Admissible values of the temperature Ts and the confinement parameter Hs = ρsRs, of a hot thermonuclear spark in DT and D2 fuels have been investigated. It is shown that it is not possible to achieve spark ignition of pure deuterium targets. Numerical simulations of precompressed DT and D2 microspheres (bare or tamped with a gold shell) have been performed and the values of the spark factor fs (ratio of thermonuclear energy gains that can be achieved in optimal core ignited and volume ignited targets) have been calculated. For a few milligrams of equimolar DT mixture the spark factor is shown to be 1.8-1.9, while for 10 mg D2 microspheres it is below 1.2.