Dependence of the transition from Townsend to glow discharge on   secondary emission
  In a recent paper Sijacic and Ebert have systematically studied the transition from Townsend to glow discharge, refering to older work from von Engel (1934) up to Raizer (1991), and they stated a strong dependendence on secondary emission $\gamma$ from the cathode. We here show that the earlier results of von Engel and Raizer on the small current expansion about the Townsend limit actually are the limit of small $\gamma$ of the new expression; and that for larger $\gamma$ the old and the new results vary by no more than a factor of 2. We discuss the $\gamma$-dependence of the transition which is rather strong for short gaps. 
Yu.P. Raizer, Ute Ebert and D.D. Sijacic