Ion gun

"I will need the weapon back once your work here is completed. It is, after all, only on loan to you, temporarily."

- Kvasir

Asgard energy weapons are the primary weapons used by vessels of the Asgard fleet.

Overview
While these weapons are significantly superior to traditional Goa'uld weapons and shields, lesser forms of Asgard weaponry are inferior to some Ancient shields: Anubis was able to upgrade Goa'uld shields using Ancient knowledge he obtained as a result of his ascension, rendering Asgard weapons considerably less effective. Ori motherships proved similarly capable of withstanding weapons fire from an Asgard O'Neill-class ship. These weapons have, however, proven quite effective against Replicator ships which have just emerged from hyperspace, before their shields could be raised.

History
The technology used in these weapons was a closely guarded secret by the Asgard as they declined to equip any Earth ship. Shortly before their extinction, however, the Asgard changed their minds and equipped the Odyssey with numerous upgrades, including Asgard plasma beam weapons. These weapons have proven to be considerably more powerful than any of their previous offerings, being able to destroy Ori motherships, Aurora-class warships, and Wraith hive-ships with only a handful of shots.

The new beam weapons are being installed on all other Daedalus-class ships, starting with the Apollo and the Daedalus herself. With these new weapons, the two ships launched an assault against the warships the Asurans were deploying in the Pegasus Galaxy, causing them to retreat to Asuras. The Daedalus, the Apollo, seven Wraith Hive-Ships, and five Travelers ships later launched an assault against Asuras with the objective of destroying the Asurans for good, which they succeeded in.

The Asgard scientist Kvasir also allowed the SGC to borrow an energy weapon after a string of alternate SG-1s began coming through the Stargate from parallel universes, but only after they promised to return it. The borrowed energy weapon was powerful enough to pierce quantum realities.