The psi-field generator is a protoss device that replicates a psionic entity's ability to focus psionic energy, developed to replicate a small number of protoss warriors who could generate plasma shields in the Aeon of Strife. These devices allow an object to generate and maintain a plasma shield. Shields generated with such devices are vulnerable to electromagnetic effects.
Underwood, Peter, Bill Roper, Chris Metzen and Jeffrey Vaughn. StarCraft (Manual). Irvine, Calif.: Blizzard Entertainment, 1998