LLE-Annual report input

Abstract

In FY18, an LLNL/GA/Michigan University team carried out the project “Exploring pair plasmas and their applications” with 2 LBS shot days on OMEGA EP. The campaign used an EP short pulse beam to produce jets of electron-positron antimatter pairs while the 2nd short pulse beam was used to producing protons to diagnose the laser-target interactions. The experiments focused on 1) inhensing the pair yield using nano-structure targets and 2) collimating/focusing positron jets through curved sheath potential from shaped target surface. The experiments successfully demonstrated both the yield enhensement and the positron focusing. The data provided basis futrue experiments on labortary astrophysics using the pair jet – plasma interaction to drive beam instabilities. A total of 14 shots were performed in the fiscal year. The EP short pulse beams (~1 kJ in 10 ps) irradiated targets of 1 mm thick Au targets, with and without the nano-structure on the laser interaction surface. It was found that for the same laser energy, positron yields and acceleration both were increased dramatically by using nano-structure. This finding is important to the future experiments and applications using laser pair jets. Previous experiments used gold targets almost exclusively. It was also found that a hemispherical shaped backsurface can focuing the positron jets through the sheath field – like that observed in proton focusing. This observation is important as it demonstrated that higher pair density can be achieved by reducing their volume, which may enable the development of instablities predicted by theory. The prior experiments showed that quasi-monoenergetic relativistic positron jets are formed during high-intensity irradiation of thick gold targets, and that these jets can be strongly collimated using the magnetized-inertial fusion electrical delivery system (MIFEDS). The external field produces a 40-fold increase in the peak positron and electron signal [4]. The positron yield was found to scale as the square of the laser energy. The FY15 results revealed another dimension of scaling on the target materials. The favorable scaling would enable the laboratory study of relativistic pair plasmas that are important to understanding some of the most exotic and energetic systems in the universe.