Hard x-ray (8-100 keV) spectrum emission from plasma produced by femtosecond laser solid target interactions and Kα x-ray conversion efficiency have been studied as a function of laser intensity (1017 W/cm~2 ～10~(19) W/cm~2), pulse duration (70 fs ～400 fs), laser pulse fluence and laser wavelength (800 nm and 400 nm). The Ag Kα x-ray conversion efficiency produced by a laser pulse at 800 nm with an intensity I = 4 * 10~(18) W/cm~2 can reach 2x10-5. We discuss the behavior of Kα conversion efficiency scaling laws as a function of the laser parameters. We found that the Kα x-ray conversion efficiency is more dependent on laser fluence than on pulse duration or laser pulse intensity. The conversion efficiency exhibits a similar value at I ～ 1 * 10~(18) W/cm~2 when we work with a high contrast laser pulse at 400 nm or with a low contrast laser pulse at 800 nm, but in the first case it presents a higher scaling law. Consequently, the use of 400 nm laser pulses could be an effective method to optimize the Kα x-ray emission via vacuum heating mechanisms.
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