Plume Dynamics and Radiation Emission from Laser Plasmas

Abstract

Laser ablated Plasma is one of the best modern and richest sources of radiations for material processing. Time resolved, time integrated (space resolved) Plume Dynamics has been investigated for the radiation emission under different pressures of air ~10-3 torr, 5mbar and 760 torr. Two different Nd: YAG laser were tightly focused on the transient 4N, annealed metallic targets (2cm x 2cm x 0.2 cm). CCD (Monochrome AVC 301 A) and ICCD (5760/ IR- UV MO 595310) based computer controlled image grabbing systems were employed for capturing plasma plume photographs. Ions, Electrons, X-Rays and Extreme Ultra Violet (EUV) were detected by Solid State Nuclear Track Detectors (SSNTDs) and Faraday Cups, Langmuir Probe, modified BPX 65 PIN Photodiode alongwith Ag filter 2 μm in thickness and SXUV-HS 5 Mo/S respectively. Signals thus obtained were stored on 4 channels, 1GHz YOKOGUAWA (DL9140) and two channel 200 MHz UNI T (UTT) digital storage oscilloscopes. Plume expansion is non-uniform as initially, the plume is spherical later on it is sharpened. Plasma intensity from copper, platinum and gold plumes is same i.e. 250 Arbitrary Units (A.U.) whereas it is 190 (A.U.) for silver and 180 (A.U.) for zinc plume. The lengths of plumes from targets range from 120 pixels to 900 pixels when delay time ranges from 530 ns to 2500 ns. During the initial stage of plume dynamics, a dark nucleus with a sharp boundary is formed which remains up to 75 ns. Plasma expands at a higher rate during this stage. A shock wave is seen at higher pressure (1 atmosphere) that remained progressing up to 500 ns. Expansion of plume is faster in radial direction than in axial direction. The plume dynamics depend on laser intensity. Ion emission study using SSNTDs shows that maximum ions flux (4.62 x 107cm–2) is for Pt whereas minimum ions flux (1.08x107 cm-2) is from Ag ions. The energy of ions emitted from Pt is largest (6.91Kev) and minimum value emitted from Ag plasma (0.88 K eV). Ion emission investigations using Faraday Cups reveals that with the increase in atomic number Z (29-79), the ion energy increases (47.61 KeV – 149.8 KeV) with the exception that Ions emitted from plasma of Platinum (Z = 78) have higher energy (149.8KeV) than those emitted from gold (z = 79). Ion flux here also (at all angles) increases with the increase in the atomic number. Electron Emission using Langmuir Probe shows that there is a small variation in the values of probe current for applied voltage -30 V to 10 V for metals used. Probe current due to Cd, Ag and Au plasmas is higher than other targets (Cu, Zn and Pt). Electron Temperature is maximum for Cu and minimum for Zinc. A valley of depth about 3 x1017 m-3 for electron density is present in the -ve potential region. There is only a small change in electron density, plasma frequency for metals having large surface binding energy (Cu, Zn and Pt), while the metals having small values of surface binding energy electron density decreases with increase in biasing voltage. Debye’s length for the metals Cd, Ag and Au is smaller as compared to metals Cu, Zn and Pt. The Debye’s length of metals with large surface binding energy is larger and metals with small surface binding energy shows opposite behavior. The numbers of particles in Debye’s length for metals Cd, Ag and Au remain same for all biasing voltages while for metals Cu, Zn and Pt the number of particles varies with biasing voltage. Debye’s length of metals Cu, Zn & Pt decrease with increase in biasing voltage. The number of particles in Debye length decreases for metals having large value of surface binding energy with increase in biasing voltage Investigations on Extreme Ultra Violet Radiation (EUV) Emission using Photodiode depict that minimum value of the peak voltage of EUV (1.0223mV) is emitted from Cd plasma and its maximum value (2.326mV) is emitted from Zn plasma. The intensity of EUV has maximum value 1.5 watt /m2 for Zinc and minimum value 5.04 watt/m2 and remaining targets platinum, gold, silver, copper and cadmium have values in between respectively. X-Rays Emission from LIP using PIN Photo Diode indicates that maximum value of peak voltage for X-Rays signals is 66.18 mV for silver and minimum value is 9.75mV for gold plasma. The intensity of X-Rays is maximum for silver (574.43 watt /m2) and minimum value for zinc (1.938 watt/m2). So Zinc is the best source for EUV whereas Silver is the best source of X-rays.