During my Ph. D. work at Tata Institute of Fundamental Research, my basic aim has been to find out the impact of surface roughness on energetic ion emission from intense laser produced solid plasmas. In contrast to the existing conventional particle accelerators, laser produced plasma have come up with the potential of developing efficient energetic ion sources at smaller dimensions. Although it is observed that under appropriate conditions short pulse laser based accelerators can give rise to high energy charged particle beams but to use them on routine basis operation the efficiency of these methods have to be improved. One of the prime challenges in this aspect is the efficient coupling laser energy into the plasma.
The surface structures are known to enhance the laser energy coupling up to 90% when compared to a conventional polished target. With the recent developments in the production of nanostructures with high quality and characteristic reproducibility, they can form the basis of the efficient coupling of laser energy into the plasma on a routine basis. The work reported in my thesis presents pathways toward exploiting these surface modulations appropriately towards achieving an efficient photon and charged particle sources depending on the user requirement.
During my thesis work, I have designed and implemented a versatile experimental set up easily adaptable to various challenges faced in this field of research. The conventional and structured surfaces are kept side by side ensuring identical experimental conditions. The X-ray energy was measured by a calibrated NaI(Tl) detector while the ejected high energy ions were measured by a channel electron multiplier in proportional mode, exploiting conventional ion arrival time measurement technique. To estimate the spatial divergence of the laser beam, four annular Faraday cups were employed.
The thesis work identifies the key mechanisms responsible for the enhanced laser energy coupling, namely, i) surface plasmon resonance and ii) "lightning rod" effect. It demonstrates that the surface plasmon resonance can be exploited to channelize major part of the absorbed laser energy to energetic ions emitted from the plasma. Whereas, the "lightning rod" factor, yields brighter harder X-ray sources with low energy (ion) debris.