Automation Of A Microwave Source For The Diagnostic Of Electron Density In Plasma Using Labviewtm

Abstract

This project is devoted towards developing a plasma diagnostic technique based on microwave sensor for the purpose of real time monitoring of electron density in low temperature plasmas. The technique is based on resonance of a small quarter wavelength antenna placed inside the plasma whose resonance frequency is highly sensitive to the dielectric constant of the plasma surrounding the probe tips. The heart of the technique is to finding the resonance frequency. This is achieved using a signal source from a voltage controlled YIG-oscillator that provides variable output frequency in the range of 2-18 GHz. The resonance detection circuit comprises of a directional coupler (Model No- CB7-20) and Schottkey Diode positioned between the YIG oscillator and the sensor. This circuit measures the corresponding reflected power in the co-axial line and displays it in the form of voltage amplitude in an oscilloscope. The resonance frequency is obtained by when a sharp fall in the reflected power is clearly observed at the resonance frequency. The project involves complete automation of the voltage controlled oscillator and developing a robust peak identification technique for the detection of resonance peak against the background noise. Based on particular application in the plasma, two important versions have been developed in the project. This includes software for (1) the time average mode for steady-state plasmas and (2) the time-resolved model using NI DAC applicable to periodic variation of electron density. The specific software developments includes interfaces between PC and instruments namely; Tektronix TDS3034C Oscilloscope, Signal Source Tektronix Model No AFG3102, National Instrument DAQ USB 6008. The important milestones achieved during this project is the (1) calibration of the microwave source (2) development of the algorithm for noise rejection in the signal (3) Analogue sweeping technique based on ramp signal obtained from a signal generator. (4) Analysis of the waveform to find the resonance peaks and (5) Online display of the electron density in continuous ( time-averaged) mode.