Nitrogen dissociation degree in a helicon plasma source inferred from microwave interferometry and langmuir probe cross measurements

摘要

Summary form only given. Using the compact helicon waves and instabilities experiment (CHEWIE) device we present a method of calculation of nitrogen dissociation degree in a helicon generated plasma by microwave interferometry and Langmuir probe measurements. The dependencies of nitrogen dissociation degree on external source parameters: input power, gas pressure and magnetic field strength are also presented. The nitrogen plasma is generated in CHEWIE helicon plasma source, which consists of a 12 cm long, 6 cm diameter Pyrex tube concentrically connected to a stainless steel expansion chamber, 30 cm long, 15 cm in diameter. The helicon antenna is a water cooled, 7 cm long Boswell saddle antenna that can handle up to 600 W of RF power over the frequency range 3 to 28 MHz. Helicon waves are generated with the help of an external magnetic field (up to 1200 G) from three electromagnets that surround the Pyrex tube. The radially averaged electron plasma density is determined by the phase shift of a 94 GHz microwave signal passing through the bulk plasma with respect to "plasma off" signal. The magnitude of total phase shift is a fraction of the wavelength. Instead of counting fringes we use the phase lock-loop functionality of the millimeter wave source to actively change the frequency during the "plasma on" time to match the interference signal of the "plasma off" time. With sufficient bandwidth in the feedback circuit this allows us to monitor any afterglow effects in the plasma in pulsed operating mode. Complementary measurements of the plasma density are accomplished with a RF compensated, cylindrical Langmuir probe. The probe consists of 0.5-mm-diam graphite rod shielded by an alumina tube. The effective collecting length of the tip is 3 mm. A series of RF filters are used to suppress interference from the helicon source in the 3-28 MHz range. Magnetic field effects on the probe characteristic are accounted by probe calibration through comparisons with microwave me-asurements in argon plasma