Electromagnetic Characterization of Energetic Ions Produced by Pulsed Laser Ablation of Solid Targets

Abstract

Laser Ion Source (LIS) is a promising candidate to be used as a pre-injector in ion accelerators and also as an ion implanter due to many advantages over established ion sources like electron beam ion source (EBIS) and electron cyclotron resonance ion source (ECRIS). LIS is still under development and its various aspects require further investigation. Important features of ions emitted from laser produced plasma such as distribution of ion energy, charge state and intensity are not well characterized. Moreover Influence of magnetic field over laser produced plasma and emitted ions, is a less investigated area. Main objective of this research work was to characterization highly charged ions produced by pulsed laser ablation of technologically important metals like Ti, Cu and W. Here the results of the series of experiments that were conducted to characterize nanosecond pulsed laser produced metal plasmas and emitted highly charged ions are reported. Nanosecond pulsed Nd:YAG laser (1064 nm wavelength) and KrF laser (248 nm wavelength) were used to irradiate Ti, Cu and W targets. Laser intensity at the surface of target was varied from 108 to 1010 W/cm2 . Laser produced plasma and emitted ions were characterized by using Ion Collector (IC), electrostatic Ion Energy Analyzer (IEA) working in time of flight configuration and Intensified Charge Coupled Device (ICCD). The charge state of Ti ions was found to increase with increase in laser fluence and maximum available ions charge in the investigated range of fluence was Ti4+. A correlation between the intensities of various ion charge states indicated that higher charge states of ions were most probably produced through stepwise ionization xxvii mechanism. In addition, energy distribution of ion charge states Tin+ (n=1-4) was in the range of 0.36-3.0 keV and the most probable ion energy was found to increase linearly with ion charge state. The estimated equivalent potential was about 310 V at the laser fluence of 20.3 J/cm2 . Mentioned results were found in conformity with the predictions of electrostatic model of ion acceleration in laser produced plasma. The axial magnetic field applied on the expanding Cu plasma revealed that the integrated ion yield, highest ion charge state, average ion energy and energy of individual ion charge states were amplified. The average ion charge, equivalent potential, electron temperature, electron density, Debye length and transient electric field were estimated from the experimental results obtained without and with application of the magnetic field. The increase of ion charge state ion yield on application of magnetic field are most probably due to the electron trapping in front of the target surface, which boosts up the electron impact ionization process. In addition, ablation rate of Cu was also increased in presence of externally applied magnetic field. The energy increment of ions on application of magnetic field is discussed in the frame work of electrostatic model for ion acceleration in laser plasma. The laser ablation of tungsten revealed that the number of available Wn+ ions charge states increases from 1+ to 6+ when laser fluence at the target was varied in the range of 3.0-19.4 J/cm2 . With the application of 0.23 T axial magnetic field at the target surface, the number of available Wn+ ions charge states increases from 1+ to 10+ in the similar range of the laser fluence. In addition, molecular oxygen ions O2 n+ were also observed when magnetic field is applied at higher values of the laser fluence. The Coulomb-Boltzmann-shifted time function was used to determine intensities and peak energies of ions of various charge states from the IC spectra. xxviii Depending on the charge state, the energies of various charge state ions were in the range of 0.6-2.7 keV. The experimental data were exploited to estimate the equivalent accelerating potential developed inside the plume. It is found that the equivalent accelerating potential inside plasma plume increases linearly with laser fluence. The comparison with literature data revealed that the equivalent accelerating potential also increases with atomic number of the target material. It might be due to the reason that the target material of high electron density allows to add more electrons in the plasma, resultantly electron density of plasma and accelerating potential increases because it depends on the densities of separated electrons and ions clouds. In this research work charge, intensity and energy distribution of important metal ions and their dependence on laser and target parameters were investigated. Basic plasma parameters were also extracted and underlying mechanism of ion acceleration within the laser plasma plume is studied. Magnetic field was found to be very effective in enhancing the charge state and energy of ions without increasing the laser power. It is also proved to be very useful in reducing the angular spread of ion energies. All these findings can contribute towards further development of LIS for various applications as well as in other fields such as Pulsed Laser Deposition (PLD), laser assisted surface modifications,