Interpretability of magnetic diagnostics in tokamks—search for a locked mode in the TFTR (tokamak)

The interpretability of magnetic diagnostics is discussed in the context of searching for a locked mode in a tokamak. Apparent external signatures of a locked-mode perturbation determined by a magnetic locked-mode diagnostic route are found under some circumstances to be at variance with the interior structure of the perturbation measured by an electron cyclotron emission diagnostic route. This discrepancy suggests the possibility that locked-mode detector signals may be misinterpreted under some conditions. It is conjectured that toroidally asymmetric ‘halo’ currents may be partly responsible for the locked-mode detector response and observed detrimental effects on the discharge.