Inductivelycoupled plasmas (ICP) in the high electron density regime of the order of 10 13 cm −3 are generated and their electron density characteristics are investigated by the terahertz timedomain spectroscopy (THzTDS) method. In this experiment, the plasma was produced by RF (13.56 MHz) with an applied RF power of 300-550 W and the argon gas pressure was in the range of 0.3-1.1 Torr. We generated the THz wave by focusing a femtosecond laser pulse in air with a DC electric field. As a plasma diagnostic tool, the THzTDS method is found to successfully provide the plasma density information in the highdensity regime, where other available plasma diagnostic tools are very limited. In addition, the analytical model based on the ambipolar diffusion equation is compared with experimental observations to explain the behavior of the electron density in the ICP source, where the plasma density is shown to be related to the applied RF power and gas pressure. The analytical result from the model is found to be in good agreement with the THzTDS result.
Self-focusing a laser beam in collisional plasma is investigated under the weak relativistic-ponderomotive nonlinearity. In this case, the plasma equilibrium density is modified and it causes generation of the nonlinearity due to the Ohmic heating of electrons, collisions, and the weak relativistic-ponderomotive force during the interaction of the laser beam with the plasma. Our theoretical and simulation results show that a significant nonlinearity in laser self-focusing can occur under the weak relativistic-ponderomotive regime for some appropriate simulation parameters
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