Positron Production in a Plasma Wakefield Accelerator

In a visit on Oct. 28, 1996 to the Center for Ultrafast Optical Studies at U. Michigan I had interesting conversations with Gerard Mourou and Don Umstadter about possible mechanisms for positron production in a plasma that is illuminated with a strong laser. I argue that positron production via a trident process (which includes Bethe-Heitler pair production) involving an isolated nucleus is possible only for extremely intense laser pulses (η > 5.74, as defined below). When lasers of much less intensity than this interact with a medium, a plasma is created and electrons can be accelerated by plasma wakefield effects, which permits positron production if electrons have been accelerated to (total) energy 3mc(1 + 0.029η)/ √ 1 + η2.