Pre-ionization controlled laser plasma formation for ignition applications

We present initial modeling and experimental results on the use of pre-ionizing pulses to enhance and control laser plasma formation in gases. The approaches are based on use of an initial pulse to achieve pre-ionization with a second pulse for generation of additional ionization and energy addition. Experimental results using a pair of nanosecond duration 1064 nm pulses are presented. In the configuration studied, the first pulse provides breakdown which also promotes additional energy deposition by the second pulse. Maximum energy deposition occurs when the inter-pulse separation is approximately 20 - 40 ns. Modeling results examine a case where the first pulse provides pre-ionization (but not breakdown) and shows that in the presence of pre-ionization, the effect of a second pulse is to increase ionization and heat the gas. Non-intrusive microwave diagnostics for ionization measurements are also presented.