Simplified representation of ionization-potential depression. (Top left) An ion in isolation. The solid concentric circles represent the electron energy levels of an ion. (Bottom left) In the atomic energy level diagram for an isolated ion, the electrons are still bound to the ion because their energies are below the energy of the continuum (dotted line). (Top right) Ions in high-density plasma. The closer spacing between circles corresponds to ionization-potential depression. The outer red circle represents the “ion sphere” used to model the extent over which the ion charge is neutralized. (Bottom right) In dense plasma, the ionization potential is reduced, and the energy level of the continuum is lowered, resulting in pressure ionization.