Recent literature: https://aip.scitation.org/doi/abs/10.1063/1.5142006 Review of Scientific Instruments 91, 034704 (2020); https://doi.org/10.1063/1.5142006  J. Cameron Pouncey and  Jane M. Lehr Not so recent http://www.intusoft.com/nlhtm/nl50.htm#SPARKGAPMODELING