Measurement of Preheat due to Fast Electrons in Laser Implosions

LLE Review, Volume 82 63 Introduction Fast electrons due to the two-plasmon-decay (2PD) instability have been measured in previous laser-interaction experiments.1,2 The main impact of these electrons in laser-fusion experiments is the possible preheat that can reduce the implosion’s effectiveness. The preheat caused by these electrons is studied by measuring the Kα line emission from highZ layers in a flat-target geometry on the OMEGA3 laser system. The Kα emission is directly related to the preheat level;4,5 for sufficiently high fast-electron temperatures (Tfast) the relationship between the Kα intensity and the preheat level is independent of Tfast. The preheat due to fast electrons in UV laser irradiation is relatively small and is usually masked by the preheat due to radiation. To overcome this problem, a target containing titanium (Ti) and vanadium (V) layers was designed so that when irradiated from the Ti side, most of the Ti-Kα is excited by radiation, whereas most of the V-Kα is excited by the fast electrons. As seen below, the thick Ti and V layers required for the Kα measurement precluded conducting this experiment in spherical geometry. We show, however, that such measurements can be used as a reference point for hard x-ray continuum detectors, which can then be used to determine the preheat in undoped spherical targets. The Kα measurements can conveniently be used as a reference point because in the case of x-ray continuum measurements Tfast must be known to determine the preheat level even at high temperatures. Measurement of Preheat due to Fast Electrons in Laser Implosions