K-α emission from femtosecond laser pulse irradiated targets is an intense short-pulse line source well-matched for X-ray diagnostic techniques with subpicosecond and micrometer resolution. The purpose of our numerical study is to propose suitable target design and appropriate laser intensity and pulse shape so that both high efficiency of laser energy transformation into K-α emission, and subpicosecond duration of the emitted x-ray pulse can be reached. Several emitting materials will be investigated in order to asses x-ray source in various spectral regions. Multilayer target design can enable to distinguish between radiation generated in corona and in cold matter clearly without significant decrease of emitted energy.