Studies of laser wakefield structures and electron acceleration in underdense plasmas

Experiments on electron acceleration and optical diagnostics of laser wakes were performed on the HERCULES facility in a wide range of laser and plasma parameters. Using frequency domain holography we demonstrated single shot visualization of individual plasma waves, produced by 40 TW, 30 fs laser pulses focused to the intensity of 1019 W cm2 onto a supersonic He gas jet with plasma densities ne < 1019 cm-3. These holographic "snapshots" capture the variation in shape of the plasma wave with distance behind the driver, and resolve wave front curvature seen previously only in simulations. High-energy quasimonoenergetic electron beams were generated using plasma density in the range 1.5× 1019 ne 3.5× 1019 cm-3. These experiments demonstrated that the energy, charge, divergence, and pointing stability of the beam can be controlled by changing ne, and that higher electron energies and more stable beams are produced for lower densities. An optimized quasimonoenergetic beam of over 300 MeV and 10 mrad angular divergence is demonstrated at a plasma density of ne ≃1.5× 1019 cm-3. The resultant relativistic electron beams have been used to perform photo-fission of U238 with a record high reaction yields of ∼3× 105 J. The results of initial experiments on electron acceleration at 70 TW are discussed.