Trace rare gas optical emission spectroscopy (TRG-OES) is carried out to investigate the excitation temperature, relative densities of active species (N, N2+) and nitrogen dissociation in inductively coupled helium admixed nitrogen plasma for different rf power (50, 100, 150 W), pressure (0.2 – 0.5 mbar) and helium percentage (10-90 %) using Ar as an actinometer (4 %). The excitation temperature is obtained from Boltzmann plot method using emission intensity of several argon lines. The dissociation of nitrogen has been investigated by both the actinometry method and the ratio of the atomic nitrogen line emission intensity at (746.83 nm) to the vibrational band (0-0) of the N2 second positive system at 337.1 nm. The excitation temperature increases with the increase in power and helium percentage and decreases with increase in fill pressure. The nitrogen dissociation as well as the relative densities of [ ] and [ ] increases with the increase in helium percentage. Optical emission spectroscopy and Langmuir probe are used to diagnose the low pressure inductively coupled Ar-N2 plasmas for different discharge parameters such as rf power (10-100W), filling pressure (0.02-0.4 mbar) and argon content (5-95%) in nitrogen discharge. Both diagnostic tools are used to obtain the plasma parameters including the excitation temperature, the density of active species in ground electronic state, dissociation fraction, electron temperature, electron number density and electron energy probability functions (EEPFs) in Ar-N2 plasmas. It is noticed that the actinometry is an efficient and reliable technique to calculate the densities of nitrogen species. It is also observed that the active species generation, dissociation fraction and electron temperature significantly depend on discharge parameters and may be used to optimize the plasma reactor. Mixture of single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs) are treated for different treatment time (0-120min) at optimum discharge conditions. Changes induced in the elemental composition, surface morphology, crystallographic structure, and structural disorder in the plasma irradiated CNTs are analyzed by EDX, FTIR, SEM, XRD and Raman spectroscopy, respectively. Ar-N2 mixture plasma treatment of CNTs leads to significantly increase the electrical conductivity, modify the microstructure and induce structural disorder and a transition of crystalline phase from well crystalline to an amorphous structure.