Current driven turbulence and microturbulent spectra in the TORTUR Tokamak

Abstract

In the TORTUR Tokamak (R=0.46 m, a-0.085 m, BT=2.9 T, Ip<or=55 kA, Te approximately=Ti<or=1 keV, n<or=1020 m-3, Zeff(imp)<2) plasmas are heated by current driven turbulence in the build-up phase to high beta values and electron temperatures a factor of two above values retrieved from scaling law studies reported in literature for ohmically heated Tokamaks. A stationary state of current driven turbulence with the same beta value can be maintained during the plateau state by virtue of relatively high loop voltages (2-4 times the classical value) giving proportionally enhanced resistive dissipation. The turbulent fluctuations of the plateau state have been observed at r/a=0.7 by collective scattering of microwaves in a wide spectral region for Ip 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.