Measurements of a DC Gas Discharge

Abstract

To investigate properties of gas discharges and to evaluate the design and implementation of a Langmuir probe diagnostic, an array of several probes was designed and inserted into an intermediate pressure direct-current (DC) gas discharge. In particular, this work was motivated by: studying electron kinetics in gas discharge plasmas and measurement of the electron energy distribution function (EEDF). First, measurements of the discharge were performed including obtaining current-voltage (IV) characteristics and determining breakdown voltages for several gases. Images of discharges were recorded using a digital camera, and emission spectra were obtained using a visible light spectrometer. Then, Langmuir probe measurements were then taken of the potential distribution in the cathode region and IV characteristics were obtained to determine plasma properties. Electron density and temperature were determined both by a graphical method and by calculation of the EEDF from the second derivative of the IV characteristic. Probe measurements were taken at a vacuum pressure of 1Torr and an electrode separation of 5cm. The results from a single experiment are reported for which the discharge voltage was set to 276V and discharge current to 0.238mA. The plasma properties obtained graphically and by the EEDF calculation were compared and their interpretation discussed. The results of the graphical analysis method indicated a plasma density of 9e14m-3 and an electron temperature of 5.4eV. The results of the EEDF method indicated a plasma density of 1.17e14m-3 and an electron temperature of 11.0eV. The results were compared to simulations performed using the open-source Boltzmann equation solver called BOLSIG. The main input parameters were: average reduced field of 170Td, and degree of ionization of 1e-8. The output from the program indicated a mean energy of 7.65eV, which falls between the values obtained graphically and by the EEDF calculation.