ALUMINUM-COATED OPTICAL FIBERS AS EFFICIENT INFRARED TIMING FIDUCIAL PHOTOCATHODES FOR SYNCHRONIZING X-RAY STREAK CAMERAS

The timing fiducial system at the Nova Two-Beam Facility allows time-resolved x-ray and optical streak camera data from laser-produced plasmas to be synchronized to within 30 ps. In this system, an A1-coated optical fiber is inserted into an aperture in the cathode plate of each streak camera. The coating acts as a photocathode for a low-energy pulse of 1-omega (gamma = 1.054-mu-m) light which is synchronized to the main Nova beam. The use of the fundamental (1-omega) for this fiducial pulse has been found to offer significant advantages over the use of the 2-omega second harmonic (gamma = 0.53-mu-m). These advantages include brighter signals, greater reliability, and a higher relative damage threshold, allowing routine use without fiber replacement. The operation of the system is described, and experimental data and interpretations are discussed which suggest that the electron production in the A1 film is due to thermionic emission. The results of detailed numerical simulations of the relevant thermal processes, undertaken to model the response of the coated fiber to 1-omega laser pulses, are also presented, which gave qualitative agreement with experimental data. Quantitative discrepancies between the modeling results and the experimental data are discussed, and suggestions for further research are given.