The UC Davis Plasma Group possesses many well-characterized, multi-meter sized devices for use in microwave-plasma interaction studies. These devices are equipped with a number of diagnostic and viewing ports. High power microwave beam(s) are introduced to the plasma via metallic horns located at the end(s) of each device. Alternating rows of north and south permanent magnets that are located along the outside walls of each device producing a type of magnetic mirror. The device is filled with a low pressure (0.1-10 mTorr) gas such as Hydrogen, Helium, Argon or Xenon.
The plasma is created via a filament discharge process. Here, a filament basket is placed near one end of the plasma device. The thoriated tungsten filaments are heated until they become strong electron emitters. The filaments are then pulsed negative with respect to a grounded mesh located in close proximity to the basket, which causes electrons to be accelerated from the filaments to the grid. The high transparency of the grids ensures that the vast majority of electrons pass through the mesh into the field-free interior of the device.
Most of the high energy primary electrons
emitted from the filaments will collide with neutral gas molecules and
ionize them to create secondary electrons and ions. Some of these
secondary electrons may in turn have sufficient energy to ionize even more
neutrals. The cusp fields created by the multi-dipole magnets reflect most
of the electrons away from the chamber walls, greatly enhancing the
chances of ionizing a neutral molecule. Thus, a plasma consisting of
ions, electrons and neutrals is formed.
Studies will primarily take place in two unmagnetized plasma devices.
The larger, utilized in our previous stimulated Brillouin scattering,
optical mixing, and four-wave mixing studies, is shown schematically
in Fig. 1 together with some of the diagnostics (both existing
and proposed). This is a 200 cm long, 75 cm diameter
stainless steel vessel. Long scale length plasmas are produced
with Ln~20 m on axis (LT~4 m on axis) with numerous axially- and radially-scanned
probe diagnostics such as Langmuir probes, RF probes, magnetic
probes, and microchannel retarding grid energy analyzers. A millimeter-wave
interferometer system enables not only detailed time-resolved
measurements of the spatial variations in plasma density, but
also permits measurements of plasma wave activity via collective
The second device is somewhat smaller, 88 cm long and 60 cm in diameter, and has been utilized extensively in our earlier resonance absorption, magnetic field generation and vpXB acceleration studies. This device produces higher density but shorter scale length plasmas (Ln~50-100 cm on axis).
UC Davis also has a wide variety of high intensity microwave sources available for microwave-plasma experiments covering a range of 1-94 GHz. Sources include moderate power (1-100 kW), long pulse duration (~50 µsec) TWT amplifiers (1-94 GHz) suitable for use in ion timescale studies (such as stimulated Brillouin scattering), and high power (1 MW - 1 GW), short pulse duration (1-10 µsec) klystron amplifiers (up to 11.424 GHz), gyrotrons/gyroamplifiers, crossfield amplifiers and magnetron oscillators (up to 35 GHz) suitable for use in electron timescale studies (such as stimulated Raman scattering). It should be noted that extensive facilities are available at UCD for the design and fabrication of both RF sources and power supplies/modulators such that additional RF sources may be built if required.