Preheating suppression for high-density compression by CO2 laser

In 10-μm wavelength laser fusion studies the reduction of target core preheating by hot electrons is considered to be the most crucial problem in high-density compression. The ability of a CO2 laser to achieve high-density compression with a new pellet structure scheme, Cannonball, is demonstrated. The high-energy tail of the electron distribution, which induces preheating, is observed to be reduced. Experiments on various target types, including Cannonball of one-dimensional or spherical shape, have been performed by using two beams of the LEKKO VIII CO2 laser system. The energy distribution of hot electrons which penetrate into the fuel pellet has been measured by Ksub(α) emissions from various coated layers on the pellet. The results show that the high-energy tail is truncated in the case of a Cannonball target, where the inner sphere is irradiated by lasers. Together with features such as high absorption, good uniformity and high hydrodynamic efficiency, this target configuration indicates the possibility of high compression by a CO2 laser. (author)