DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	364: Absolute plasma response null measurement by multiple betaN steps
Name:	Carlos Paz-Soldan paz-soldan@fusion.gat.com	Affiliation:	Columbia University
Research Area:	Stability & Disruption Avoidance	Presentation time:	Not requested
Co-Author(s):	S & DA team	ITPA Joint Experiment :	No
Description:	This proposal aims to demonstrate a new technique to experimentally determine optimum error field control (EFC) currents that is compatible with high-performance plasma scenarios. The measured optimum current levels will be compared to results with existing techniques. The technique is predicted to be able to determine the absolute plasma response null (which has been previously shown to be near optimum correction levels) using multiple betaN steps. The coil current prior to the betaN step is varied (per step), and the coil current is quickly scanned after the betaN step to discover the relative plasma response null. Because the structure of the measured plasma response after the step is proportional to the coil current before the step, repeating the scan with different initial conditions allows an experimental determination of the absolute plasma response null. It is proposed to do this experiment with and without a known (proxy) error field. The proxy field case will serve as a benchmark of the technique and ensure the plasma response null is where it is predicted by modeling (and the experiment described in RoF#188, if it occurs). The proxy-off case can then be confidently interpreted and yield the required current to null the plasma response to the machine intrinsic error. It is possible to skip the proxy step and jump straight to the intrinsic only case, though at a cost of reduced confidence in the final result.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	The execution of the experiment requires stepping a stable plasma from a lower beta to a higher beta, if possible several times within a discharge. The coil currents before and after the step will be pre-programmed to map out the plasma response in the compass scan space after each step. This information will yield the absolute plasma response null, and thus a prediction of the optimal error field correction currents for that equilibrium. This proposal will complement #188, where a rotation optimization will be used to independently determine optimal correction currents. The discharge scenario should be similar to that of the 2011 TBM experiment so that the results from the two may be compared. The reference shot is 147135, although extra betaN steps will be added. These measurements may also be useful for the new 3D magnetics diagnostic as the plasma response will be synchronously detectable. This could also be tried for n=2 if the n=1 experiment is a success.
Background:	The current technique used most often used to optimize error field correction in high-performance scenarios is dynamic error field correction (DEFC), which minimizes the change in the plasma response to a step in betaN using feedback. This process, however, is fundamentally iterative and requires several dedicated discharges. Furthermore, there is no transparent way to ascertain if the results are indeed optimal or if a feedback error took has taken. The present technique is theoretically possible to accomplish in a single (long-pulse) discharge, requires no deleterious instability, and relies on our knowledge of the plasma response functional form. Furthermore, it is entirely pre-programed so could become a routine tool for the future with minimal control room tweaking.
Resource Requirements:	BetaN control is critical. Six 3D power supplies will need to be available (4 SPAs and 2 Cs).
Diagnostic Requirements:	ECE, CER, MSE, SXR, Thomson, 3D magnetics.
Analysis Requirements:	Normal suite of 3D field analysis tools.
Other Requirements:	--