Measurements of hohlraum-produced fast ions

Abstract

We report the first fast ion measurements in indirect-drive experiments, which were taken on OMEGA hohlraum and halfraum shots using simple filtered CR-39, a nuclear track detector, and a charged-particle spectrometer. Protons are observed in two energy regimes that are associated with different fast ion production mechanisms. In the first, resonance absorption at the hohlraum wall early in the laser pulse accelerates runaway electrons. In the second, fast electrons are produced with high energy from the two-plasmon decay instability in the exploding laser entrance hole, or from stimulated Raman scattering in the underdense gas fill. In both cases, the runaway electrons set up a strong electrostatic field that accelerates the measured ions. The former mechanism is observed to have an energy conversion efficiency ∼(0.6-4)×10-4 into fast protons depending on the hohlraum and drive. The latter mechanism has an estimated conversion efficiency from the main drive of ∼(0.5-2)×10-5 depending on the assumptions made.