Emission characteristic of long laser pulse keV X-ray source

Multi-keV X-ray sources generated by irradiating thick solid target with a 2 ns-duration laser pulse are investigated on Shenguang II laser facility. With the long laser pulse, the significant three-dimensional expansion of the plasma leads to that the longitudinal scale length of the X-ray emission region is finally determined by the laser spot size and the emission volume is proportional to the third power of the laser spot size. Theoretical analysis shows that X-ray flux can be enhanced by increasing the laser spot size because in the steady state the X-ray flux is proportional to the five-third power of the laser spot size under the same laser power. The scaling law is in good agreement with the experimental data. It is also shown that short wavelength laser is suitable for the multi-keV X-ray sources. (authors)