A diagnostic system based on the heavy ion beam probe technique has been developed for the Elmo Bumpy Torus. A variety of singly charged alkali metal ions have been used in the probing beam; detection of multiply charged reaction products allows a continuous spatially resolved monitor of the plasma space potential. Typical spatial resolution is 1 cm−3; time resolution is on the order of 1 s. The potential measurement is direct and properly referenced to the plasma ground. An investigation into possible sources of error in the measurement points to the conclusion that the uncertainty is an order of magnitude less than the experimentally measured potentials.
The initial processes excited directly by high-frequency (HF) heating waves include parametric decay instabilities, which decay the HF heating wave to a frequency-downshifted Langmuir/upper hybrid sideband together with an ion acoustic/lower hybrid wave as the decay mode, and oscillating two stream instabilities, which decay the HF heating wave to two oppositely propagating Langmuir/upper hybrid sidebands and purely growing mode/field-aligned density irregularities. These instabilities provide effective channels to convert heating waves to electrostatic plasma waves in the F region of the ionosphere. The following-up parametric instabilities include the cascades of Langmuir pump waves into Langmuir sidebands and ion acoustic waves/lower hybrid waves, and the decay of upper hybrid waves to Langmuir sidebands and ion decay modes, as well as the filamentation of those HF electrostatic waves to generate field-aligned density irregularities. The instability thresholds, growth rates, angular distribution and regions of excitation are determined.
Two secondary parametric instabilities providing cascade channels for the
Langmuir sidebands of the oscillating two-stream instability (OTSI) and parametric
decay instability (PDI), which are excited by O-mode high-frequency (HF) heating
waves, are studied. The first one decays a Langmuir pump wave into a Langmuir
sideband and an ion acoustic decay mode. Both resonant and nonresonant cascade
processes are considered. Nonresonant cascade of Langmuir waves proceeds
at the same location and is increasingly hampered by the frequency mismatch effect.
Resonant cascade takes place in different resonant locations to minimize the
frequency mismatch effect, but it has to overcome the severe propagation loss of the
mother Langmuir wave in each cascade step. This process produces a narrow spectrum
of frequency-downshifted (from the HP wave frequency) plasma waves. The
second employs the lower-hybrid wave as the decay mode. Only the nonresonant
cascade is of interest, because the propagation loss of the mother Langmuir wave
in each resonant cascade step is far too severe. This is a three-dimensional coupling
process, because the wavevectors of coupled three waves have to be matched in
three-dimensional space, rather than matched in the conventional way on the plane
of the pump wavevector and the geomagnetic field. A broad spectrum of frequency-downshifted
plasma waves can be produced by this process in a narrow altitude
range preferentially located near the matching heights of Langmuir sidebands of
the OTSI and PDI.
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