Current driven turbulence and microturbulent spectra in the TORTUR Tokamak

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Abstract
In the TORTUR Tokamak (R=0.46 m, a-0.085 m, BT=2.9 T, Ipe approximately=Ti20 m-3, Zeff(imp)p approximately=30 kA corresponding with plasmas in the safe q range: q(a)=7. Three frequency domains of turbulence have distinguished with rather arbitrary separation, interpreted as follows: (a) A low-frequency region (0-0.7 MHz) containing the frequently reported electrostatic drift modes and low-frequency electromagnetic modes which may account for a considerable fraction of the electron heat diffusivity chi e(r); (b) Unstable magnetic microtearing modes driven by a supercritical electron temperature gradient characterized by real frequencies in the interval 0.7-3 MHz. Indirect evidence for the presence of this magnetic tearing exists by the observation of distinct satellites in the Thomson scattering spectrum; (c) A magnetic fluctuation spectrum in the high-frequency region (5-55 MHz) which can be ascribed to current driven modes from non-thermal electron tails around 50 keV. All spectral domains exhibit slow time periodicities synchronous with the development of high-energy tails of the ions and with the Mirnov-type small sawteeth found on the loop voltage, electron cyclotron emission signals and soft X-rays.