Extrapolator force field

The Extrapolator Force Feild was created when The Doctor converted the Tribophysical waveform macro-kinetic extrapolator into a force feild to protect the TARDIS from the Dalek Fleet. Captain Jack Harkness then linked it to the Game Station's defences to protect the top six floors with the feild. This forced the Dalkes to enter at floor 494. (DW: The Parting of the Ways)