Numerical modeling of electron beam-plasma interactions
Beam plasma interactions in density gradients can produce narrow hf structures.
The \hf spikes
are seen in experiments with a hot cathode as electron
source, and in particle in cell simulations. Using a new parameterization
technique for non-Maxwellian distribution functions in homogeneous
plasmas, it is found that the spatial growth in the spike cannot be explained
by a kinetic beam-plasma instability. Waves entering denser plasma are reected.
The interference, of a forward wave and a backward wave, produce
a narrow region of standing waves surrounded by regions dominated by
travelling waves. A dynamic uid model, incorporating the static electric
eld and the inhomogeneous electron density, is solved by numerical methods.
The solution reproduces a standing wave pattern with the maximum
amplitude at a position with a local plasma frequency close to, but below,
the oscillation frequency. The oscillation frequency is found to be a normal
mode of a short-circuited diode. The amplitude of the eld in the diode,
and thus the amplitude at the spike, is determined by the total ac current,
which is limited by the dc electron density level close to the cathode. To incorporate
kinetic e ects as the driving instability in the beam and resonant
damping, a new method for eigenmode analysis in inhomogeneous diodes
is developed. No applications of this method for the spike are reported but
it is shown that singular space charge structures, producing corners in the
electric eld, can form in the re ection regions.
School:Kungliga Tekniska högskolan
Source Type:Doctoral Dissertation
Date of Publication:01/01/1999