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Experiments investigating the pulse‐width dependence of thermal x‐ray conversion efficiencies (hν≪1.5 keV) in laser‐heated gold plasmas are described. The results show that the instantaneous ratio of the emitted x‐ray flux to the laser energy deposition rate increases throughout a 4‐ns laser pulse. The studies were carried out using single arms of the 10‐beam Nova laser facility. The irradiance was ∼4–5×1014 W/cm2 in the target plane as we varied the laser pulses’ FWHM from 2 to 4 ns. The laser pulses were nominally flat‐topped and contained between 1 and 2 kJ of (1)/(3) μm light. Time‐resolved plasma radiation was monitored with a broadband, streaked x‐ray spectrograph that has a roughly 30‐ps time resolution and channels that are roughly 100–150 eV wide. One‐dimensional numerical models run with the lasnex code produce a conversion efficiency that is nearly constant throughout the laser pulse. We discuss various approximations made in the one‐dimensional models and conclude that none of them are a likely explanations for the increase of conversion efficiency with time. A preliminary two‐dimensional model of a disk heated by a 3‐ns pulse shows that the conversion efficiency increases throughout the pulse. The increase is due to soft x rays emitted from outside the laser spot. Further experiments and modeling will be carried out to assess these two‐dimensional effects.