Plasma gas identification using the single Langmuir probe at the PUPR mirror-cusp

Abstract

Plasma can be produced by ionizing gas particles. Different gases will produce different plasma parameters. Using the single Langmuir probe, the characteristics of argon, nitrogen, hydrogen and residual gas (no gas injected) can be compared. Using a mass spectrometer, we can identify the elements present in the chamber and relate the probe characteristic curve with its respective elements. Each gas has its own characteristic appearance; by looking at the form of the curve it is possible to identify the gas producing the plasma. Plasma parameters may vary due to a number of factors; probe position, microwave power, magnetic confinement, gas and pressure are the factors that we can control. In order to compare gases, operation parameters must remain the same for all tests. Plasma tests were administered on different gases using a microwave power of 198 W to heat the gas and initiate ionization and a coil current of 390 A, while running in mirror mode; that is, the current flowing through two parallel solenoid magnets (coils) used to confine plasma runs in the same direction, making the magnetic field complementary in the direction of the probe. The coils, distance from center to center was fixed at 60 cm. In mirror mode, plasma is confined in a hot electron ring produced between the coils. The ring contains the highest plasma density in the chamber, but the center remains at lower density. The probe was submerged into the plasma 110 cm from the flange, the collector (disc) was 14 cm from the center of the chamber and 18.12 cm from the electron ring. Low power (4% of a 5 kW microwave generator) was used to protect the probes from burning at that close range. The pressure at the chamber was 2.9E−4 Torr. Good readings were taken and analyzed. Measurements with variable microwave power and variable magnetic field strength were also studied to determine whether there is a linear relationship between the machine setup and plasma conditions.