Optimization of a laser-plasma x-ray source for contact x-ray microscopy

Presented are the detailed characteristics of a laser-plasma x-ray source that is successfully used for producing x-ray images of biological specimens in water by contact microscopy. The conversion efficiencies of laser energy to soft x rays are measured for various target materials using a 0.53 μm, 500 ps laser pulse. Yttrium target showed the highest conversion efficiency of around 2.5%/sr to the ‘‘water window’’ region x rays at 3×1012 W/cm2. Angular distributions of x rays and ions are studied to find the best angular position of specimen for reducing the target debris. The effective x-ray spectrum contributing to making an image of a specimen in water is discussed. It is shown that an extremely small laser could be employed in high-resolution contact microscopy.