Relativistic spherical plasma waves
  Tightly focused laser pulses as they diverge or converge in underdense plasma can generate wake waves, having local structures that are spherical waves. Here we report on theoretical study of relativistic spherical wake waves and their properties, including wave breaking. These waves may be suitable as particle injectors or as flying mirrors that both reflect and focus radiation, enabling unique X-ray sources and nonlinear QED phenomena. 
S. S. Bulanov, A. Maksimchuk, C. B. Schroeder, A. G. Zhidkov, E.   Esarey, and W. P. Leemans
physics.plasm-ph