A scheme for obtaining high gain-length product in recombination x-ray laser by using a low density target

A scheme for obtaining high gain-length product(GL) of recombination x-ray laser is proposed and theoretically studied, in which a thin fiber with a density less than the critical one is driven by a short pulse laser. The features of laser-produced plasma and the gain coefficients of the transition from n = 3 to n = 2 of the H-like ions for a solid fiber and a low-density cylindrical target of carbon are shown. The Sobolev escape probability in a self-similarly expanding cylindrical geometry is used to evaluate the trapping effect on the gain coefficient. According to the simulations there are three obvious advantages of the low-density target, i.e., wider gain region, longer lasing duration and no drift of the gain region. In addition, only a few joules are needed for getting saturated GL value.