Experimental Study of the Shear Alfven Resonance in a Tokamak

Studies of a new rf heating technique in tokamaks, Shear Alfven Resonance Heating, have been performed on the Tokapole II tokamak at the University of Wisconsin. High power heating experiments have been preceded by careful identification of the resonance and its properties at low power. According to MHD theory, the shear Alfven resonance manifests itself as a resonant enhancement of the wave magnetic field perpendicular to the equilibrium field at the location in the non-uniform plasma where the frequency, (omega), and parallel wavelength match the Alfven speed v(,A), i.e. (omega) = k(,(PARLL))v(,A). Experiments on the Tokapole II device have demonstrated the existence of the shear Alfven resonance in a tokamak by direct probe measurement of the wave magnetic field within the plasma. The resonance is experimentally identified as a radially localized enhancement of the poloidal wave magnetic field. The radial location of the resonance agrees with a 2-D MHD calculation which includes toroidicity and non-circularity of the plasma cross-section. Other properties of the resonance such as polarization, radial width, risetime to saturation, and resonant enhancement over the driving vacuum rf fields are found to be in good agreement with theory. The resonances are driven by a novel launching structure which utilizes the four existing poloidal divertor rings within the vacuum vessel of Tokapole II. RF currents at (TURN)1.2 MHz are driven through the mechanical supports of the divertor rings, and are superposed upon the transformer -induced equilibrium field shaping currents. Proper phasing of the currents in each ring allow approximation of poloidal mode numbers of m = 1,2, or 4, and the toroidal structure of the antenna currents yields a step-wise approximation to toroidal mode numbers n = 1 and 2. However, a broad n and m spectrum is generated. These studies have provided the experimental base from which high power heating experiments utilizing the shear Alfven resonance can be pursued.