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Plasma studies

Plasma produced during laser ablation and sputtering of the different optoelectronic materials are characterized which helps to find the plume conditions under which good quality thin films are formed in sputtering and pulsed laser ablation (PLA). Our plasma studies include the estimation of basic plasma parameters such as electron/ion density, electron temperature, time of flight (TOF) and the relative population studies of different species present in the plasma. Estimation of these parameters is important for understanding the energy transport in plasma. We use optical emission spectroscopy (OES) and Langmuir (single, triple and emissive) probes for plasma studies.

The set-up for imaging of the plasma emission spectra at various spatial positions of the plume and recording of the temporal evolution [time of flight (TOF)] of specific emissions from the plume are shown schematically in fig 1. The light flux was collected by the optic fibre bundle, which can be moved along the length of the image of the plume using X-Y translator. The other end of the fibre bundle abuts at the entrance slit of the monochromator. The wavelength dispersed spectra was imaged either by the CCD or PMT, the former aligned laterally and the later axially to the exit slit of the monochromator. The swing mirror near the exit slit allows the wavelength dispersed light to pass either to the PMT or CCD. The monochromator, PMT and CCD are controlled by software, (Spectramax for Windows).

The experimental set-up for collecting ion and electron TOF transients in laser ablated plasma plume is schematically shown in the figure 2. The metallic tip of the Langmuir probe is positioned perpendicular to the propagation direction of the plume. The arrangement allows the recording of TOF transients along the propagation direction of the plume, for various laser fluences and vacuum/ambient gas pressures. The Langmuir probe assembly consists of a Tungsten wire, 125 μm in diameter and 5 mm in length, supported by a glass sleeve.

The RF plasma is characterized by both Langmuir probe and OES techniques The Langmuir probe technique has some fundamental difference to that of laser plasma, owing to the difference in the plasma equilibrium and due to the presence of magnetic field and RF electric field inherent in the RF magnetron plasma. Ordinary tungsten fiber, as used in LIP cannot be used here as the probe current recorded will be erroneous due to RF interference. This can be rectified by using an RF compensated probe which filters out the perturbation imposed by the RF frequency, 13.56 MHz and may be its harmonic (27.1 MHz) from the collected probe current. Spectroscopic methods of Plasma study is employed on liquid phase pulsed laser ablation [LP-PLA] in which a solid target is immersed in a liquid medium and the laser beam is focused through the liquid onto the target surface. It gives us an insight of how nanoparticles are formed when cavitation bubbles expanded due to confinement of plasma generated in the solid-liquid interface in LP-PLA. Transmission of optical windows in tokomak gets severely attenuated by the deposition of impurity particles like C and Fe from the plasma .Cleaning of optical windows is essential to improve the optical transmission characteristics of tokomak windows for determining the plasma parameters by means of spectroscopic methods. A proper optics for scanning the laser beam along the whole area through the backside of the contaminated tokomak window is developed for cleaning it without breaking tokomak vacuum. This is automized by monitoring the plasma produced during the ablation of film deposited in the window by laser induced breakdown spectroscopy [LIBS] using lab view.

Plasmas are conductive assemblies of charged particles, neutrals and fields that exhibit collective effects. Further, plasmas carry electrical currents and generate magnetic fields. We use optical emission spectroscopy (OES) and Langmuir probe for plasma studies. Investigations have been carried out on plasma produced during laser ablation and sputtering of the different optoelectronic materials which helps to find the plasma conditions under which good quality thin films are formed in sputtering and pulsed laser deposition (PLD). Our plasma studies include the estimation of basic plasma parameters such as electron/ion density, electron temperature, time of flight (TOF) spectra of different ionic species and the relative population studies of different species present in the plasma. Estimation of these parameters are important for understanding the energy transport in plasma. RF plasma