  A new non-disruptive error field (EF) assessment technique not restricted to
low density and thus low beta was demonstrated at the EXTRAP-T2R reversed field
pinch. Stable and marginally stable external kink modes of toroidal mode number
n=10 and n=8, respectively, were generated, and their rotation sustained, by
means of rotating magnetic perturbations of the same n. Due to finite EFs, and
in spite of the applied perturbations rotating uniformly and having constant
amplitude, the kink modes were observed to rotate non-uniformly and be
modulated in amplitude. This behavior was used to precisely infer the amplitude
and approximately estimate the toroidal phase of the EF. A subsequent scan
permitted to optimize the toroidal phase. The technique was tested against
deliberately applied as well as intrinsic error fields of n=8 and 10.
Corrections equal and opposite to the estimated error fields were applied. The
efficacy of the error compensation was indicated by the increased discharge
duration and more uniform mode rotation in response to a uniformly rotating
perturbation. The results are in good agreement with theory, and the extension
to lower n, to tearing modes and to tokamaks, including ITER, is discussed.
