Assessment of Error Field Control with the ``\boldmath{$n=1$} Coil''*

Small deviations from the nominally axisymmetric field of a tokamak (termed the `error field') can drastically alter plasma performance. Improved performance can be achieved using arrays of non-axisymmetric coils to optimize the error field. One metric for evaluating error field control is the locked-mode density limit in Ohmic plasmas, where better compensation allows operation at lower density. Based on this metric, not all coils are equally beneficial for error field control. On \hbox{DIII-D}, the best correction was achieved by using a circular coil (called the `$n=1$ coil') placed above and off-center to the machine in conjunction with an array of midplane saddle coils (called the \hbox{`C-coil'}) [1]. We use recently developed analysis techniques that include the plasma response to examine why this coilset proved so successful on \hbox{DIII-D}. The distinct roles of resonant and non-resonant error fields will be assessed. \vskip6pt\noindent [1] J.T. Scoville and R.J. La Haye, Nucl. Fusion {\bf 43}, 250 (2003).