The interaction of an intense (> 1018 W/cm2)The interaction of an ultra-short high intensity laser pulse with solids results in a hot dense plasma, which is an excellent source of bright short-lived hard x-rays and energetic particles. A quasi-static charge-separation sheath electric field is generated by the collective displacement of a large number of laser generated hot electrons. The ions get accelerated, according to their charge-to-mass ratio, in such fields until charge neutrality is restored. Thus, the laser driven ion acceleration mechanism is a secondary process determined by the hot electron dynamics. Typically, in the laser-plasma based ion accelerator, the sheath electric field is of the order of TV/m.
Protons which have the highest q/m ratio among the ions experience the maximum acceleration. Although, the target material is not composed either of hydrogen or the carbon, the target surface impurities under standard experimental conditions result in efficient acceleration of these ionic species.
The sheath field is proportional to the square root of the product of the electron density and its temperature. So, the efficiency of the present ion acceleration scheme (Target Normal Sheath Acceleration) strongly depends on the enhanced hot electron generation. Typically, the interaction of an obliquely incident p-polarized intense short laser pulse couples 30-40% of its energy to the hot electrons and a few percent of the laser energy subsequently gets channelled to the ions. To have viable applications, the energy conversion efficiency to the ions needs to be increased substantially. In order to improve the ion acceleration efficiency either laser pulses of improved parameters are required or novel engineered targets needs to be used such that more laser energy coupling is possible into the hot electrons and hence to the ions. Nanometric or micrometric modulations (regular or random) on the target front surface, in which side the laser beam is incident, provide better laser energy absorption as well as hot electron generations. Hence, the use of structured targets can in turn increase the ion acceleration efficiency compared to the conventional flat targets under the same laser parameters.
In typical laser-solid interactions at laser intensity 1017 - 1019 W/cm2, a plethora of ionic charge states (H+, Cn+, On+ etc.) are produced and their energy distribution is also very broad ranging from eV to multi- MeV. A simple time-of-flight (TOF) technique is not adequate to measure the ion energy spectra due to its inherent limitation on the charge/mass resolution and thus, the TOF signal is very badly convoluted of signals from different ionic species and very little information can be discernable. Hence, a Thomson parabola ion spectrometer (TPIS) becomes an automatic choice to characterize such laser produced ion beams. A TPIS transforms/converts the ion energy to a spatial position in the perpendicular plane to its propagation direction. A position sensitive ion detector can record the ion tracks and the energy spectra can be determined very easily. Thus, such spectrometers are generally equipped with MCP, CR-39 or radio-chromic film (RCF) as the position sensitive detector. However, the MCP provides online measurements and hence suitable with the high repetition rate laser systems. [For further details, see Probing strong field ionization of solids with a Thomson parabola spectrometer, Malay Dalui, T Madhu Trivikram, Ram Gopal and M Krishnamurthy, Pramana -J. Phys, 82(1), 111-120 (2014).]