Waveguide admittance for radiation into plasma layers--Theory and experiment

Measured admittance of an
X
-band waveguide radiating into dielectric and plasma layers is compared with variationally computed admittance figures. The waveguide admittance measurements for polystyrene sheets of varying thicknesses compared closely with calculations. The admittance is computed first by assuming the principal waveguide mode, and second by using a superposition of sine and shifted cosine waves as trial functions for aperture fields. Although the aperture fields are shown to differ significantly in the two cases, the computed admittance data are nearly the same because of the stationary character of the admittance expression. The admittance measurements for plasma layers are made during the diffusion controlled afterglow of a pulsed discharge contained in a bell-shaped vessel. Thin Teflon sheets are used to confine the plasma layers to an approximate size of
5 \times 5 \times \frac{3}{4}
inches. The electron density of the plasma profile is measured by Langmuir probes at various times during the afterglow. The plasma density was measured over approximately 70 percent of the thickness of the plasma layers, and was found to be nearly constant in this range which excluded regions near the boundary. Numerical solutions of the equations for ambipolar diffusion during the afterglow in a rectangular geometry show that the lateral plasma density variations are negligible in the vicinity of the waveguide. The measured admittance and plasma density data are shown to agree with calculations made for homogeneous plasma layers, if a correction is made for elevated ion temperatures for times of approximately 10 to
20 \mu
s following the discharge. Several computational models consider plasma stratifications near the boundary of the plasma layer but they do not improve the agreement between measurement and calculations.