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Measurement of preheat due to fast electrons in laser implosions

The emission of hard x-rays, coincident with 3(omega) L/2 emission, has been measured in laser-interaction experiments on the OMEGA laser system, indicating fast electrons due to the two-plasmon-decay instability. The purpose of the present experiment was to determine the preheat level due to these electrons. In a calibration experiment a flat target containing titanium and vanadium layers was irradiated and the preheat was determined using two signatures: the intensity of K(alpha) lines from the high-Z target layers and the spectrum of hard x-rays measured by an array of BaF2 scintillation detectors. The intensity of both signals can be directly related to the preheat energy; therefore, the two preheat measurements can be cross calibrated. This comparison lends support to the determination of preheat in spherical target experiments, where only the hard x-ray diagnostic is available. For CH shell implosions and square laser pulses the preheat energy deposited in the target amounts to approximately 0.1 percent of the laser energy. The maximum rise in temperature due to the preheat is approximately 10 eV. Finally, the temperature of fast electrons is deduced from the measured spectrum of MeV protons accelerated from the surface of the target; the result agree with the temperature deduced from hard x-rays, namely, T approximately 100 to 200 keV.