Spectroscopic measurements of target preheating on OMEGA

The preheating of laser-heated microballoon targets has been measured by ti me-resolved x-ray and extreme ultraviolet (euv) spectroscopy on the 30 kJ, 60-beam (maxima), 351 nm, laser-fusion system at the University of Rocheste r Laboratory for Laser Energetics. Thin coatings of aluminum overcoated wit h magnesium served as indicators. Both the sequence of the x-ray line emiss ion and the intensity of euv radiation were used to determine a preheating peaking at similar to 10 nsec prior to onset of the main laser pulse, with a power density less than or similar to 1% of the main pulse. The measureme nts are supported by numerical modeling. Further information is provided by absorption spectra from the aluminum coating, backlighted by continuum fro m the heated surface. The exact source of the preheating energy remains unk nown at present, but most likely arrives from early laser leakage through t he system. The present target diagnostic is particularly useful when all be ams cannot be monitored directly at all laser wavelengths. (C) 2000 America n Institute of Physics. [S1070-664X(00)01804-8].