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Title	107: Error field detection by application of a "spiraling" error field
Name:	Daisuke Shiraki shirakid@fusion.gat.com	Affiliation:	Oak Ridge National Lab
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):	N. Logan, E. Olofsson, F. Volpe	ITPA Joint Experiment :	No
Description:	The n=1 intrinsic error field can be inferred from a single discharge by the application of a slow "spiraling" error field (uniform rotation with growing amplitude) in the presence of a 2/1 locked mode. The mode comes into torque balance such that it locks to the total n=1 error field, which is the sum of the static intrinsic error field and the spiraling applied error field. The intrinsic error field is deduced from analysis of the locked mode phase relative to the applied phase of the spiraling field, as well as an analysis of the torque experienced by the mode from the total error field.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	A non-disruptive 2/1 locked mode will be created by a beta ramp in a low-rotation (balanced injection) plasma. Once the mode is locked, a slow (~1Hz) n=1 spiraling error field will be applied by the I-coils to vary the total EF and therefore the phase of the locked mode. The amplitude of the spiraling field will begin at less than the expected intrinsic EF (~1kA in the I-coils) and increase to greater than this value over a few cycles, efficiently sampling the amplitude-phase EFC operating space. The inferred value of the EF from this method gives an empirically optimized set of currents for n=1 EFC. Upon optimization of the EFC currents, a rotating EF can be super-imposed to the optimized EFC currents in an identical discharge, and the quality of the EFC can be assessed by the uniformity of the locked mode rotation. The same technique will be used with the C-coils to optimize the currents for C-coil EFC. If time permits, a spiraling n=2 field applied to a 3/2 locked mode may be used to study the intrinsic n=2 EF.
Background:	Sampling a large range of amplitude and phase with continuous torque measurements due to a spiraling EF ensures a wide range of rotation behavior while providing a detailed mapping of the EFC contours in a single discharge. The torque experienced by the locked mode from the spiraling EF can be calculated based on a model for the perturbed currents in the island, or computed directly through measurement of the Maxwell stress tensor. The new 3D magnetics should allow improved measurements of the Maxwell stress. This technique provides an alternative to (and can be benchmarked against) the current method of error field detection based on the low-density locked mode threshold. The spiraling EF technique has the advantage of only requiring a single non-disruptive discharge, and not being limited to low-density ohmic plasmas.
Resource Requirements:	I240- and C-coils with SPAs
Diagnostic Requirements:	3D magnetics, ECE
Analysis Requirements:	--
Other Requirements:	--