Measurement and qualitative interpretation of the ra dial scale of turbulence in JET plasmas

Introduction and Method Turbulence generally dominates transport in tokamak plasmas [Doyle2007, Conway2008]. It is believed to play a central role in the formation of internal transport barriers (ITBs) and the edge transport barrier through the reduction of anomalous transport by sheared poloidal flow. In JET, there is ongoing work on the influence of rotational shear on ITB dynamics [Crombe2009], and recent studies on the radial electric field indicate that its increase does not necessarily precede the L-H transition [Andrew2008]. The experimental characterization of turbulence in JET is thus necessary to, for instance, complement such flow shear studies and contribute to the understanding of ITB and H-mode physics. Here, measurements of the radial correlation length of density fluctuations made during different phases of JET plasmas are discussed in relation to changes in transport and confinement. Both correlation length L and coherent reflection G are directly calculated from raw radial correlation reflectometry data, which relies on the variation of coherence with the separation between the cutoff positions of two probing microwave beams [Figueiredo2008]. A spectral approach is used to calculate G and provide some sensitivity to variations of δn/n without requiring the DC components of the reflectometer signals [Figueiredo2008], which are currently unavailable. Calculations use the magnetic field from reconstructed equilibria and electron density profiles from the high resolution Thomson scattering diagnostic. Although realistic modelling of correlation reflectometry in JET will be required to obtain the actual turbulence correlation length Lδn and level δn/n from L and G [Kramer2003], it is possible to make a qualitative interpretation of variations in L and G based on published simulations [Kramer2003] and on the understanding of the interplay between turbulence level and reflectometer correlation length [Kramer2003, Gusakov2004]. In fact, a variation of L signifies more than just a variation of Lδn. For higher turbulence levels complicated interference patterns arise and the phase of the microwaves becomes increasingly chaotic and poorly localized [Mazzucato1996]. Consequently, coherence is lost between the reflectometry signals from the two probing beams, which leads to lower values of L. Simulations in [Kramer2003] show that if G does not change too much, as in the analyses presented here, to a variation of L corresponds a matching variation of Lδn and an opposite variation of δn/n —