Turbulent fluctuations with the electro gyro-scale in the national spherical torus experiment

Various theories and numerical simulations support the conjecture that the ubiquitous problem of anomalous electron transport in tokamaks may arise from an electron gyro-scale turbulence driven by the Electron Temperature Gradient (ETG) mode. To check whether such a turbulence is present in NSTX plasmas, measurements of turbulent fluctuations were performed with coherent scattering of electromagnetic waves using a novel scattering geometry with a radial resolution of ± 2.5 cm. Measurements in plasmas heated by high harmonic fast waves (HHFW) confirm the existence of turbulent fluctuations with the electron gyro-scale in the range of k -perpendicular r_s = 8 - 16. This seems to suggest that neither the Ion Temperature Gradient mode nor the Trapped Electron Mode is the source of measured fluctuations. Agreement with numerical results from the linear gyrokinetic GS2 code supports the conjecture that the observed turbulence is caused by the ETG mode. (author)