Measurements of Preformed Plasma Generation and Its Suppression Inside a Cone in a Cone-in-Shell Target for Fast Ignition

Abstract

We have investigated preformed plasma generation inside a cone on cone-in-shell targets for fast ignition scheme. We focused on the study of preformed plasma generation via irradiation by an implosion laser. In the GEKKO-XII laser system (Institute of Laser Engineering, Osaka University), a large fraction of laser energy is frequency-doubled to a 527 µm wavelength light with KDP crystals, but unconverted fundamental light (λ = 1.053 µm) is ultimately a possible heating source for the target. We measured the temperature at the inside tip of the cone in several experimental conditions to verify the effect of unconverted light. The experimental results indicated that the direct irradiation inside the cone with the unconverted fraction is the primary source of preheating effects. The results also suggest that preheating is suppressed by "long" cone targets whose sizes are typically larger than the focal points of incident unconverted light.