Measurements of hot-electron temperature in laser-irradiated plasmas

In a recently published work [Yaakobi et al., Phys. Plasmas 19, 012704 (2012)] we reported on measuring the total energy of hot electrons produced by the interaction of a nanosecond laser with planar CH-coated molybdenum targets, using the Mo K-alpha emission. The temperature of the hot electrons in that work was determined by the high-energy bremsstrahlung [hard X-ray (HXR)] spectrum measured by a three-channel fluorescence-photomultiplier HXR detector (HXRD). In the present work, we replaced the HXRD with a nine-channel image-plate (IP)-based detector (HXIP). For the same conditions (irradiance of the order of 10(14) W/cm(2); 2-ns pulses), the measured temperatures are consistently lower than those measured by the HXRD (by a factor similar to 1.5 to 1.7). We supplemented this measurement with three experiments that measure the hot-electron temperature using K-alpha line-intensity ratios from high-Z target layers, independent of the HXR emission. These experiments yielded temperatures that were consistent with those measured by the HXIP. We showed that the thermal X-ray radiation must be included in the derivation of total energy in hot electrons (E-hot), and that this makes E-hot only weakly dependent on hot-electron temperature. For a given X-ray emission in the inertial confinement fusion compression experiments, this result would lead to a higher total energy in hot electrons, but preheating of the compressed fuel may be lower because of the reduced hot-electron range. Published by AIP Publishing.