Debye Length

The distance over which a plasma screens out electric fields; below it, collective behaviour dominates.

The Debye length is the scale over which a plasma shields internal electric fields. Structures larger than it behave collectively as a plasma; it is a basic yardstick of plasma physics.

The Debye length is the scale below which a plasma's charges screen out electric fields; structures larger than it behave collectively as a plasma. It is tiny in a fusion plasma, which is why the plasma stays quasineutral almost everywhere and why the sheath at a wall is so thin.