Beam Physics Seminar
There is always some ambient gas in electron beam devices and
background ionization is ubiquitous. For long pulse times, the
electrostatic potentials associated with this ionization can reach
significant levels and give rise to such observed phenomena as
phase noise in microwave tubes. Observations of noise in microwave
tubes such as coupled-cavity traveling wave tubes and klystrons
have been discussed in the literature and will be briefly reviewed.
In order to explain this phenomenon, a 1D hybrid model of the ion
effects in microwave tubes has been developed in which the electron
beam is treated as a fluid using the beam envelope equation, and
the ions generated by beam ionization are treated as discrete
particles. The effect of secondary electrons is neglected. The
ionization rate depends on the ambient gas pressure and species
as well as on the electron beam current and energy. Based on this
rate, ions are created and distributed on an axial grid on each
time step. The ion charge is then mapped onto the grid, and Poisson's
equation is then solved in 1D under the assumption that the transverse
scale lengths are less than the betatron wavelength of the electron
beam. The ion charge distribution is then used to integrate the
beam envelope equation that updates the beam equilibrium. The ion
motion is then integrated subject to the wall potential, the
space-charge potential of the electron beam, and the self-consistent
ion potential. This process is iterated over any desired pulse time.
The coupling between the ionization and the electron beam equilibrium
is responsible for the phase noise. In effect, the ion motion
between and within the different wells as well as ion draining to
the cathode and collector results in a dynamic retuning of the
electron beam equilibrium. Oscillations are observed on many
time scales. The fastest time scale oscillation is related to
the bounce motion of ions in the axial potential wells formed by
the scalloping of the electron beam. Slower oscillations are
observed to correlate with the well-to-well interactions induced
by the ion coupling to the electron equilibrium. These oscillations
have observable effects on ion dumping to the cathode or collector.
*Work supported by the Office of Naval Research.
Talk slides: PDF