Plasma Gas Identification Using the Single Langmuir Probe

Abstract

A novel method to identify mass composition of plasmas was discovered at Polytechnic University of Puerto Rico Mirror Cusp (PUPR-MC) plasma machine using a Single Langmuir probe characteristic. This is a comparative method similar to a mass spectrometer. Argon, nitrogen, hydrogen and residual gas (no gas injected) characteristics can be compared using the Single Langmuir probe. Different gases will produce different plasma parameters. Using the mass spectrometer we can identify the elements present in the chamber and relate probe characteristic curve to its respective element. Each gas has its own characteristic appearance. By looking at the shape of the curve it is possible to identify the must abundant gas in the plasma chamber. Plasma parameters may vary due to a number of factors. Probe position, microwave power, magnetic confinement, gas and pressure are factors that can be controlled. In order to compare gases, using a Microwave source at 198 W to heat the gas and initiate ionization, a coil current of 390 A operating in mirror mode was used. It means that the current flows in the same direction through the two parallel solenoid magnets (coils) used to confine plasma. Distance between coils was fixed at 60 cm. At Mirror mode, plasma is confined to a hot electron ring produced between the coils. The ring contains the highest plasma density in the chamber; therefore the center remains at a lower density. The probe was immersed into plasma at 110cm from the flange, the collector (disk) was placed at 14cm from the center of the chamber, and 18.12cm apart from the electron ring. Low power (4% of 5KW microwave generator) was used to protector probes from burning at close range. The pressure chamber was 2.9x10-4 Torr. Averages of plasma parameters for each gas were obtained using a Single Langmuir probe. After studying results, every curve showed the respective characteristic for each gas. Without a mass spectrometer, it was possible to identify a gas used to produce plasma with only the I-V obtained from a Single Langmuir probe.