Numerical simulation of thermonuclear flash with isobaric ignitor for laser spherical targets with different dimensions and temperatures are performed and the target energy gain as a function of ignitor parameters is obtained. As a result the requirements for ignitor parameters are elaborated under which the spark ignition provides an essential increase in the gain as compared with volume ignition. Calculations are carried out with the use of TERA code based upon self-consistent solution for system of kinetic equations for thermonuclear particles and radiation, obtained by the Monte Carlo method, and of hydrodynamic equations.