Influence of laser focal position on X-ray and ion emission of copper plasma

X-ray emission from copper plasma produced by a sub-nanosecond Nd:glass laser was studied as a function of distance of the target from the focus position. Optimization of soft (0.7-1.56 key) and hard (3.2-5 key) X-ray emissions as a function of the laser focal position was studied. In addition, a thallium acid phthalate (TAP) crystal spectrometer with spectral resolution of 30 mA was also developed to study variation in X-ray line emission in the spectral range of 1.291-1.610 key (7.7-9.6 angstrom) as a function of laser focal position. It is observed that the maximum soft X-ray emission is on either sides of the focus, indicating a dependence on plasma volume, whereas hard X-ray emission shows a single peak close to the 'best focus' position. The line X-ray emission intensity with respect to laser focal position also shows a double hump structure as in the case of soft X-ray emission. This indicates that the line emission is also a function of plasma volume. Scaling of X-ray yield with laser intensity has also been determined. Ion emission was also studied as a function of focal position variation. It is observed to match well with the trend shown by X-ray emission. (C) 2011 Elsevier B.V. All rights reserved.