Asymmetric-field mode locking in Alcator C-Mod

Full text: Asymmetric field coils have been installed on Alcator C-Mod and used to establish the locked mode threshold and scaling for this compact size device. The magnitude and phase of the intrinsic field error has been identified and is found to be consistent with a comprehensive model of the sources of field errors. There proves to be no major difference between the fractional (m=2,n=1) field level (typically 1.e-4 of toroidal field) for mode locking on Alcator and that on JET (which has 4.3 times the linear dimensions) for typical densities on each machine. This result is a first direct experimental indication based on size scaling that locked modes should not be more problematic for ITER than for existing devices. Initial results are consistent with a linear scaling of the total asymmetric field locking threshold with density, as has been observed elsewhere, and within a factor of 2 of the scaling previously deduced from the toroidal field scaling on JET or DIII-D using Connor-Taylor constraints. Specifically chosen dimensionlessly similar plasmas in C-Mod and JET are under investigation to determine more definitively the field and size scalings. (author)