  This work addresses the identification of zonal flows in fusion plasmas.
Zonal flows are large scale phenomena, hence multipoint measurements taken at
remote locations are required for their identification. Given such data, the
Biorthogonal Decomposition (or Singular Value Decomposition) is capable of
extracting the globally correlated component of the multipoint fluctuations. By
using a novel quadrature technique based on the Hilbert transform, propagating
global modes (such as MHD modes) can be distinguished from the non-propagating,
synchronous (zonal flow-like) global component. The combination of these
techniques with further information such as the spectrogram and the spatial
structure then allows an unambiguous identification of the zonal flow component
of the fluctuations. The technique is tested using gyro-kinetic simulations.
The first unambiguous identification of a zonal flow at the TJ-II stellarator
is presented, based on multipoint Langmuir probe measurements.
