Experimental investigation of the propagation properties of an edge fluctuation in the KT-5C tokamak

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. (Author)