It is found on the KT-5C tokamak that an edge fluctuation can be
excited using
a pair of floating Langmuir-probe pins. This fluctuation propagates along
the
magnetic field, with its amplitude decreasing drastically when it is off
the
magnetic field line, which is similar to the results obtained from experiments
on
the TEXT tokamak. Moreover, our experimental observations indicate that
the
excited fluctuation propagates with the electron directional motion, and
is
mainly manifested in an electron density fluctuation. Physically, this
excited
fluctuation could be considered as modulation of the external electric
field by
electrons, and is carried by the electron directional motion wherever it
goes.
These results are in good agreement with the proposed ballistic model that
can
be derived by solving the linearized Vlasov equation.