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Abstract
The characterization of a new beam source of metastable argon atoms is reported in this dissertation. The source, the surfajet, is based on expanding a plasma sustained by electromagnetic surface waves in a quartz tube through a simple converging nozzle and extracting a beam from the supersonic free-expansion jet.
The 2.45 GHz surface waves are produced by a surfatron wave launcher designed to launch the fundamental transverse magnetic (TM) cylindrical mode in the plasma-dielectric waveguide. The waves travel along the interface between the plasma and the discharge tube (4 mm i.d., 6 mm o.d.) to a cooled supersonic nozzle located 10.5 cm from the launcher. The wave fields sustain the argon plasma column, whose length depends on the microwave power absorbed in the plasma up to a threshold value of power corresponding to the plasma extending to the nozzle exit. An increase of absorbed power beyond this threshold produces a linear rise in the gas temperature at the nozzle and excitation in the expansion jet. Vacuum ultraviolet spectroscopy of the jet indicates lack of significant Ar$\sb2\sp{*}$ excimer formation.
An antenna is used to measure axial profiles of the radial electric field intensity of the surface wave. The profiles indicate the presence of a reflected wave for powers exceeding the threshold. Axial profiles of the surface wave phase are also measured, from which electron density profiles are determined. For powers exceeding the threshold, a peak occurs in the electron density profile corresponding to the reflected wave.
The intensity and velocity distribution of metastables in the beam are determined for 190 $\mu$ and 350 $\mu$ diameter nozzles from time-of-flight measurements fitted to a model distribution. The beam velocity is found to be proportional to the square root of the source temperature, as predicted by the continuum treatment of the expansion. The beam thermal velocity decreases with the product of the source pressure and the nozzle diameter, with the lowest achieved parallel beam temperature being 23 K. The terminal parallel speed ratio increases with the pressure-diameter product and reaches a maximum of 8.5. The absolute intensity of metastables in the beam is determined through the calibration of the channeltron multiplier that serves as the particle detector. The maximum intensity of 6.2 $\times$ 10$\sp{14}$ s$\sp{-1}$ sr$\sp{-1}$ with a speed ratio of 4.1 is achieved for a discharge with a 13.9 sccm mass flow through a 350 $\mu$ diameter nozzle. The metastable intensity can be correlated to the absorbed power per unit mass flow; this is explained by a simple model of a recombining plasma in the expansion jet.
