
54 Electrical Discharge Lamps
contains argon to act as the buffer gas at a pressure of 3
–5
Torr. The role of the
buffer gas is to reduce electrode sputtering, prevent charged species from reach-
ing the walls, and provide an easier breakdown at a lower striking voltage.
Mixtures such as mercury and argon are called Penning mixtures due to the
proximity of one component’s metastable levels (argon’s metastable levels at
11.53eV and 11.72 eV) and the other’s ionization potential (mercury’s ionization
potential at 10.44 eV). Breakdown can occur at lower electric elds for Penning
mixtures due to the energy transfer processes that take place. Energy ...