The saturated slate of a low frequency coherent, m=2, global mode has been studied in the Encore tokamak using probe techniques. The mode is found to have large fluctuations in density, electron temperature, space potential and magnetic field. The equilibrium plasma characteristics were also determined with probe measurements. Magnetic probes were used to determine the radial profile of the poloidal magnetic field, from which the rotational transform and current density profiles could be deduced. A Langmuir probe was used to measure radial profiles of the density, electron temperature and space potential.
By comparing the experimental measurements with the predictions of a computer code. the mode was identified as a finite βτ modified drift wave. The code is based on a linear, two-fluid theory of the coupling of drift and shear-Alfven modes. Of the two shear-Alfven solutions and the drift branch solution, it was found that the drift wave solution best fit the observed frequency of the mode and the relative amplitudes of the density, space potential and magnetic fluctuations. The identification of the mode as a finite βτ modified drift wave means that the mode is more closely related to the higher frequency, turbulent fluctuations observed on larger machines. rather than the lower frequency, coherent Mirnov oscillations.
