The role of metastable states in critical ionization velocity discharges

Despite the possible fast formation of a hot electron tail distribution due to plasma‐wave interaction, ionization in Alfvén's critical ionization velocity (CIV) process has to come from electron impact ionization. Excited states may play an important role in this process. By solving coupled rate equations numerically, we have examined the time evolution of ionization in a CIV process, finding that metastable states of some of the gases contribute substantially to the rapidity of the process. The metastable states serve the function of storing energy from collisions between low‐energy electrons and ground state neutrals. Subsequent collisions between the metastable states with low‐energy electrons lead to pooling of the energy, such that ionization can occur rapidly. Competing processes are evaluated for their effect in quenching CIV.