In the far field of the intensity distribution of the beam delivered by a two-stage transient–collisional excitation X-ray laser (XRL), a non-expected interference pattern that is stable from shot to shot has been discovered. It is demonstrated that the interference is caused by the emergence of an imaginary source in the amplifying plasma, which is phase matched to the radiation of the generator. The observed phenomenon is called an X-ray coherent mirage. To explain the obtained results, a new theoretical approach is developed. The basic essential conditions for formation of the X-ray mirage are formulated, and possible applications are discussed. This paper details the experiments, including the formulation of the necessary and sufficient conditions for formation of the X-ray mirage, and possible applications are discussed.

Effects of an implosion nonuniformity with [ell ] = 1 ([ell ]: Legendre polynomial mode number) on the hot spark formation were investigated in a series of direct-drive implosion experiments at the Gekko-Xll glass laser (Yamanaka et al., 1987). The implosion dynamics and the performance from the early to final stage of the implosion were observed with a variety of X-ray imaging and neutron diagnostics. A drive nonuniformity in the implosion with [ell ] = 1 was observed in the shape of the accelerated target at the early stage of the implosion. At the final stage of the implosion, the resultant nonuniformity with [ell ] = 1 was also observed as a geometrical shift of core plasmas from the center of the chamber. The observed neutron yield and X-ray emission properties at the final stage of the implosion were significantly degraded with an increase of the implosion nonuniformity with [ell ] = 1. The experimental results were compared with one-dimensional (1-D) and two-dimensional (2-D) hydrodynamic simulations. As a result, it was found that the implosion nonuniformity with [ell ] = 1 shifts the whole implosion dynamics towards its direction and prevents the confinement of the gas fuel considerably. However, the experimentally observed degradation in the hot spark formation, such as reductions in neutron yield and features in X-ray emission, can be reproduced in 2-D simulations not with an asymmetric perturbation of [ell ] = 1 only but with multimode nonuniformities such as [ell ] = 1 coupled with some additional middle-mode ones (e.g., [ell ] = 6). Such a complex spike structure caused by the multimode nonuniformities was found to be essential for the experimentally observed rapid cooling of the hot spark.