Identification and measurement of core plasma turbulence by observation of fast electrons in TEXT-U

A method to investigate the nature and level of the turbulence in the core of hot tokamak plasmas is discussed. The method is based on the energy-resolved observation of superthermal electrons by means of vertical electron cyclotron emission. The sensitivity of the electron distribution function and of the related emission spectra to both electrostatic and magnetic turbulence is first investigated theoretically. Then, the method is applied to the study of electron cyclotron resonance heated plasmas in the TEXT-U tokamak. All the measured spectra appear to be dominated by effects due to magnetic rather than to electrostatic turbulence. The experimental spectra are compared with the results of 3D Fokker - Planck simulations including radial diffusion, and the corresponding levels of magnetic turbulence are deduced.