by a tranquilizer dart. The shield was ineffective against the Kull Warrior, however, and he was able to easily pass through it.
A related type of technology acts as a two-way barrier and is completely invisible until there is an attempt to penetrate it, at which time it glows a crackled blue and takes on the characteristics of a solid wall. The force shield or energy barrier operates on a frequency oscillation principle and does not allow penetration by objects or weapons of any kind in either direction, however, while wearing the Atanik armbands, SG-1 was able to move fast enough to be able to see the oscillation intervals and to pass through the barrier. First encountered on Hathor's planet, this type of force shield is used most often to defend a perimeter or to surround prisoners. The SGC has acquired this technology and has used a variation of the bluish Tok'ra force field both as a one-way shield for detaining prisoners such as Khalek, and as a two-way shield such as the one created by Carter to restrict the range of Merlin's phase-shift technology, a barrier which protected her as she was drawn into an alternate universe.
A version of the same technology, glowing red instead of blue, was also used by Aris Boch and Malikai. Often prisoners are detained by a similar invisible wall or white beam that produces an electrical shock when touched. Morgan Le Fay used a similar force field of Ancient design as one of several safeguards to ensure that the Sangreal would be claimed by someone of virtue. A treasure chest in the forest acted as bait and activated a one-way force field trap designed to capture the greedy and release the charitable.
The protective shields of the Ori are tremendously powerful. Priors use personal protective force fields that weapons cannot penetrate. Adria herself was protected by a necklace, a powerful pendant that held a piece of the holy city of Celestis and kept her safe. Her personal shield glowed blue when force was used against it, and she was even able to expand her shield to protect others, as she did against the fire of the Sangreal dragon. Ori warships use a force field of extraordinary power. Not until the Odyssey acquired the advanced technology of the Asgard was any weapon effective against it. When activated, the shield also prevents the transport beam from getting a lock on a target within the field.
On Kallana, the Ori used their technology to establish a force field dome around the area of the stargate, but rather than for protection, this force field was designed to collapse the planet into a micro-singularity. Approximately every 30 minutes, the field would expand in predictable increments lasting about 16 seconds each, during which time the force field could be penetrated by a transport beam. Inside the dome, the environment was extremely toxic, and the field fed upon weapons fire until the interior had turned into superheated plasma and the force field enveloped the planet, allowing the Ori to establish a beachhead.
Other races have also used force fields as protective barriers. The planet P3X-289 had been rendered unlivable by an environmental disaster, however a force field created an artificial dome that acted as a barrier against the toxic atmosphere while allowing solid matter to pass through. A small chamber on Praclarush Taonas had also once been protected by a dome-like force field that was eventually covered by molten rock. Anubis himself required a special force shield, in the form of a fluid black mask, to contain his essence as he existed between mortal existence and ascension.
First Introduced: The Nox
Significant Episode Reference: The Serpent's Lair, Into the Fire, Deadman Switch, Upgrades, Window of Opportunity, Full Circle, Fallen, Orpheus, Revisions, Evolution, Chimera, Lost City, Beachhead, Prototype, Counterstrike, The Quest, The Road Not Taken, Unending