On the stability of strong double layers

For a strong, stationary double layer to be maintained, the electron flux Φ e and the ion flux Φ i must satisfy the Langmuir condition Φ e /Φ i = √ m i / m e . However, this condition alone does not determine whether or not the double layer will remain in stable equilibrium at a given position. A new stability condition is obtained and the results of several laboratory double layer experiments are examined using this condition.