Chemistry Reference
In-Depth Information
To increase frequency the output frequency of these devices might be
doubled or tripled by driving further nonlinear solid-state elements.
2. Electron tube sources such as BWOs (backward wave oscillator), car-
cinotrons or klystrons.
3. Electrically or optically pumped gas lasers.
Semiconductor and electron tube sources are tunable in a range of at least a few
GHz. Therefore an offset of several GHz can readily be achieved making them
especially suitable for heterodyne systems with PLL circuits for high-frequency
stability [105,117].
6.2.5 W
AVE
T
RANSMISSION
The probing wave must be guided from the source to the plasma and to the detector,
antennas providing the interface between guided and free-space propagation at the
plasma column. The antennas used in plasma diagnostics are typically aperture anten-
nas whose size determines the beam pattern. They are designed to provide narrow
beam width of about (10-20)
◦
. The radiating structures are horns which are flared
sections of a waveguide to meet the free space characteristic impedance. Lenses
can be used to transform the divergent wave front of the horn into a plane wave
or conversely focus a plane wave to approximately a point. Alternatively elliptical
reflectors are used to reconcentrate power from one focal point of the ellipsoid where
the feed horn is mounted to the other focal point. They are the standard focusing ele-
ments in quasi-optical systems (Gaussian optics) which can be used with advantage
to form slim beams through the plasma [103,117,124,125]. The angular resolution or
the spot size obtained are limited by the wave nature of the radiation. At millimeter
wavelengths beam diameters are of the order of 1 cm.
In case distances are short between the source and the plasma, standard funda-
mental mode waveguides can be used. For larger distances oversized waveguides
are used instead to avoid resistive losses which for mm waves are typically 5 dB
per meter. Oversized waveguides have diameters at least 10 times the wave-
length. Taper sections are needed to connect them reflection free to standard size
waveguides.
6.2.6 F
INAL
R
EMARKS
The aim of this chapter on microwave interferometry was to make the reader
familiar with one of the most important plasma diagnostic system to determine
the line-integrated electron density. The basic physics behind was briefly reviewed
and an overview was given of the various types of mm-wave interferometers in use.
The experimental difficulties have been discussed and ways out have been shown.
The main microwave components are reviewed very briefly. References are included
where special designs are discussed in detail demonstrating the large variety of
possibilities of experimental realization.
