PRODUCTS

SPECTROSCOPIC DIAGNOSIS OF MICROWAVE PLASMA INTERACTION

The schematic of experimental set-up built at IPR (Gandhinagar, Gujarat) consists of plasma forming electrodes and optical diagnostic system. It consists of parallel-plate rectangular stainless-steel electrodes of 7.5 cm x 16.0 cm dimensions mounted in a cylindrical chamber of 40 cm diameter and 50 cm height. The plasma facing surface of the electrodes was masked with PVC dielectric sheet (225 mm thickness). The emitted light from the plasma was focused by a lens to an optical fiber, and fed to the entrance slit of a visible spectrograph fitted with CCD camera (Andor Technologies). The observed spectrum was stored in a PC for further analysis.

Experimental set-up for Microwave-Plasma Interaction

The Typical Optical Emission Spectrums of selected neutral Helium lines.

On varying the applied voltage at fixed operating frequency (~9.5 kHz), corresponding electron density and temperature was obtained as shown in figure above.

Left Image: Typical oscilloscope trace of detected microwave signal before and after the plasma operated at 4.5 kVp-p discharge voltage and ~9.5 kHz operating frequency.
Right Image: The variation of microwave signal profile as the discharge current varies after the plasma initiation. It is observed that as the discharge current rises and electron density builds-up in 10 the plasma, the microwave tends to attenuate.

Left Image: The applied voltage was varied and the microwave attenuation was measured at various voltages. The variation was plotted. It is predicted from the graph that the attenuation increases with the applied voltage. The increase in the microwave attenuation is due to the increased plasma density with the applied voltage. At higher electron plasma density, more electrons are available to attenuate microwaves through collisions.
Right Image: The variation of plasma density with the applied voltage is plotted.

The variation in microwave attenuation with electron-neutral collision frequency was plotted at various increasing applied voltages. It is predicted that the attenuation increases with the applied voltage at each collision frequency. The abrupt variation was observed in attenuation level with collision frequency.