Observation of Flat-Top Velocity Distributions of Electrons in Turbulently Heated Plasmas

Velocity distributions of the electrons have been measured by a Thomson-scattering method with an eight-channel polychoromator in the initial stage of a linear turbulent-heating experiment. Flat-top distributions were observed associated with high-frequency fluctuations ($\omega \lesssim {\omega }_{\mathrm{pi}}$) just before the onset of fast heating of the electrons. Those results were compared with computer simulations and theoretical analysis based on quasilinear process.